The digestive system consists of the digestive tract plus the digestive glands.
< Oral cavity >
Tooth is a hard, calcified structure and used to break down food. The teeth and the maxilla/mandible form fibrous joint (Fig. 2-4).
Twenty deciduous teeth are substituted by 32 permanent teeth.
Among the deciduous teeth, the medial incisors erupt first (6 months after birth). Among the permanent teeth, the 1st molars erupt first (6 years after birth).
The tongue manipulates food for mastication, and is used for swallowing (Fig. 4-17) (Fig. 4-20). Another major function of the tongue is enabling speech.
The intrinsic muscles of tongue in three directions give rise to a variety of tongue shapes, and the extrinsic muscles pull the tongue up and down, back and forth.
Fig. 4-7. Dorsum of tongue.
On the dorsum of tongue, four kinds of papillae exist. The small filiform papillae and the large fungiform papillae in one’s own tongue can be observed with naked eyes. With exception of the filiform papillae, the fungiform, foliate, vallate papillae include microscopic receptor which is called the taste bud.
A sulcus is the boundary between anterior 2/3 and posterior 1/3.
The frenulum of tongue decides length of the tongue’s free part.
Lateral to the frenulum of tongue, mucosa is protruded, where the submandibular duct and sublingual duct release saliva, respectively.
Fig. 4-10. Submandibular gland.
The submandibular gland surrounds the free posterior border of the mylohyoid muscle (Fig. 3-20). The submandibular duct arises from deep part of the gland.
The parotid duct opens on the protruding mucosa, which is lateral to the maxillary 2nd molar (Fig. 4-3).
Fig. 4-12. Parotid gland, adjacent structures.
The par (near) + otid (ear) gland is located near the ear. The parotid gland surrounds the mandible. Unlike the submandibular duct (Fig. 4-10), the parotid duct arises from superficial part of the gland. Therefore, the parotid duct can be injured by an external impact. The parotid gland contains VII (Fig. 13-74).
The palate is divided into two parts, the anterior bony hard palate, and the posterior fleshy soft palate (Fig. 4-15). The muscles in the soft palate will be shown soon (Fig. 4-18).
< Pharynx, esophagus >
The pharynx consists of the nasopharynx (posterior to nasal cavity), the oropharynx (posterior to oral cavity), and the laryngopharynx (posterior to larynx).
Fig. 4-15. Three parts of pharynx.
Anterior borders of the nasopharynx, oropharynx, and laryngopharynx are the choana, fauces, and laryngeal inlet, in that order.
Among three parts of the pharynx, only the oropharynx can be seen outside without using any instrument. At this time, structures around the fauces (e.g., palatine tonsil) are visible (Fig. 12-16).
Food in the oral cavity should be prevented from entering the nasal cavity. Therefore, when we swallow, the soft palate elevates, blocking the passage between the oropharynx and nasopharynx.
Fig. 4-18. Muscles of soft palate.
In order for the soft palate to firmly separate the nasopharynx and oropharynx (Fig. 4-17), the soft palate should be made “tense” by the “tensor” veli palatini. Simultaneously, the soft palate should be “elevated” by the “levator” veli palatini.
The auditory tube connects the tympanic cavity (Fig. 14-33) to the nasopharynx (Fig. 4-23). The two muscles (tensor veli palatini and levator veli palatini) originate from the inferior wall of the auditory tube. Contraction of the two muscles pulls the origin down, which eventually opens the collapsed auditory tube (Fig. 5-8).
When one tries to swallow one’s own saliva, one can hear a loud pop sound of the auditory tube opening (Fig. 5-8).
Food must not enter the larynx either. As the larynx moves upward, the epiglottis (containing epiglottic cartilage) (Fig. 5-10) blocks the laryngeal inlet between the laryngopharynx and larynx (Fig. 4-15). Elevation of the larynx is caused by contraction of the suprahyoid muscles (Fig. 3-19) (Fig. 3-20).
The pharynx is a part of the digestive tract (Fig. 4-1).
Fig. 4-22. Muscles of pharynx.
The term “constrictor” is used only in the pharynx. This is why the terms “constrictor” as well as “pharyngeal constrictor” are acceptable.
The superior constrictor is for constricting the oropharynx while the inferior constrictor is for constricting the laryngopharynx (Fig. 4-15); the middle constrictor intervenes. When swallowing, the constrictors contract sequentially to let the food down like peristalsis (Fig. 16-30). Thus you can swallow food even when you are standing on your head.
Fig. 4-23. Posterior view of opened pharynx.
Once posterior wall of the pharynx is removed, posterior view of the nasal septum (Fig. 5-4), choana, soft palate (Fig. 4-15), and opening of auditory tube (Fig. 14-35) can be seen in the nasopharynx. Successively, the fauces (Fig. 12), epiglottis, and laryngeal inlet (Fig. 4-15) are to be found in the oropharynx and laryngopharynx.
Fig. 4-24. Esophagus.
Food passes through the esophagus (from the pharynx to the stomach), aided by peristaltic contractions (Fig. 16-30) of the skeletal and smooth muscles of the esophagus. The esophagus is situated behind the trachea (Fig. 4-15) and heart and passes through the diaphragm (esophageal hiatus) (Fig. 3-26).
< Stomach, intestine >
The stomach (gastro) is an initial, dilated part of the gastrointestinal tract.
The stomach secretes peptic acid and gastric juice to aid in food digestion.
Fig. 4-27. Stomach.
Border between the esophagus and stomach is called the cardia (meaning heart), since this part is close to the heart (Fig. 10-4). Border between the stomach and duodenum is called the pylorus (meaning gatekeeper) which has pyloric sphincter (Fig. 4-28).
Between the cardia and pylorus, the stomach is divided into the fundus, body, pyloric antrum, and pyloric canal. Inside the stomach, there are the longitudinal folds that disappear when the stomach is distended by food.
The duodenum is the first and shortest segment of the small intestine to receive partially digested food from the stomach. The duodenum plays a vital role in the chemical digestion of food with bile and pancreatic juice (Fig. 4-53) in preparation for absorption in the small intestine.
Fig. 4-29. Small intestine.
The small intestine consists of the duodenum, jejunum, and ileum. The junction between the duodenum and jejunum is suspended by a ligament. The boundary between the jejunum and ileum is roughly estimated with the length ratio (jejunum : ileum = 2 : 3). Unlike the above figure, the jejunum and ileum are so convoluted that the small intestine is longer than the large intestine (Fig. 4-33).
The jejunum is shorter than the ileum but is more important in absorption.
The large intestine also has its own morphological characteristics.
Fig. 4-32. Characteristics of large intestine.
The teniae coli in the large intestine are the three thickened parts of the external longitudinal muscle (Fig. 4-43). The thick teniae coli do not fully elongate during the development of the large intestine, which results in the wrinkles (haustra) (Fig. 8-33). The large intestine has omental appendices, which are fat protrusions surrounded by the peritoneum (Fig. 4-41).
Fig. 4-33. Large intestine.
A remarkable structure inside the end of the small intestine is the ileocecal valve. This valve prevents the reflux of food from the large intestine into the small intestine.
The large intestine consists of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. The appendix is a blind-ended tube connected to the cecum.
The descending colon is in the abdominal cavity; the sigmoid colon and rectum are in the pelvic cavity. The border between the abdominal and pelvic cavities is the pelvic inlet (Fig. 1-35) (Fig. 3-64).
The large intestine as well as the stomach are not vital organs in terms of nutrient absorption, whereas the small intestine is. Without the small intestine, we would not be able to properly digest or absorb nutrients which are necessary for our survival (Fig. 4-29).
The rectum is regarded as the reservoir of feces. Whereas the sigmoid colon and rectum are in the pelvic cavity, the anal canal is in the perineum (Fig. 3-64) (Fig. 4-33).
Fig. 4-36. Anal canal.
To maintain fecal continence, the internal anal sphincter (smooth muscle) and the external anal sphincter (skeletal muscle) contract. The internal anal sphincter is an expansion of the internal circular muscle (Fig. 4-43).
During defecation, the abdominal wall muscles (skeletal muscle) contract to increase intraabdominal pressure (Fig. 3-29) and the large intestine muscle (smooth muscle) contracts for peristalsis (Fig. 16-30); concurrently, the sphincters relax.
Fig. 4-38. Visceral and parietal peritonea.
Abdominal organs are classified into two groups: the intraperitoneal organs which are hung by mesentery and the retroperitoneal organs which are not. The intraperitoneal organs are usually located anterior and are mobile. Visceral peritoneum blankets the intraperitoneal organs completely and forms the mesentery. These three words (intraperitoneal organ, mesentery, and visceral peritoneum) always accompany one another.
Parietal peritoneum covers the retroperitoneal organs partly and is in contact with the abdominal wall. The peritoneal cavity contains a small amount of serous fluid that acts as a lubricant (Fig. 8-35).
Fig. 4-39. Rotation of stomach.
During its development, the stomach is rotated at 90 degrees counterclockwise in the inferior view, and locations of the liver, stomach, and spleen become to resemble those in adult (Fig. 4-40). Two mesenteries of the stomach are the lesser omentum and greater omentum. The “lesser and greater” curvatures are the stomach margins where the “lesser and greater” omenta attach (Fig. 4-40).
Fig. 4-40. Lesser and greater omenta.
The lesser omentum connects the liver with the stomach and duodenum. The greater omentum connects the stomach with the diaphragm, spleen (Fig. 4-39), and transverse colon (Fig. 4-33).
Fig. 4-41. Disappearance of dorsal mesentery by fusion.
During development of the intestine, peritoneal fusion occurs; as a result, the duodenum, ascending colon, and descending colon become retroperitoneal organs which do not have any mesentery (Fig. 4-38). Surely, the duodenum, ascending colon, and descending colon are located posterior.
The gastrointestinal tract is main portion of the digestive tract (Fig. 4-1), because of its clinical significance.
Regarding the wall of gastrointestinal tract, the outmost layer (serosa) is the peritoneum (Fig. 4-38).
Fig. 4-43. Muscular layer of intestine.
Concerning the muscular layer, the intestine possesses the external longitudinal muscle and the internal circular muscle that are smooth muscles (Fig. 3-1). (Exactly, the stomach has one more layer (innermost oblique muscle).) The muscular layer carries out peristalsis and moves food down the tract (Fig. 16-30).
The portal vein delivers the absorbed nutrients from the gastrointestinal tract to the liver (Fig. 10-67).
< Liver, pancreas >
The liver has a wide range of functions, including storage of nutrients such as glucose (Fig. 9-19), and detoxification of various substances such as alcohol.
The liver which performs important functions is protected by the ribs (Fig. 1-18) and is located below the diaphragm (Fig. 3-26).
Fig. 4-48. Lobes of liver.
An imaginary sagittal plane passes the gallbladder and inferior vena cava (Fig. 10-68) divides the liver into the right and left lobes. The portal triad (portal vein (Fig. 4-45), hepatic artery, common hepatic duct (Fig. 4-49)) is each divided to be distributed in the right and left lobes.
Fig. 4-49. Passage of bile.
Above figure shows the distribution of the right and left hepatic ducts to the right and left lobes. The cystic duct from the gallbladder is the border between the common hepatic duct and common bile duct.
The liver produces bile that aids lipid digestion in the small intestine. The gallbladder stores and concentrates bile.
The authors do not know why “gall” and “bladder” are put together without spacing, unlike the “urinary bladder” (Fig. 6-17).
The color of bile is dark green to yellowish brown.
Fig. 4-53. Pancreatic duct, common bile duct.
The common bile duct to convey bile descends and meets the pancreatic duct to convey pancreatic juice too, opening into the duodenum (Fig. 4-28). The secretion of bile and pancreatic juice is regulated by certain hormone, which is produced when the incoming food is detected.
Fig. 4-54. Parts of pancreas.
The pancreas is divided into the uncinate process, head, body, and tail. Like the neighboring duodenum, the uncinate process, head, body of pancreas are retroperitoneal (Fig. 4-41). On the other hand, the tail of pancreas is intraperitoneal like the attached spleen (Fig. 4-39).