The sensory system is transducer from the world to the realm of mind, creating perception of the world around us. This chapter deals with eye and ear.
< Eye >
The retina lines the back 2/3 of the eyeball.
An image from the visual field is created on the retina (through the lens), which serves the same function as the electronic sensor in a digital camera. The image on the retina initiates the impulse that is conveyed through the cranial nerve, II (Fig. 13-68).
The “center” of retina is the fovea “centralis” that can sense the color very well thanks to the massed receptors.
On the contrary, the optic disc, which is the blind spot, has no receptor. The optic disc, connected to II, is located medial to fovea centralis (Fig. 13-68).
The iris, ciliary body, and choroid are continuous.
Fig. 14-7. Iris muscles.
The pupil is the narrowest portion of light path to the retina. The size of the pupil is adjusted by the iris which contains the sphincter pupillae and dilator pupillae.
In Peace state, Parasympathetic nerve of III stimulates the sphincter pupillae to yield the small pupil. Imagine the Peaceful sunlight. Reversely, sympathetic nerve stimulates the dilator pupillae (Fig. 13-61) (Fig. 14-7).
Fig. 14-9. Ciliary muscle.
The ciliary muscle inside the ciliary body (Fig. 14-6), innervated by Parasympathetic nerve of III, contracts to thicken the lens. Then one can Peacefully look at a close object like a comic book.
If the inside of eyeball were white, we would see the same object over and over. To prevent such situation from happening, the choroid is black.
The sclera forms posterior 5/6 of the eyeball. It is continuous with the cornea (1/6).
The white color of sclera means it is opaque.
In the meantime, the cornea is transparent to make the iris and pupil observable through it.
Everyone’s pupil is black because inside of the eyeball is dark (without flash) (Fig. 14-11).
In case of glaucoma, aqueous humor’s pressure is increased. Then the humor may exert excessive pressure on II behind the eyeball, which leads to blindness.
Vitreous humor keeps the lens and retina in their places. Dusts in the vitreous humor do not make a serious problem.
Fig. 14-19. Actions of extraocular muscles (innervating nerves).
There are six extraocular muscles that rotate the eyeball.
The superior oblique muscle and lateral rectus are not innervated by the central III.
Fig. 14-21. Extraocular muscles.
The superior oblique muscle hooks around the “trochlea”, so the innervating IV is named the “trochlear” nerve. The lateral rectus results in the “abduction” of the eyeball (Fig. 14-19), so the innervating VI is named the “abducens” nerve.
The extraocular muscles are delicate enough to aim even a moving object.
Fig. 14-24. Muscles of superior eyelid.
The eyelid has extremely thin skin and subcutaneous tissue. Muscles to open and close the eyelid are levator palpebrae superioris and orbicularis oculi (Fig. 3-9), respectively.
Some East Asians undergo the double eyelid surgery to make the eyes appear larger.
III innervates the levator palpebrae superioris (Fig. 14-24) as well as the four extraocular muscles (Fig. 14-21). VII innervates the orbicularis oculi as well as the other facial muscles (Fig. 3-9).
Fig. 14-27. Conjunctiva.
The conjunctiva lines the inside of the eyelid (Fig. 14-24) and covers the anterior sclera (Fig. 14-13).
Fig. 14-28. Lacrimal apparatus.
Tears are constantly secreted from the lacrimal gland (Fig. 13-72) to clean and lubricate the eyeball. Tears pass through the nasolacrimal duct to arrive at the nasal cavity (Fig. 5-2).
< Ear >
The ear consists of the external, middle, and internal ears.
Fig. 14-30. External ear.
The curved external acoustic meatus becomes straight when the auricle is pulled posteriorly. The middle ear is the tympanic membrane and its inside, tympanic cavity (Fig. 14-33).
The tympanic cavity contains auditory ossicles: malleus, incus, stapes in the order of sound conduction.
Fig. 14-32. Movement of auditory ossicles.
The auditory ossicles play the role of a lever. Simply put, the shift of stapes is shorter but stronger than that of malleus. A stronger shift is needed to push the perilymph inside the oval window (Fig. 14-36). Liquid in the internal ear is not as mobile as air of the external ear.
Fig. 14-33. Tympanic cavity.
The tympanic cavity can be thought of as a square box. In the figure above, its lateral wall including the tympanic membrane (Fig. 14-32) is opened like a lid to expose the inside.
Fig. 14-34. Tympanic cavity, its connections.
Posterior and anterior walls of the tympanic cavity have openings of the mastoid antrum and auditory tube, respectively. The mastoid antrum and successive mastoid cells are located in the mastoid process of temporal bone (Fig. 1-11) (Fig. 3-16). The tympanic cavity is connected to the nasopharynx through the auditory tube (Fig. 4-19) (Fig. 5-8).
The medial wall of the tympanic cavity includes oval window which is gate of the scala vestibuli in the internal ear. What is the scala vestibuli?
Fig. 14-36. General form of bony and membranous labyrinths.
The bony labyrinth is a complex canal found in the temporal bone. The bony labyrinth surrounds the membranous labyrinth like a tunnel encircling an oil pipe. The bony labyrinth contains perilymph, and the membranous labyrinth contains endolymph.
Fig. 14-37. Bony and membranous labyrinths of cochlea.
Unlike the general form (Fig. 14-36), the scala vestibuli and scala tympani (bony labyrinth) surround the cochlear duct (membranous labyrinth).
An easy way to remember two scalas is introduced in the cartoon above, showing capital letters.
The oval window pushed by the stapes (Fig. 14-32) creates perilymph vibration in the scala vestibuli and scala tympani, which produces endolymph vibration in the intervening cochlear duct.
Let’s summarize sound entering the ear. Sound is air vibration in the external ear, solid vibration in the middle ear, and liquid vibration in the internal ear.
The impulse perceived in the cochlear duct proceeds to the cerebral cortex through the cochlear nerve, a part of VIII (vestibulocochlear nerve) (Fig. 13-75).
Fig. 14-41. Bony and membranous labyrinths of vestibule, cochleaa.
Another bony labyrinth is the vestibule, which envelops two membranous labyrinths (Fig. 14-36), the globular saccule and oval utricle.
The “vestibule” is origin of the term scala “vestibuli” which is connected to it. Likewise the “tympanic” cavity is origin of the term scala “tympani” (Fig. 40-38).
When a car accelerates forward, the endolymph inside the saccule and utricle (Fig. 14-41) of the rider flows backward. The resultant impulse proceeds to the cerebral cortex by way of the vestibular nerve, a part of VIII.
The remaining bony labyrinth (semicircular canal) resembles its membranous labyrinth (semicircular duct) (Fig. 14-41) just like the general form (Fig. 14-36).
When one spins, endolymph in the semicircular duct stimulates receptor; the vestibular nerve is also responsible for the impulse.
Three semicircular ducts (Fig. 14-41) are situated in the XY-plane, XZ-plane, and YZ-plane.
The sense from the saccule, utricle, and semicircular ducts is essential for the body balance. It is closely related with the vestibulocerebellum (Fig. 13-28).