< Anterolateral abdominal wall >
Fig. 6-1. Anterolateral abdominal wall muscles.
Among the anterolateral abdominal wall muscles, the terms “external and internal oblique muscles” are conveniently employed instead of the official long terms “external and internal abdominal oblique muscles.” This is acceptable because there are no such oblique muscles in other regions of the body. The obliquus capitis muscles do not confuse because they are written in Latin (Fig. 1-15).
The three anterolateral abdominal wall muscles can be studied in relation to the thoracic wall muscles. The external oblique muscle matches the external intercostal muscle, the internal oblique muscle matches the internal intercostal muscle (Fig. 5-1), and the transversus abdominis matches the transversus thoracis (Fig. 5-3). A thoracoabdominal nerve (Fig. 5-5) passes just deep to the internal intercostal muscle (Fig. 5-4) and to the internal oblique muscle (Fig. 6-5).
Moreover, the direction of muscle in each layer is alike between the abdominal and thoracic walls (Figs. 5-1, 6-1). The external oblique muscle has the origin at ribs and the insertion at iliac crest. So the direction of the muscle is same as the external intercostal muscle.
Reversely, the internal oblique muscle has the origin at iliac crest and the insertion at ribs. Transversus abdominis has the two origins of ribs and iliac crest. The thoracolumbar fascia which is another origin of the internal oblique muscle and transversus abdominis (Fig. 1-8) is not drawn in Fig. 6-1.
All three muscles have common insertions which are the linea alba and the pubic crest (Figs. 6-5,8).
Fig. 6-2. Actions of anterolateral abdominal wall muscles excluding rectus abdominis (anterior view).
The action of the three muscles is to rotate the trunk and laterally flex the trunk. The muscles are attached to the ribs to allow for this movement (Fig. 6-1).
Fig. 6-3. Action of the anterolateral abdominal wall muscles excluding the rectus abdominis (horizontal plane).
Another notable action is to compress the abdominal cavity since the muscles surround the abdominal cavity in total. For this movement, the muscles are attached to the linea alba (Figs. 6-1,5).
Fig. 6-4. Schematic (left) and realistic (right) drawings of rectus abdominis.
Anterolateral abdominal wall muscles also include the rectus abdominis. The rectus abdominis has its origin at the pubic crest and insertion at the xiphoid process and adjacent ribs (to be exact, costal cartilages (Fig. 5-1)). As a result, this muscle allows for sit-ups (forward flexion of the trunk).
The lateral border of the rectus abdominis is called the linea semilunaris. This is because the border is curved like a half moon, even though it resembles a crescent more.
The bilateral rectus abdominis muscles have a white midline border called the linea alba. Each rectus abdominis is made up of four muscle bellies and intervening tendinous intersections. A strenuous exercise of these muscles induces hypertrophy of the muscle bellies; the linea alba and tendinous intersections thus become relatively depressed. With the thinning of the overlying subcutaneous tissue, the muscles appear as a six-pack.
Fig. 6-5. Rectus sheath superior to arcuate line (top) and inferior to arcuate line (bottom) (horizontal plane).
Just as deep back muscles are encircled by the thoracolumbar fascia (Fig. 1-8), the rectus abdominis is encircled by the rectus sheath. Similar to the thoracolumbar fascia, the rectus sheath functions as the aponeurosis of other muscles.
The external and internal oblique muscles form the anterior layer of the rectus sheath, while the internal oblique muscle and transversus abdominis form its posterior layer. The anterior and posterior layers arrive at the linea alba, so the linea alba is regarded as the insertion of the three muscles (Fig. 6-1).
There is an exception worth noting. Inferior to the arcuate line (Fig. 6-4), all three muscles contribute only to the anterior layer of the rectus sheath. Its posterior layer below the arcuate line does not exist.
Fig. 6-6. Rectus sheath (sagittal plane).
In other words, the arcuate line is the inferior border of the posterior layer of rectus sheath. A sagittal view of the muscle and rectus sheath also displays that the anterior layer sticks to the tendinous intersections unlike the posterior layer. This anatomy is supportive of the appearance of the rectus abdominis, the six-pack (Fig. 6-4).
A deeper structure is the “transversalis” fascia that is derived from the deep fascia of “transversus” abdominis (Figs. 6-5,8).
The rectus abdominis as well as the lateral three muscles are innervated by the thoracoabdominal nerves (Figs. 5-5, 6-5). The linea alba is an ideal incision site when approaching the peritoneal cavity because it would not damage any nerves or the inferior epigastric artery (Fig. 6-30).
< Inguinal canal, scrotum >
Fig. 6-7. Aponeurosis of external oblique muscle.
Unlike Fig. 6-1, this figure demonstrates actual features of the insertion of the external oblique muscle. Its aponeurosis has a free inferior margin between the anterior superior iliac spine and pubic tubercle. This called is the inguinal ligament which serves as the boundary between abdomen and lower limb (Fig. 5-5). The aponeurosis has a gap above the pubic crest. This is called the superficial inguinal ring.
Fig. 6-8. Sheets forming inguinal canal (anterosuperior view).
There are three meaningful sheets that form the inguinal canal. The superficial sheet is the aponeurosis of the external oblique muscle. The intermediate sheet is the fusion of the internal oblique muscle and the transversus abdominis. The common aponeurosis of these two muscles attached to the pubic crest is called the inguinal falx (Fig. 6-1). The deep sheet is the transversalis fascia. During dissection, the transversalis fascia cannot be distinguished from the parietal peritoneum that lies directly underneath it (Fig. 6-9).
The defects of the superficial and deep sheets are the superficial and deep inguinal rings. The two rings serve as openings of the inguinal canal. The superficial inguinal ring is medial to the deep inguinal ring, which means that the inguinal canal is oblique. This obliquity prevents the small intestine from herniating through the inguinal canal (Fig. 6-30).
Fig. 6-9. Shortening of male gubernaculum.
To understand the inguinal canal, one must first understand its embryology. In the case of male, a string called gubernaculum connects the testis with the scrotum, a portion of skin. During the development, the gubernaculum gets shorter and shorter until it disappears. Thus the gubernaculum pulls the testis down into the scrotum. During the descent, the testis drags the following three structures into the scrotum.
First, the testis takes along with the spermatic cord including ductus deferens (Figs. 6-12, 7-24). To put it another way, the spermatic cord elongates as much as the gubernaculum shortens.
Second, the testis takes along with the parietal peritoneum to the scrotum. The peritoneum that firmly covers the testis (tunica vaginalis) is like the peritoneum that firmly covers the gastrointestinal tract (tunica serosa) (Figs. 6-15,34).
Fig. 6-10. Tunica vaginalis.
As a result of development, the tunica vaginalis coats the testis and epididymis (Fig. 6-12).
Fig. 6-11. Isolated tunica vaginalis. (External structures are omitted.)
Further development leads to the obliteration between the original parietal peritoneum and the tunica vaginalis. If not, gravity and intraabdominal pressure would push the serous fluid or even the small intestine down into the scrotum. This type of intestinal descent is called an indirect inguinal hernia (Fig. 6-30).
Third, the testis takes along with the three sheets (Fig. 6-8). The extension of the external oblique muscle is the external spermatic fascia which starts from the superficial inguinal ring (Figs. 6-7,9).
The extension of the internal oblique muscle is the cremaster (muscle) (Fig. 6-9). It is not difficult to observe the continuity of the two muscle bellies during the dissection of a male cadaver. (There is no connection of the transversus abdominis with the cremaster.)
Even though the cremaster is a skeletal muscle like the internal oblique muscle, this exceptional muscle involuntarily elevates the testis when the temperature drops. Male readers may try to evoke this involuntary elevation by stroking the medial upper thigh. This is the cremaster reflex by the femoral branch (somatic sensory nerve) and genital branch (somatic motor nerve) of the genitofemoral nerve (Fig. 6-55). (Other examples of involuntary skeletal muscles are the tensor tympani and stapedius in the tympanic cavity (Fig. 4-51).)
The “internal” spermatic fascia is frequently mistaken as being the extension of the “internal” oblique muscle. However, the “internal” spermatic fascia is the extension of the transversalis fascia (Fig. 6-9).
The internal spermatic fascia starts from the deep inguinal ring (Fig. 6-8). At the deep inguinal ring, only the internal spermatic fascia covers the spermatic cord. At the superficial inguinal ring, the internal spermatic fascia, cremaster, and external spermatic fascia all cover the spermatic cord. In males, the deep inguinal ring is smaller than the superficial one (Fig. 6-9).
To understand the male inguinal canal between the superficial and deep inguinal rings, two figures (Figs. 6-8,9) should be carefully compared with each other.
The spermatic cord looks like a TIE coming out of the inguinal canal. The coverings of spermatic cord are connected with TIE (Transversalis fascia, Internal and External oblique muscles) (Fig. 6-9).
The main content of the spermatic cord is undoubtedly the ductus deferens; additional important contents are the testicular artery (Fig. 6-48), testicular vein (Fig. 6-53), and the genital branch of genitofemoral nerve to innervate cremaster (Fig. 6-55), etc. In addition to the spermatic cord, the ilioinguinal nerve (sensory nerve) (Fig. 6-55) passes both the male and female inguinal canal.
Fig. 6-12. Testis, epididymis.
In the testis, the seminiferous tubules produce sperms. The tubule of 0.2 mm diameter can be grossly identified, like the pulmonary alveolus. Sperms advance toward the epididymis through the rete testis and efferent ductules. Just as milk stored in the lactiferous sinus is secreted (Fig. 2-6), sperms stored in the epididymis are ejaculated.
The long journey of the sperms (ejaculation) is made possible by the peristalsis of the smooth muscle of the ductus deferens (Figs. 7-24,26), not by the movement of the sperm tail. The word “deferens” means “carrying away” in Latin.
Fig. 6-13. Not shortening of female gubernaculum.
In the case of a female, the gubernaculum connects the ovary with the labium majus (Fig. 7-32). The underlying reason is that the ovary and labium majus are homologous organs of the testis and scrotum (Fig. 6-9). Unlike the male gubernaculum, the female one never gets shorter.
The proximal part of the gubernaculum between the ovary and uterus becomes the ligament of ovary (Fig. 7-35). The distal part becomes the round ligament of uterus that passes the inguinal canal (between the deep and superficial inguinal rings).
With the exception of the ilioinguinal nerve (Fig. 6-55), the content of the female inguinal canal (round ligament) is definitely thinner than that of the male inguinal canal (spermatic cord). Consequently, the female inguinal canal is thinner than the male one. Thus, the herniation of the small intestine through the inguinal canal (indirect hernia) (Fig. 6-30) is less likely to occur in female.
The cross sections of the round ligament of uterus and the round ligament of liver (Figs. 6-21,35) are both round. Such is also true for the teres minor and major (Fig. 2-4), since the word “teres” means round.
< Peritoneum >
Fig. 6-14. Visceral and parietal peritonea.
In order to give an explanation of the peritoneum, we must familiarize ourselves with a few definitions. Abdominal and pelvic organs are classified into two groups: intraperitoneal organs which are hung by mesentery and retroperitoneal organs which are not (Fig. 7-34). Intraperitoneal organs are oftentimes located anterior and are mobile.
Visceral peritoneum blankets the intraperitoneal organs completely and forms the mesentery. These three words always accompany one another: visceral peritoneum, intraperitoneal organ, and mesentery.
Parietal peritoneum covers the retroperitoneal organs partly and is in contact with the abdominal wall and pelvic wall (Fig. 7-34).
Fig. 6-15. Mesenteries of foregut, midgut, hindgut.
In gross anatomy, horizontal plane is viewed from the inferior. The tradition is followed by the horizontal (axial) plane of the CT and MRI. However, in embryology and neuroanatomy, horizontal plane is viewed from the superior (cranial). We have chosen to use the inferior view even when discussing embryology and neuroanatomy in this book, to offer some sense of unity.
Recall that during embryonic development, there used to be bilateral peritonea (Fig. 5-8). The two peritonea surround the foregut, midgut, and hindgut to configurate the ventral and dorsal mesenteries. The development of the three guts from the umbilical vesicle (yolk sac) is briefly introduced below.
Fig. 6-16. Head and tail folds to form foregut, midgut, hindgut.
Due to the head fold and the tail fold, umbilical vesicle gets into the embryo to develop into the foregut, midgut, and hindgut. The oropharyngeal membrane between the foregut and stomodeum (primitive oral cavity) will rupture to become the fauces that is the border between the pharynx and the oral cavity (Figs. 3-8,43). The cloacal membrane between the hindgut and proctodeum (anal canal below the pectinate line (Fig. 7-39)) will undergo another change (Fig. 6-29).
Fig. 6-17. Three arteries to foregut, midgut, hindgut.
The foregut, midgut, and hindgut are fed by the celiac trunk, superior mesenteric artery, and inferior mesenteric artery respectively (Fig. 6-48). (In fact, the foregut also includes pharynx and esophagus which are not supplied entirely by the celiac trunk.) After the development, two boundaries between the three guts become the middle of the duodenum (Figs. 6-42,43) and the middle of the transverse colon (Figs. 6-43,44). Residual umbilical vesicle is destined to disappear (Fig. 6-24).
Fig. 6-18. Mesenteries of foregut.
On the foregut level, the liver develops in the ventral mesentery and divides the ventral mesentery into the falciform ligament and the lesser omentum (Fig. 6-21). The lesser omentum is further categorized into the hepatogastric and hepatoduodenal ligaments, because the foregut contains the stomach and duodenum (Figs. 6-17,22).
Simultaneously, the spleen develops in the dorsal mesentery that is divided into the greater omentum and splenorenal ligament (Fig. 6-21).
Fig. 6-19. Mesentery of midgut, hindgut.
On the midgut and hindgut level, the ventral mesentery simply disappears and only the dorsal mesentery remains. This implies that the double peritoneal cavities merge into single peritoneal cavity (Fig. 6-14).
Fig. 6-20. Parietal and visceral peritonea.
The box in the figure above shows the single, united peritoneal cavity. While the six external surfaces represent the parietal peritoneum, the Γ (large gamma)-shaped four internal surfaces represent the visceral peritoneum. The parietal peritoneum is removed to demonstrate the visceral peritoneum and the intraperitoneal organs in the figure below.
Fig. 6-21. Visceral peritoneum, intraperitoneal organs.
As previously described, on the foregut level, both the ventral and dorsal mesenteries remain (Fig. 6-18). However, on the midgut and hindgut level, only the dorsal mesentery remains (Fig. 6-19). Compare this figure with others (Figs. 6-18,19).
The inferior border of the falciform ligament contains the round ligament of liver (formerly, umbilical vein) (Fig. 6-36) which exists between the umbilicus and portal triad (more precisely, portal vein) under the liver (Fig. 6-35). Roughly speaking, the inferior border of the lesser omentum contains the portal triad that connects the liver with the duodenum (Fig. 6-23).
The superior surface of the liver that is in direct contact with the diaphragm is called the bare area of liver because this is not covered by the peritoneum. Around the bare area, one can observe bilateral coronary ligaments including the triangular ligaments (Fig. 6-25).
Fig. 6-22. Lesser and greater omenta.
The lesser omentum is divided into the hepatogastric and hepatoduodenal ligaments. The greater omentum is subdivided into the gastrosplenic, gastrophrenic, and gastrocolic ligaments. Figs. 6-18,21 portray the gastrosplenic and gastrophrenic ligaments close to reality. The gastrocolic ligament, however, can be comprehended by further explanation that will be given in the following section (Fig. 6-25).
On a basic level of anatomy, ligament is a modified fibrous membrane of articular capsule that connects a bone to another bone (Figs. 2-58,59). However, in the peritoneum, there are three different types of ligaments. Among them, the coronary ligament, a border between the parietal and visceral peritonea (Figs. 6-21,25), may remind the readers of the pulmonary ligament, a border between the parietal and visceral pleurae (Fig. 5-29).
Fig. 6-23. Rotated stomach to change peritoneum, peritoneal cavity.
Let us have a close look at the Fig. 6-23(A). The foregut is rotated at 90 degrees counterclockwise in the inferior view (Fig. 6-39), and the locations of the liver, foregut, and spleen start to resemble those in adult (Fig. 6-22). Eventually, the term, splenorenal ligament may be understood as “renal” signifies the left kidney which is an original retroperitoneal organ.
An oblique plane is chosen in which the portal triad appears instead of the liver and falciform ligament (Fig. 6-23(B)). The peritoneal cavity is divided into the greater and lesser sacs with the boundary “omental foramen” behind the portal triad. The lesser sac has the synonym “omental bursa.” It is because the lesser sac is posterior to the lesser and greater “omenta”; and the lesser sac plays the “bursa” role for the stomach movement (B).
During dissection, students should tear the lesser omentum and slide a finger into the lesser sac. At the same time, students should put another finger in the several places of the greater sac to feel the two fingers meet at the omental foramen, greater omentum, and splenorenal ligament (Fig. 6-23(B)).
Let us direct our attention to the development of the gastrointestinal tract. The superior mesenteric artery is located at the center of the gastrointestinal tract, and it provides blood to the midgut (Figs. 6-16,17).
The superior mesenteric artery also acts as the axis of the rotation of the gastrointestinal tract. The cecum in the midgut is the spearhead of the rotation. The cecum rotates at 270 degrees counterclockwise in the anterior view, then descends to the proper place (Fig. 6-27).
Simultaneously, the stomach rotates along the cardia axis (Fig. 6-31) clockwise in the anterior view, situating the liver superior and right to the stomach.
The bottom left of the Fig. 6-24 demonstrates the gastrointestinal tract in a late state of development. However, one must note that unlike in this figure, the actual small intestine is much more elongated than the large intestine.
After the development of the gastrointestinal tract, in the sagittal plane, the liver is located superior to the stomach, and the transverse colon is located inferior to the stomach. The lesser sac is still posterior to the lesser omentum, stomach, and greater omentum like in the horizontal plane (Fig. 6-23).
The peritoneum has a tendency to fuse with one another. The greater omentum fuses with itself and with the dorsal mesentery of the transverse colon. As a result, a portion of the greater omentum, which connects the stomach and transverse colon, becomes the gastrocolic ligament (Fig. 6-22). If one picks up the gastrocolic ligament (looking like a cooking apron) of a cadaver, one is holding two layers of the greater omentum, or four layers of visceral peritoneum (Fig. 6-14).
Dorsal mesentery of the ascending colon fuses with the parietal peritoneum to form the fusion fascia and paracolic gutter.
Whenever bowling, anatomists think about the paracolic gutter.
The same peritoneal fusion (Fig. 6-26) happens in the duodenum and descending colon. As a result, the duodenum, ascending colon, and descending colon become retroperitoneal organs absent of any mesentery (Fig. 6-14). If one were to spread the fusion fascia in cadaver, these retroperitoneal organs will artificially become intraperitoneal organs.
The boundaries between the descending colon, sigmoid colon, and rectum are clear based on their definitions. The descending colon is an abdominal, retroperitoneal organ; the sigmoid colon is a pelvic, intraperitoneal organ; and the rectum is a pelvic, retroperitoneal organ (Fig. 7-10). The border between the abdominal and pelvic organs is the pelvic inlet (Figs. 7-1,34).
The last story on the peritoneum is the region around the umbilicus. If the anterior parietal peritoneum is viewed from the peritoneal cavity, three kinds of folds of peritoneum come into sight.
The median umbilical fold includes the median umbilical ligament extending from the urinary bladder to the umbilicus.
To understand the median umbilical ligament, comprehension of the embryology is crucial. The cloacal membrane between the hindgut and proctodeum has been introduced (Fig. 6-16). The cloacal membrane is divided into anal and urogenital membranes, by the grown tissue from the connecting stalk. Concurrently, the hindgut develops not only into a part of the digestive tract (proximal anal canal, etc.) but also into a part of the urinary tract (proximal urethra, urinary bladder). The anal membrane ruptures to remain as anal valves (Fig. 7-39); the urogenital membrane also ruptures to allow the urine passage (Fig. 7-25,33).
Allantois between the urinary bladder and the umbilicus is obliterated to become the median umbilical ligament. This is why the median umbilical ligament connects the urinary bladder and the umbilicus (Fig. 6-28).
Keep your eyes on the Fig. 6-28. The medial umbilical fold includes the medial umbilical ligament from the umbilical artery to the umbilicus (Fig. 7-21). This ligament priorly used to be the umbilical artery, allowing the fetal circulation (Fig. 6-36).
The lateral umbilical fold includes the inferior epigastric artery. The inferior epigastric artery from the external iliac artery (Fig. 7-19) feeds the rectus abdominis (Fig. 6-6) from behind, anastomosing with the superior epigastric artery (Fig. 5-7).
There is the deep inguinal ring, lateral to the inferior epigastric artery. Therefore, the ductus deferens in male (Fig. 6-9) or the round ligament of uterus in female (Fig. 6-13) hooks around the inferior epigastric artery.
The inferior epigastric artery, rectus abdominis, and inguinal ligament, together form the inguinal triangle. Due to the intraabdominal pressure (Fig. 6-3), the small intestine may escape from the abdominal cavity. When the small intestine pushes the inguinal triangle around the superficial inguinal ring, it is called a direct inguinal hernia. When the small intestine passes through the deep inguinal ring, inguinal canal, and superficial inguinal ring, it is an indirect hernia. As the names suggest, direct hernia takes the shorter route, whereas indirect hernia takes the longer route to exit the abdominal cavity (Figs. 6-8,9). The inferior epigastric artery is the criterion to differentiate the two types of the inguinal hernia.
< Digestive system >
The border between the esophagus (stratified squamous epithelium) and the stomach (simple columnar epithelium) is called the cardia, since this part is close to the heart. Recall that in the diaphragm, the esophageal hiatus (near cardia) is not distant from the caval opening (near heart) (Fig. 5-47).
Just superior to the cardia, the internal circular muscle (smooth muscle) of the esophagus is well developed to act as the sphincter. If this sphincter is slackened, gastric acid may flow upward and damage the esophagus.
The border between the stomach and duodenum is called pylorus which has a pyloric sphincter. After the digestion of food in the stomach, the pyloric sphincter physiologically gets relaxed to allow food to enter the duodenum. The Latin word “pylorus” means “gatekeeper.”
The stomach is divided into fundus, body, pyloric antrum, and pyloric canal.
Inside the stomach, there are longitudinal folds (rugae) that disappear when the stomach is distended by food.
Lesser and greater curvatures are the margins where the lesser and greater omenta attach (Figs. 6-21,22).
Fig. 6-32. Parts of small intestine.
The small intestine consists of duodenum, jejunum, and ileum. “Duode” in duodenum means twelve, since its length is same as the breadths of twelve fingers. Students should try to measure the length of the duodenum of cadaver by holding it with their fist (four fingers) three times. Despite its shortness, the duodenum is curved and further categorized into the superior, descending, inferior, and ascending parts.
The junction between the duodenum and jejunum is suspended by the suspensory ligament of duodenum, which originates from the right crus of diaphragm (Fig. 5-47).
At the end of the small intestine, a remarkable inside structure is the ileocecal valve. The valve prevents the reflux of food from the large intestine into the small intestine (Fig. 6-27).
The boundary between the jejunum and ileum is roughly estimated with the length ratio (jejunum : ileum = 2 : 3).
The jejunum is shorter but is more important in digestion and absorption. It has a few following morphologic characters. The jejunum has a thicker wall and a larger lumen. It also has more numerous circular folds that are greater in size. Additionally, abundant blood supply to the jejunum gives it a reddish color, and its mesentery has less fat since the fat is not helpful in the function.
The large intestine also has its own morphological characteristics. Compared to the small intestine, the length of the large intestine is short, but its cross sectional area is “large.” This is why we call it the “large” intestine.
The teniae coli in the large intestine are the three thickened parts of the external longitudinal muscle (smooth muscle). The thick teniae coli do not fully elongate during the development of the large intestine (Fig. 6-24) which results in the wrinkles (haustra). Note that the haustra differ from the circular folds: the circular folds are visible only inside the intestine (Fig. 6-33).
The teniae coli and hausta are pleural forms of the tenia coli and haustrum, respectively.
Lastly, the large intestine has omental appendices, which are fat protrusions surrounded by the peritoneum. “Omental” appendices are named after the greater “omentum” (gastrocolic ligament) of the transverse colon (Figs. 6-22,25), where the appendices are often encountered. Omental appendices are abundant also in the sigmoid colon; this structure can be said to be somewhat characteristic of intraperitoneal large intestine.
These traits of the large intestine in addition to the differences between the jejunum and ileum are important when performing the surgery of the abdominal cavity with restricted visual range.
An important function of the large intestine is the absorption of water from the ingested food. The stomach as well as the large intestine are not vital organs in terms of food absorption, whereas the small intestine is. Without the small intestine, we would not be able to properly digest or absorb food which is necessary for survival.
In the anterior view of the liver, falciform ligament seems likely to divide the liver. However, an imaginary plane that is shifted to the right and passes the gallbladder and inferior vena cava divides the liver into right and left lobes. This plane will allow us to divide the liver in halves of equal volumes.
The left lobe seems to include both the quadrate and caudate lobes in the inferior view. However, the caudate lobe is independent from the left and right lobes considering its vasculature. Detailed liver segments in the right and left lobes are not introduced in this book.
One can discover the portal triad in the porta hepatis (hilum of liver) that is between the quadrate and caudate lobes (Fig. 6-21). The portal triad consists of the portal vein (Fig. 6-45), common hepatic duct (Fig. 6-39), and proper hepatic artery (Fig. 6-42).
Among the portal triad, the vein is the thickest and the artery is the thinnest. One can think of the POrtal vein as being situated POsteriorly, and the common hepatic Duct as occupying the Dexter (right).
During the dissection, students should identify the round ligament of liver (connecting umbilicus to portal vein) in the falciform ligament (Fig. 6-21). Also, students should try to identify the ligamentum venosum (connecting portal vein to inferior vena cava). A tip to memorize these details is that the inferior Vena Cava is in contact with the ligamentum Venosum and Caudate lobe.
Fetal circulation must be understood to learn the embryological structures in the thorax and abdomen. The term “fetus” replaces “embryo” at the beginning of the 9th week (after fertilization). At that point, the organs are formed yet still growing.
The fetus is passively supplied with oxygen and nutrients. The umbilical vein which receives oxygen and nutrients from the placenta passes the umbilical cord and umbilicus and ends at the portal vein.
Why is the umbilical “vein” a “vein”? The placenta, made of the maternal tissue (from uterus) and fetal tissue (from zygote), is the real boundary between the mother and fetus. It means that the umbilical cord is undoubtedly a fetal tissue, and the blood vessel to the fetal heart must be called the umbilical “vein” and not the umbilical artery. The umbilical “vein” conveys oxygenated blood like the pulmonary vein (Fig. 5-35).
After birth, the umbilical vein is obliterated to become the round ligament of liver between the umbilicus and the portal vein (Figs. 6-21,35). Once the baby is delivered, its umbilical cord between the umbilicus and placenta is not taken care of because it naturally necrotizes off.
A majority of the blood in the portal vein need not enter the fetal liver which hardly works for orally ingested food. So the bypassing anastomosis, ductus venosus, is between the portal vein and the inferior vena cava. After birth, the ductus venosus is obliterated to become the ligamentum venosum (Fig. 6-35).
Most blood in the right atrium is not required to reach the fetal lung which does not work for the oxygenation of the blood. One bypass is the oval foramen, which is obstructed to become the oval fossa after birth (Figs. 5-15,16).
Another bypass is the ductus arteriosus which connects the left pulmonary artery to aortic arch. After birth, the ductus arteriosus is blocked and becomes the ligamentum arteriosum (Fig. 5-13). By the Latin inflection, arteriosum becomes “arteriosus” and venosum becomes “venosus” when placed after the term “ductus.”
The fetal blood, filled with carbon dioxide and body wastes, travels to the placenta by the way of umbilical artery. After parturition, the umbilical artery from the branching point of the superior vesical artery to the umbilicus is obliterated and becomes the medial umbilical ligament (Figs. 6-28, 7-21).
There are two medial umbilical ligaments (Fig. 6-28) (formerly, umbilical arteries) because of the bilateral internal iliac arteries (Figs. 7-19,21). On the other hand, there is only one round ligament of liver (Figs. 6-21,35) (formerly, umbilical vein) because of the single portal vein (Fig. 6-45). The two umbilical arteries and the singular umbilical vein can be found in the umbilical cord as well.
In the anatomy of the liver, emptying the blood in the liver into the inferior vena cava is accomplished by the three hepatic veins (Figs. 6-35,53). Unlike the portal triad, the hepatic veins only exist inside the liver since the inferior vena cava is attached directly to the liver (Fig. 6-35). Fig. 6-36 is only a schematic drawing that shows the hepatic vein outside the liver.
Right and left hepatic veins drain the blood from right and left lobes respectively; middle hepatic vein intervenes between the two lobes (Fig. 6-35). The tributaries of the three hepatic veins are intersegmental veins, while the branches of the portal triad are segmental (such as segmental arteries). The same architecture occurs in the lung (Fig. 5-36).
The components of the liver and the lung can be compared. The portal vein (Fig. 6-45) is equivalent to the pulmonary artery (Fig. 5-35), while the hepatic artery (Fig. 6-42) is equivalent to the bronchial artery (Fig. 5-37). The hepatic vein is definitely equivalent to the pulmonary vein. The hepatic duct (Fig. 6-39) is equivalent to the bronchus (Fig. 5-36) despite different functions of two ducts. Before birth, the ductus venosus and the ductus arteriosus serve as equivalent byroads (Fig. 6-36).
Recall that the stomach has undergone a 90 degrees rotation (Figs. 6-18,23). The duodenum is rotated 90 degrees also, to become a retroperitoneal organ (Fig. 6-26). At the same time, the common bile duct that enters the duodenum rotates an additional 180 degrees (in total, 270 degrees). Consequently, dorsal and ventral pancreatic buds that previously were located opposite from one another come in contact. Consider that the top figure is a left side view, while the bottom figure is an anterior view.
Thereafter, the dorsal and ventral pancreatic buds coalesce to form the pancreas, pancreatic duct, and accessory pancreatic duct. This embryology lets us understand why the common bile duct passes posterior to the superior part of the duodenum and the accessory pancreatic duct.
The gallbladder which concentrates the bile juice produced in the liver is not a vital organ. Dissectors cannot miss the gallbladder and adjacent ducts due to their green color from the bile juice. The green color usually spreads even to the adjacent peritoneum of the cadaver.
The cystic duct from the gallbladder is the border between the common hepatic duct and the common bile duct (Fig. 6-39). The common bile duct descends and meets the pancreatic duct to become hepatopancreatic ampulla which is surrounded by a sphincter (Fig. 6-40).
The opening of the hepatopancreatic ampulla is the major duodenal papilla in the descending part of duodenum. While the stomach has longitudinal folds (Fig. 6-31), the small intestine has circular (transverse) folds (Fig. 6-33). However, the duodenum also has a longitudinal fold beneath the major duodenal papilla (Fig. 6-40).
The minor duodenal papilla, which is the opening of accessory pancreatic duct, is located anterior and superior to the major duodenal papilla. This is because the AcceSsory pancreatic duct is Anterior and Superior to the hepatopancreatic ampulla (Fig. 6-40).
The pancreas is divided into the uncinate process, head, body, and tail. Like the neighboring duodenum, the pancreas is a retroperitoneal organ. In contrast, the spleen attached to the pancreas is an intraperitoneal organ. This seemingly contradicting phenomenon is solved by the fact that the tail of the pancreas passes the splenorenal ligament so as to get in touch with the spleen (Fig. 6-23). The pancreas is thus incompletely retroperitoneal.
The spleen purifies the blood like lymph node that cleanses the lymph. The spleen has no functional relationship with the pancreas with which it is in contact.
The topographic relation between the pancreas and the superior mesenteric artery and vein indicates the obliquity of the blood vessels. The superior mesenteric artery from the abdominal aorta (Fig. 6-48) passes the dorsal mesentery toward the anterior and inferior abdominal organs (Figs. 6-17,19,43).
After the having identified the spleen, students should try stripping its capsule inside the peritoneum. The capsule will get stripped only partially.
< Arteries, veins, lymph
The celiac trunk for the foregut has extremely complex branches; hence its name is celiac trunk, not celiac artery. Just after it arises from the abdominal aorta (Fig. 6-48), it trifurcates. One of them is the left gastric artery for the left part of the lesser curvature of the stomach.
Another is the splenic artery, from which the short gastric artery for fundus (Fig. 6-31) and the left gastroomental artery for the greater curvature arise. For convenience, let us say that the greater “omentum” monopolizes the gastro“omental” arteries. Thus, the lesser omentum is not supplied by the gastro“omental” arteries but by the gastric arteries.
The last is the common hepatic artery, which bifurcates into the gastroduodenal and proper hepatic arteries. The gastroduodenal artery further bifurcates into the right gastroomental artery for the greater curvature and the superior pancreaticoduodenal artery for the proximal duodenum. With no doubt, the right and left gastroomental arteries constitute anastomosis.
The proper hepatic artery sounds as if it is only for the liver. However, the proper hepatic artery gives off the right gastric artery for the lesser curvature of stomach. Thereafter, the proper hepatic artery proceeds to be a member of the portal triad (Fig. 6-35), then splits into the right and left hepatic arteries.
The gallbladder is habitually drawn below the right lobe of the liver (Fig. 6-39), probably because the cystic artery is the branch of the right hepatic artery. If we were to be exact, the gallbladder is located between the right and left lobes of the liver (Fig. 6-35).
The word “cyst” meaning bladder is used for gallbladder (e.g., cystic artery) and urinary bladder (e.g., cystitis). Authors do not know why “gall” and “bladder” are put together without spacing, unlike urinary bladder.
Branches of the superior mesenteric artery for midgut will be explained, according to the sequence of the intestine.
The inferior pancreaticoduodenal artery for the distal duodenum anastomoses with the superior pancreaticoduodenal artery from the celiac trunk (Fig. 6-42). It reminds us that the border between the foregut and midgut is the middle of duodenum (Fig. 6-17).
Both the jejunal and ileal arteries configurate arterial arcades and vasae rectae. The jejunal artery has less arterial arcades than the ileal artery, which is beneficial for immediate and increased blood supply to the jejunum; thus the jejunum is redder than the ileum (Fig. 6-33). The simpler procedure is, the more effective it is.
The ileocolic artery is for the distal ileum, cecum, appendix and proximal ascending colon. (The ileocecal artery is an incorrect term, considering the ileocecal valve between ileum and cecum (Figs. 6-27,32).) The right and middle colic arteries are for ascending and transverse colons, respectively.
In the digestive tract, the proximal end (opposite to the distal end) is the oral cavity; in the respiratory tract, it is the nasal cavity; in the urinary tract, it is the kidney; in the blood vessel, it is the heart. The word “proximal” involves the concept of “early” in the time elapsed.
While the main stream of the superior mesenteric artery becomes the ileocolic artery, the main stream of the inferior mesenteric artery becomes the superior rectal artery. The inferior mesenteric artery for the hindgut has only a few branches.
Note that the superior and inferior mesenteric arteries give off the middle and left colic arteries, respectively. It signifies that the border between the midgut and hindgut is the junction of transverse and descending colons. However, the border has been mentioned to be the middle of the transverse colon (Fig. 6-17). In fact, the border is the point that is shifted to the left from the middle of transverse colon (Fig. 6-24).
The descending colon and the rectum are retroperitoneal organs (Fig. 6-26). Therefore, among the inferior mesenteric artery’s branches, only the sigmoid artery passes along the dorsal “mesentery” of the sigmoid colon. If it were not for the sigmoid artery, the name, inferior “mesenteric” artery would not survive.
Along the small and large intestines, there are arteries that are continuous by anastomosis. The arteries are the superior mesenteric, ileocolic, right colic, middle colic, left colic, and sigmoid arteries (Figs. 6-43,44).
The portal vein delivers the absorbed nutrients from the gastrointestinal tract to the liver. In the viewpoint of the gastrointestinal tract, this blood vessel is a vein leaving the capillary; however, in the viewpoint of the liver, it is an artery coming to the capillary. So the portal vein is both an artery and a vein.
However, the portal “vein” is named as a vein, because it has the features of a vein: low blood pressure and thin vessel wall.
Precisely speaking, the portal vein is the short vein entering the liver as a member of the portal triad (Fig. 6-35). The portal vein’s tributaries are the superior mesenteric vein (Fig. 6-41) and the splenic vein. The splenic vein’s tributary is the inferior mesenteric vein. You can see that the two mesenteric veins have quite different fates from the mesenteric arteries (Fig. 6-48).
Gastric veins drain to the portal vein directly or indirectly via the splenic vein and superior mesenteric vein.
Most lymph nodes in the abdominal cavity are similar to the arteries, for their names, distributions, and connections. For instance, the lymph in the gastric and gastroomental nodes drains to the celiac node, which corresponds to the celiac trunk (Fig. 6-42). The exceptional one is the pyloric node that receives the lymph from the right gastroomental node.
However, the three big lymph nodes have dissimilar connections to arteries. Lymph in the inferior mesenteric node approaches the superior mesenteric node. Thereafter, the lymph ascends to meet the lymph from the celiac node and forms the intestinal lymphatic trunk.
The intestinal lymphatic trunk ends at the chyle cistern. “Chyle” means absorbed dietary fat in the gastrointestinal tract. Such chyle is transported through the lymphatics, rather than through the portal vein (Fig. 6-45).
Besides the intestinal lymphatic trunk, the two lumbar lymphatic trunks join the chyle cistern. While the “intestinal” lymphatic trunk conveys the lymph from the abdominal cavity (mostly “intestine”), the lumbar lymphatic trunks convey the lymph from the abdominal wall, pelvis, perineum, and lower limbs (Fig. 7-22). The chyle cistern leads to the thoracic duct, the biggest highway of the lymph (Fig. 5-46).
Consequently, the thoracic duct contains all the lymph from the lower body (abdomen, pelvis, perineum, and lower limbs). Then, the terminal portion of the thoracic duct travels to the left side to receive the lymph from the left jugular (Fig. 3-31), left subclavian (Fig. 2-21), and left bronchomediastinal (Fig. 5-38) lymphatic trunks. In conclusion, the thoracic duct receives lymph from the whole body except the right head, right neck, right upper limb, and right thorax, for which the right lymphatic duct is responsible.
To see the names of veins which receive the lymph, refer to Fig. 3-30.
Abdominal aorta resembles a man with a partner; whereas the inferior vena cava resembles the partner.
The best way to remember the abdominal aorta’s branches from the publication of Goldberg & Ouellette, 2011 is introduced here.
In detail, the man’s head and trunk are the abdominal aorta, the upper limbs are the renal arteries, and the lower limbs are the common iliac arteries. The eyes are the inferior phrenic arteries, the nose is the celiac trunk (Fig. 6-42), the mouth is the superior mesenteric artery (Fig. 6-43), the nipples are the testicular/ovarian arteries, and the umbilicus is the inferior mesenteric artery (Fig. 6-44). The four pairs of trunk hairs are the lumbar arteries, and the penis is the median sacral artery.
< Nerves >
Whereas the somatic motor and somatic sensory nerves (Fig. 2-10) are distributed in the abdominal wall (Fig. 6-5), the visceral motor and visceral sensory nerves (Fig. 3-17) are distributed in the abdominal cavity. For the distribution of the visceral nerves, the celiac, superior mesenteric, and inferior mesenteric plexuses exist at the levels of corresponding arteries, in front of the abdominal aorta (Fig. 6-48). There are the superior and inferior hypogastric plexuses for the pelvic cavity and perineum.
Concerning the sympathetic nerve, the greater and lesser (thoracic) splanchnic nerves (Fig. 3-18) enter the celiac and superior mesenteric plexuses; the lumbar splanchnic nerve enters the inferior mesenteric and superior hypogastric plexuses; the sacral splanchnic nerve (Fig. 7-18) enters the inferior hypogastric plexus.
The sympathetic splanchnic nerves are named after the paravertebral ganglia (thoracic, lumbar, and sacral ganglia) (Figs. 5-43, 7-18), while the parasympathetic splanchnic nerve (pelvic splanchnic nerve) is not.
The thoracic splanchnic nerves and lumbar splanchnic nerves (from the 1st–2nd lumbar ganglia) include the preganglionic fibers (Route B in Fig. 3-18). However, the lumbar splanchnic nerves (from the 3rd–5th lumbar ganglia) and sacral splanchnic nerves involve the postganglionic fibers (Route C in Fig. 3-18).
The greater splanchnic nerve is thick (Fig. 5-43), so its prevertebral ganglia are prominent. The celiac ganglion in the celiac plexus especially is the largest prevertebral ganglion. On the other hand, the superior cervical ganglion is the largest paravertebral ganglion (Fig. 3-19). Both ganglia can be easily recognized in a dissected cadaver.
Regarding the parasympathetic nerve, X (anterior and posterior vagal trunks) (Fig. 5-44) joins the celiac, superior mesenteric, inferior mesenteric, and superior hypogastric plexuses, while the pelvic splanchnic nerve from SN2–SN4 (Fig. 7-18) joins the inferior hypogastric plexus.
The hypogastric plexuses will further be discussed in the next chapter on pelvis and perineum (Fig. 7-17).
Just as the sympathetic postganglionic fibers from the cervical ganglia (Fig. 3-19), the sympathetic postganglionic fibers from celiac, superior mesenteric, and inferior mesenteric ganglia travel along with the respective arteries (celiac trunk, superior and inferior mesenteric arteries) (Figs. 6-42,43,44). Thanks to the arteries, the postganglionic fibers are easily and appropriately distributed to the abdominal organs.
Parasympathetic preganglionic fibers from the plexuses travel along with the arteries as well. On the cadaver, the branches of the arteries have an uneven wall surface because of the autonomic nerve.
In the case of the gastrointestinal tract, the parasympathetic ganglion is located in the wall of stomach or intestine, just before smooth muscle. It means that the postganglionic fibers of parasympathetic nerve are awfully short (Fig. 3-17).
Reversely, the visceral sensory nerve from the abdominal cavity moves up along with the sympathetic or parasympathetic nerves (Fig. 3-17). It comes naturally that the visceral sensory nerve passes the celiac, superior mesenteric, and inferior mesenteric plexuses.
< Kidney >
The right kidney is situated slightly lower than the left because the right kidney is pressed down by the liver. The humorous reason is that “right” having one more letter than “left” is heavier. The right tricuspid valve has one more cusp (Fig. 5-21); the right lung has one more lobe (Fig. 5-32).
Kidneys are located at the spaces between floating ribs (R11, R12) and lumbar vertebrae. The angular space is called the costovertebral angle.
The kidney and adrenal gland (namely, suprarenal gland), which are retroperitoneal organs (Fig. 6-23), can be damaged if a person gets punched hard around the costovertebral angle. Hence they are protected by the surrounding pararenal fat and perirenal fat. The border between two kinds of fats is called the renal fascia which is hardly identifiable during the dissection of the embalmed cadaver.
The adrenal (endocrine) gland is composed of the adrenal cortex and the medulla, which are discernible by microscopy. The kidney, covered by renal capsule, is divided into renal cortex, renal medulla, and renal sinus.
To be specific, only the renal pyramids are the renal medulla, since the intervening renal columns belong to the renal cortex. The color difference between the renal cortex and medulla results from the glomeruli, which are found only in the renal cortex.
The cortex and medulla finally produce urine that drops from the renal papilla, which is at the tip of the renal pyramid. Urine then passes the minor calyx, major calyx, and renal pelvis, all of which are located in the renal sinus. The renal sinus contains not only the urinary tract but also renal artery, renal vein, and fat.
Do you remember the comics, in which a man’s upper limb (renal artery) is behind a woman’s upper limb (renal vein)? At the hilum of the kidney, the renal vein (Fig. 6-53), renal artery (Fig. 6-48), and ureter are serially located from the anterior to the posterior.
Their initials, VAU reminds us of VAN (vein, artery, and nerve) of the neurovascular bundle. Among VAN, the vein is at the most dangerous location (for example, external jugular vein (Fig. 3-32)). However, the renal vein is not the case. Thick veins of the body trunk are routinely placed in a safe place. Another case is the inferior vena cava (Fig. 6-38).
Urine descends through the ureter by the peristaltic movement of the smooth muscle of the ureter.
Like the esophagus (Fig. 5-42), ureter has three sites where it is narrowed: initial site (hilum of kidney), terminal site (intramural part), and the site where it crosses the common iliac artery (Fig. 6-52). The three site are where the renal stones are frequently caught and found in radiographs.
Right and left testicular/ovarian arteries branch off from the abdominal aorta symmetrically (Fig. 6-48). On the other hand, the right testicular/ovarian vein enters inferior vena cava, while the left one enters the left renal vein. During the development, a vein is more changeable than an artery; the left testicular/ovarian vein couldn’t reach the inferior vena cava which is shifted to the right (Fig. 6-38), so reached the left renal vein instead.
< Posterior abdominal wall >
In posterior abdominal wall, there are three muscles. Quadratus lumborum runs from the iliac crest to the R12 and lumbar vertebrae, so it depresses the R12 and flexes the lumbar vertebrae laterally.
Iliacus from the iliac fossa and the psoas major from the lumbar vertebrae slide down and arrive at the lesser trochanter. Since the iliacus and psoas major have the identical insertion and function (flexion of the femur), they are often regarded as a single muscle, called iliopsoas (Fig. 8-9).
It is not recommended to find the psoas minor which serves little purpose. Instead, students should notice that the psoas major involves the lumbar plexus, of which some nameless branches innervate the posterior abdominal wall muscles.
Branches of the lumbar plexus are from the anterior rami of the lumbar nerves (Fig. 2-12). The ilioinguinal nerve from LN1 passes deep to the internal oblique muscle like the thoracoabdominal and subcostal nerves (Fig. 6-5). The “ilioinguinal” nerve literally travels along the “iliac” crest to enter the “inguinal” canal (Fig. 6-8). The ilioinguinal nerve is not included in the spermatic cord, for reasons that the nerve does not go to testis and it is just for the dermatome of LN1 on the inguinal ligament (Fig. 5-5).
The genitofemoral nerve from LN1 and LN2 divides into the genital and femoral branches. The genital branch is included in the spermatic cord because it innervates the cremaster of male (Fig. 6-9). The femoral branch is a cutaneous nerve (Fig. 3-3).
Remaining nerves (from LN2–LN5) for the lower limb belong to the branches of the lumbosacral plexus (from LN2–SN3). Those nerves will be further described in the pelvis, perineum chapter (Fig. 7-15).