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the variations in size of the pupil from variations in the degree of contraction depend upon different intensities of light. If the light be intense, the circular fibers contract and diminish the size of the pupil; if the light diminishes in intensity, the circular fibers relax and the pupil enlarges.
Point of Most Distinct Vision.- While all portions of the retina are sensitive to light, their sensibility varies within wide limits. At the macula lutea, and more especially in its most central depression, the fovea, where the retinal elements are reduced practically to the layer of rods and cones, the sensibility reaches its maximum. It is at this point that the image is found when vision is most distinct. The macula and fovea are always in the line of direct vision. From the macula toward the periphery of the retina there is a gradual diminution in sensibility and a corresponding decline in the distinctness of vision. In those portions of the retina lying outside the macula the indistinctness of vision depends not only on diminished sensibility, but also upon inaccurate focusing of the rays.
Blind Spot.—Although the optic nerve transmits the impulses excited in the retina by the ethereal vibration, the nerve fibers themselves are insensitive to light. At the point of entrance of the optic nerve, owing to the absence of the rods and cones, the rays of light make no impression. This is the blind spot. As this spot is not in the line of vision, no dark point is ordinarily observed in the field of vision, that circular space before a fixed eye within which objects are perceptible.
The rods and cones are the most sensitive portions of the retina. A ray of light entering the eye passes entirely through the various layers of the retina and is arrested only upon reaching the pigmentary epithelium in which the rods and cones are imbedded. As to the manner in which the objective stimuli, light, and color so-called, are transformed into nerve im. pulses, but little is known. It is probable that the ethereal vibrations are transformed into heat, which excites the rods and cones. These, acting as highly specialized end organs of the optic nerve, start the impulses on their way to the brain, where the seeing process takes place. As to the relative function of the rods and cones, it has been suggested, from the study of the facts of comparative anatomy, that the rods are impressed only by differences in the intensity of light, while the cones in addition are impressed by qualitative differences or color,
Accessory Structures. The muscles which move the eyeball are six in number: the superior and inferior recti, the external and internal recti, the superior and inferior oblique muscles. The four recti muscles, arising from the apex of the orbit, pass forward and are inserted into the sides of
the sclerotic coat; the superior and inferior muscles rotate the eye around a horizontal axis; the external and internal rotate it around a vertical axis.
The superior oblique muscle, having the same origin, passes forward to the inner and upper angle of the orbital cavity, where its tendon passes through a cartilaginous pully; it is then reflected backward and inserted into the sclerotic just behind the transverse diameter. Its function is to rotate the eyeball in such a manner as to direct the pupil downward and outward.
The inferior oblique muscle arises at the inner angle of the orbit and then passes outward and backward, to be inserted into the sclerotic. Its function is to rotate the eyeball and direct the pupil upward and outward.
By the associated action of all these muscles, the eyeball is capable of performing all the varied and complex movements necessary for distinct vision.
The eyelids, bordered with short, stiff hairs, shade the eye and protect it from injury. On the posterior surface, just beneath the conjunctiva, are the Meibomian glands, which secrete an oily fluid; it covers the edge of the lids, and prevents the tears from flowing over the cheek.
The lachrymal glands are ovoid in shape, and situated at the upper and outer part of the orbital cavity ; they open by from six to eight ducts at the outer portion of the upper lids.
The tears, secreted by the lachrymal glands, are distributed over the cornea by the lids during the act of winking, and keep it moist and free from dust. The excess of tears passes into the lachrymal ducts, which begin by two minute orifices, one on each lid, at the inner canthus. They conduct the tears into the nasal duct, and so into the nose.
THE SENSE OF HEARING.
The Ear, or Organ of Hearing, is lodged within the petrous portion of the temporal bone. It may be, for convenience of description, divided into three portions, viz. : 1. The external ear. 2. The middle ear. internal ear or labyrinth.
The External Ear consists of the pinna or auricle and the external auditory canal. The pinna consists of a thin layer of cartilage, presenting a series of elevations and depressions; it is attached by fibrous tissue to the outer bony edge of the auditory canal; it is covered by a layer of integument continuous with that covering the side of the head. The general
shape of the pinna is concave and presents a little below the center a deep depression, the concha. The external auditory canal extends from the concha inward for a distance of about one and a quarter inches. It is directed somewhat forward and upward, passing over a convéxity of bone, and then dips downward to its termination; it is composed of both bone and cartilage and lined by a reflection of the skin covering the pinna. At the external portion of the canal the skin contains a number of tubular glands, the ceruminous glands, which in their conformation resemble the perspiratory glands. They secrete the cerumen or ear wax.
The Middle Ear, or Tympanum, is an irregularly shaped cavity hollowed out of the temporal bone and situated between the external ear and the labyrinth. It is narrow from side to side but relatively long in its vertical and antero-posterior diameters; it is separated from the external auditory canal by a membrane, the membrana tympani; from the internal ear it is separated by an osseo-membranous partition which forms a common wall for both cavities. The middle ear communicates posteriorly with the mastoid cells, anteriorly with the naso-pharynx by means of the Eustachian tube. The interior of this cavity is lined by mucous membrane continuous with that lining the pharynx.
The membrana tympani is a thin, translucent, nearly circular membrane, measuring about two-fisths of an inch in diameter, placed at the inner termination of the external auditory canal. The membrane is inclosed within a ring of bone which, in the fetal condition, can be easily removed, but in the adult condition becomes consolidated with the surrounding bone. The membrana tympani consists primarily of a layer of fibrous tissue, arranged both circularly and radially, and forms the membrana propria; externally it is covered by a thin layer of skin continuous with that lining the auditory canal ; internally, it is covered by a thin mucous membrane. The tympanic membrane is placed obliquely at the bottom of the auditory canal, inclining at an angle of 45°, being directed from behind and above downward and inward. On its external surface this membrane presents a funnel-shaped depression, the sides of which are somewhat convex.
The Ear-bones. Running across the tympanic cavity and forming an irregular line of jointed levers, is a chain of bones which articulate with each other at their extremities. They known as the malleus, incus, and stapes.
The form and position of these bones are shown in Fig. 28.
The malleus consists of a head, neck, and handle, of which the latter is attached to the inner surface of the membrana tympani; the incus, or anvil