The Organs of Special Senses

Transcription

1 8 The Organs of Special Senses Special senses are those other than touch, pain, temperature, and proprioception. Vision, hearing, and equilibrium are the special senses discussed in this chapter. The Eye No text on anatomy of the head is complete without a chapter dealing with the eye. Many feel that vision is the most important of the senses. Indeed, without vision, the practice of the health professions would be almost impossible. Vision is an acutely specialized sense and must be protected with great care. The eye is composed of two major parts: the bulb and the accessory organs. It is housed in the protective bony confines of the orbit. The orbital rim of bone is quite thick, but the walls of the orbit are very thin. In fact, a direct blow to the bulb of the eye can cause it to expand laterally, medially, and inferiorly fracturing the orbital walls while the globe remains intact. The Bulb Three layers comprise the bulb of the eye: an outer fibrous layer, an intermediate layer, and an internal layer. 1. Outer Layer The outer fibrous layer is divided into two parts, the cornea anteriorly and the sclera posteriorly. The cornea is transparent and forms only one-sixth of the bulb. It projects somewhat beyond the sclera. The external surface of the sclera is white. It is quite tough, and the extrinsic muscles of the eye attach to its surface. The sclera and a small portion of the cornea are covered by the conjunctival membrane. This is the mucosa of the eye and will be discussed in more detail under accessory organs. The conjunctival membrane reflects onto the inner surface of the lids. Note that the sclera becomes continuous with the fibrous sheath of the optic nerve. The ciliary muscle and the iris are attached at the sclerocorneal junction where the sclera joins the cornea. 2. The Intermediate Layer This layer is the vascular layer which is pigmented. It is comprised of the choroid, the ciliary body and the iris. 1

2 The choroid is a vascular, brown membrane. Its superficial surface is attached to the sclera and its deep surface is attached to the retina. It is composed of a dense capillary plexus of small arteries and veins. It provides nutrition for the bulb and conveys vessels and nerves to the ciliary body and iris. The arterial supply arises from the ciliary and muscular branches of the ophthalmic artery (Chapter 3). The veins ultimately empty into the ophthalmic veins in the orbit. The ciliary body found anterior to the choroid is thicker than the choroid. It contains the ciliary muscle and also is the source of attachment of the suspensory ligament of the lens. When the ciliary muscle contracts, it causes the ligaments of the lens to relax and make the lens more convex to accommodate for near vision. The iris, found anterior to the ciliary body, is a circular contractile disc suspended between the cornea and the lens. Its aperture is known as the pupil. Normally, the iris is pigmented and separates the space between the lens and the cornea into a posterior and an anterior chamber. It contains circular and radiating muscle fibers which are controlled by the autonomic nervous system. The circular fibers, the sphincter pupillae, constrict the pupil and the radiating fibers, the dilatator pupillae, cause the pupil to widen. 3. The Internal Layer This layer is the retina. It is a delicate membrane of nervous tissue. Posteriorly, it is continuous with the optic nerve. Anteriorly, it extends nearly as far as the ciliary body. On the surface of the posterior wall, and corresponding to the optical axis, where vision is perfect, is the macula. The central depression in the macula is the fovea centralis. To the nasal side of the macula is the optic disc which is the point of entrance for the central artery of the retina. The optic disc is the blind spot and is insensitive to light. The retina is ten cell layers thick. It consists of nerve cells and connective tissue all of which take part in the transmission of light to the optic nerve. 4. The Lens This is a colorless, transparent, laminated structure that is devoid of vessels. It is biconvex and is suspended from the ciliary body by a ligament and is positioned just posterior to the iris. The shape of the lens and thus its refractory ability, is altered by the ciliary muscle. 5. Aqueous Humor This is an alkaline watery fluid, secreted by specialized vessels in the ciliary body. It fills the anterior and posterior chambers and is reabsorbed in a specialized vein, the canal of Schlemm, located in the cornea near the attachment of the iris. 2

3 6. The Vitreous Body Filling the large interior of the globe behind the lens is a gelatinous, transparent material. It is avascular, and it is attached to the retina from which it gains it nutrition. The hyaloid canal is a remnant of an embryonic artery which runs through the vitreous humor between the optic disc and the lens. The Accessory Organs The accessory organs include the extraocular muscles, fat, fascia, eyebrows, eyelids, lacrimal apparatus, and conjunctiva. The extraocular muscles of the eye are the superior and inferior rectii, the medial and lateral rectii, the superior and inferior oblique and the levator palpebrae superioris. These were discussed in Chapter 4. The bulb is suspended in and separated from the bony orbital wall by the extraocular muscles, fascia, and fat. They protect the bulb, cushion it, and allow for considerable mobility. The eyebrows are merely two crescent-shaped eminences composed of thickened skin and fascia. They overlie the supraorbital ridges and contain numerous short thick hairs. The frontalis, corrugator, and orbicularis muscles attach to the eyebrows (see Chapter 2). The eyelids protect the eyes. They are composed of skin, muscle, glands, fascia, hair (eyelashes), and mucosa. The levator palpebrae superioris and orbicularis oculi are the muscles found within the lids. Also within the substance of the lids are two thin plates of connective tissue, the tarsal plates. They give shape and support to the lids. The levator palpebrae superioris muscle inserts into the superior tarsal plate. Vertically placed glands, the tarsal glands are seen on the inner surface of the tarsal plates. The angles where the upper and lower lids meet are called the medial and lateral canthi. At the medial canthus, on the margin of each lid, is seen the lacrimal papilla. This is a small elevation which is pierced by the punctum lacrimale, a small opening which leads to the lacrimal duct. At the depths of the medial canthus is found the elevated caruncula. The lacrimal apparatus keeps the mucosa of the bulb moist. It consists of the lacrimal gland, excretory ducts, and collecting ducts. The lacrimal gland produces tears. It lies in the lacrimal fossa of the frontal bone (Chapter 2) and rests on the tendons of the extraocular muscles. Several small excretory ducts pierce the mucosa of the upper eyelid and empty into the area where the conjunctiva of the sclera reflects onto the conjunctiva of the upper eyelid. This is known as the fornix conjunctiva. The lacrimal ducts begin at the punctum lacrimale and empty into the lacrimal sac. This then drains into the nasolacrimal duct which empties into the inferior meatus of the nasal cavity. When tearing is more than the lacrimal sac can drain, the tears spill out over the lids, particularly at the lateral canthus. Lack of sufficient tears causes the cornea to dry, crack and become susceptible to infection. The conjunctiva or conjunctival membrane over the cornea lines the inner surface of the lids and reflects onto the sclera and cornea. Histologically, it is mucosa. It is transparent. 3

4 Vision Light rays of the image are refracted (bent) as they pass through the cornea, aqueous humor, lens, and vitreous humor. In order to form objects on varying distances, the lens may change shape by relaxation of the ciliary muscle, the pupil may dilate or constrict, and the eyes may diverge or converge. Once the image reaches the retina, it is ready to be conducted to the brain for the conscious realization of vision. Vision is realized in the occipital lobes of the brain. Nerves from the retina traveling to the brain, cross at the optic chiasma and pass into the optic tracts. Before the impulse gets into the optic tracts, it first synapses in the thalamus, an elevated area on either side of the third ventricle. The mixing of the fibers takes place in the optic chiasma and in the optic tract. Knowing this helps the diagnostician identify the location of lesions that may cause various amplitudes of blindness. For example, if one optic tract is injured or destroyed, the patient would be partially blind in both eyes. Moreover, damage to one optic nerve produces total blindness of only one eye, but there is a decrease in visual realization at both occipital lobes. The Ear This sensory organ is composed of three major parts: the external ear, the middle ear, and the inner ear. Hearing is not the only activity performed by the ear. Cosmesis and equilibrium are two other important functions. 1. The External Ear The external ear has two major parts: the auricula (or pinna) and the external auditory meatus. The pinna projects from the side of the head, collects air vibrations, and directs them into the external auditory meatus. (In humans, because of the small size of the pinna, this function is doubtful.) The rim around the posterior and superior perimeter is the helix. The antihelix lies just anterior to the inferior arm of the helix. Just in front of the antihelix is a deep cavity, the concha. Anterior and lateral to this, the tragus projects over the opening to the meatus. The antitragus is found opposite the tragus and separated from it by the intertragic notch. Just inferior to the antitragus is the lobule. The auricula is made of cartilage covered with skin except that there is no cartilage in the lobule. The muscles of the auricula are rudimentary and are discussed in Chapter 2. The ligaments consists of various fibrous bands connecting the pinna to the side of the head and connecting the various cartilages together. Blood supply to the pinna is via the posterior auricular artery and branches from the occipital and superficial temporal arteries. Sensory nerve supply comes from the branches of the vagus nerve, cervical nerves, and the auriculotemporal branch of the mandibular nerve. (See Chapter 3 and 4). The external auditory meatus passes from the depth of the concha to the tympanic membrane. It is a slightly S-shaped cylindrical canal. Superficially, it is formed by cartilage and more deeply by bone. The cartilage portion is about 8 mm long and the bony portion is 4

5 about 16 mm long. The skin of the external auditory meatus is thin. Numerous glands in the cartilaginous portion secrete earwax (cerumen), which prevents cracking and infection of the thin skin in this area. Note that the mandibular condyle is just anterior to the external auditory meatus. One can palpate and examine the movements of the condyle, testing for pain, mobility, and crepitus, by gently placing a finger in the external auditory meatus while the mandible is in function. The tympanic membrane can be visualized by peering through a speculum gently placed in the external auditory canal. 2. The Middle Ear This is an irregular space within the body of the temporal bone. It is often called the tympanic cavity and it is filled with air. The auditory tube connects the tympanic cavity to the nasopharynx and allows for equal air pressure in both areas. The auditory tube (eustachian tube) is about 36 mm long. It passes anterior, downward, and medially from the middle ear to the nasopharynx. The osseous portion is 12 mm long and is the superior lateral length, while the 24-mm long cartilage portion attaches to the nasal pharynx. The torus tubarius guards the posterior portion of the pharyngeal opening. The tube is covered with mucous membrane. There are three tiny bones in the tympanic cavity which form a means of conveyance of air vibrations from the tympanic membrane to the inner ear. From lateral to medial they are the malleus, the incus, and the stapes. Posteriorly, the tympanic cavity communicates with the mastoid air cells, a cribriform substance of which the mastoid process of the temporal bone is composed. Laterally, the tympanic cavity is bounded by the tympanic membrane (ear drum). One end of the malleus, the manubrium, is attached to the tympanic membrane. The other end, the head, articulates with the incus. The incus attaches to the stapes. As the tympanic membrane vibrates from sound impulses, it causes the malleus to move and thus the incus and stapes react to convey sound waves to the inner ear. There are two small muscles in the middle ear, the tensor tympani and the stapedius. The former increases the tension on the tympanic membrane. The latter controls movement of the stapes. Together, they serve to decrease movement of the bones of the middle ear and therefore protect the inner ear from loud noises. They are supplied by the mandibular branch of the fifth nerve and the facial nerve respectively (see Chapter 4). The entire tympanic cavity is lined with mucous membrane. This mucosa covers all the little bones, muscles and nerves and lines the medial surface of the tympanic membrane. It also lines the mastoid air cells, the auditory tube and is continuous with the pharyngeal mucosa. Among the important structures found in the tympanic cavity are the chorda tympani nerve and the lesser petrosal nerve on their way to parasympathetic ganglia. Sensory nerve supply to the tympanic cavity is via the tympanic plexus composed of fibers from the glossopharyngeal nerve and fibers from the sympathetic nerves following the carotid tree. 5

6 Arterial supply is mainly via the tympanic branch of the maxillary artery and stylomastoid branch of the posterior auricular artery. Small branches from the middle meningeal, ascending pharyngeal, and internal carotid arteries also contribute blood. Venous drainage ends in the superior petrosal sinus and pterygoid plexus. 3. The Inner Ear The inner ear is quite complex. It is often called the labyrinth, and has two major parts: the membranous labyrinth and the bony labyrinth. The bony labyrinth has three parts: the cochlea, the semicircular canals and the vestibule. These are cavities in the petrous part of the temporal bone. The cavities are filled with perilymph, a clear fluid, in which the membranous labyrinth is suspended. The intracranial position of the labyrinth is well demonstrated. One can see that the cochlea is anterior and medical, the semicircular canals are posterior and lateral, and the vestibule is in between. The vestibule is the middle portion of the labyrinth and lies medial to the tympanic cavity. The semicircular canals and cochlea all open into and communicate with the vestibule by their respective openings. Besides the openings for the canals and the cochlea, there are two major openings in the vestibule; the oval window which contacts the stapes and the round window which is closed by a membrane known as the secondary tympanic membrane. These take part in the cochlear function of hearing. There are three semicircular canals. One is vertical and placed transversely to the long axis of the petrous portion of the temporal bone; a second is also vertical and directed posteriorly; a third is placed horizontal, posterior, and laterally. They are termed superior, posterior, and horizontal semicircular canals respectively. The cochlea looks like a snail shell. It is oriented in front of the vestibule, and parallel to the same plane as the vestibule, then spirals two and three-fourths times around a conical axis. The axis of the spiral is called the modiolus. This axis points horizontal, forward, and lateral. Therefore, the apex points anterior and lateral and the base points medially and relates to the internal auditory meatus. The spiral canal, transcribed by the cochlea is then divided into the scala tympani and scala vestibuli by a membrane and a ridge of bone. The membranous labyrinth is housed within the bony labyrinth. It is filled with endolymph and surrounded by perilymph and contains three basic divisions, the semicircular ducts, the saccule and utricle, and the cochlear duct. Removing the bony labyrinth makes anatomy of the membranous labyrinth more clear. The semicircular ducts are quite similar to their respective bony canals except that the membranous ducts are much smaller. Sensory endings of the vestibular nerve can be seen innervating the ductal system. 6

7 The utricle and saccule lie in the bony vestibule. They, too, contain sensory nerve fibers from the vestibular nerve. The canals, utricle and saccule, are all connected and contain the same endolymphatic fluid. The cochlear duct is triangular in cross section and is a thin coiled membranous canal that follows the configuration of the bony cochlea. It is housed in the bony and membranous structures that divide the bony cochlea into the scala tympani and scala vestibuli. The cochlear nerve is formed by the confluence of the numerous nerve fibers arising from the wall of the cochlear duct. The cochlear duct, on its inner surface, contains the sensory receptors for hearing. Together they are called the organ of corti. Cell bodies of the cochlear nerve are located in the spiral ganglia found in the modiolus, and along the course of the cochlear canal. The cell bodies of the vestibular nerve are located in the superior and inferior ganglia of scarpa. Blood supply to the labyrinth is via branches of the basilar artery and posterior auricular artery. Venous drainage empties into the superior petrosal sinus or into the transverse sinus. Hearing If we unwind the cochlea we can more easily understand the physiology of hearing. As the tympanic membrane vibrates from sound, the bones of the middle ear correspondingly move. As the base of the stapes moves, the motion is communicated to the perilymph in the scala vestibuli and is transmitted all the way to the tip of the cochlea. Then it returns via the scala tympani to the round window. The endolymph in the cochlear duct is thus put into motion by the movement of the membrane that separates the duct from the scalae. The organ of corti is therefore stimulated and the impulse passes to the spiral ganglion, out to the cochlear nerve, and then to the brain. Equilibrium As the head changes position, the fluid in the semicircular canals and in the saccule and utricle develop wave patterns. These waves stimulate nerve endings of the vestibular nerve and the impulses are transmitted to the central nervous system. Vertigo (dizziness), the inability to maintain balance, may indicate a pathologic condition of the vestibular portion of the inner ear. 7