Part 86

a, optic nerve; b, sclerotic (b′, thickened “white zone”); c, cornea; d, choroid; e, iris; f, ciliary body; g, retina; h, crystalline lens; i, zonula ciliaris; j, anterior chamber of eye; k, vitreous humor.]

=Coats of the Eye= (Fig. 167).--The outer coat of the eyeball is formed by the =sclerotic= (or sclera) (b) and the =cornea= (c). The sclerotic is the whitish opaque portion covering the caudal three-fourths of the eye. It forms a tough sac serving as a protection for the structures within. The cornea (c) is the circular transparent portion of the outer coat, covering the free surface of the eye; it is a direct continuation of the sclerotic. Just before passing into the cornea the sclerotic is much thickened, forming a broad white zone (b′) about the eyeball near its equator. To this zone, at its caudal margin, are attached the rectus muscles of the eyeball. The sclerotic has many pigment-cells on its inner surface, giving it a brownish appearance and forming the =lamina fusca=.

The =cornea= (c) is transparent and more strongly curved than the sclerotic. It is thicker than most of the sclerotic coat, but not so thick as the white zone of the latter.

Within the sclerotic is the second coat of the eye, formed by the =choroid= (d) and the =iris= (e). The choroid (d) is a vascular layer, and contains much pigment, giving it a dark color. It lines the inner surface of the sclerotic, but does not cover the inner surface of the cornea. At the white zone of the sclerotic it is thrown into a large number (about seventy) of meridional folds, the plicæ ciliares or ciliary folds, which together constitute the so-called =ciliary body= (=corpus ciliare=) (f). This forms a circular zone on the inner surface of the white zone of the sclerotic, and is continuous with the =iris= (e). The choroid coat is furnished in certain parts of its area with a layer of cells containing crystals, which give it a brilliant color, forming the =tapetum=. The color of the tapetum is a metallic yellowish blue or green; it is this layer which causes the cat’s eye to “shine” in the dark.

The =iris= (e) is a continuation of the choroid. It projects inward from the white zone of the sclerotic, forming a circular curtain lying some distance behind the cornea, and perforated by an opening, the =pupil=. The iris is usually yellow in color on its outer surface, darkly pigmented within. The pupil varies normally in size and form according to the amount of light to which the eye is subjected. In a cat killed with chloroform the pupil is very large and circular; in the living animal it is elliptical with the long axis dorsoventral, or when much contracted it is a mere dorsoventral slit.

The inner coat of the eye is formed by the =retina= (g). This is the part of the eye which is sensitive to light; it is formed by an expansion of the optic nerve (a). In a preserved eye it is usually soft and opaque, and may be seen to line the caudal half of the inner surface, extending apparently to the ciliary body. Here it seems to end as a free margin, the =ora serrata=; it really becomes thin and passes onto the surface of the ciliary body, forming the =ciliary portion= of the retina, and onto the inner surface of the iris, where it forms the uvea. Near the centre of the retina the entrance of the optic nerve is marked by a small round spot, the =blind spot=.

The =Crystalline Lens= (h).--The lens is a biconvex transparent body situated within the eye a little in front of the ciliary body and just behind the iris. It is more strongly convex in front than behind. The lens is surrounded by a thin transparent sac, the =capsule= of the lens (capsula lentis). The capsule of the lens is attached all around the equator of the lens by fibrous bands to the ciliary body, this attachment forming the =zonula ciliaris= (or zonula Zinni) (i), by which the lens is suspended.

=Chambers of the Eye.=--The lens (h) and zonula ciliaris (i) form a partition dividing the eye into two parts. The cavity in front of the lens contains a fluid, the =aqueous humor=. This cavity is partly subdivided by the iris into the anterior and posterior chambers of the eye.

The part of the eye lying behind the lens is much larger than the part in front of the lens. It is lined chiefly by the retina, its front boundary being the lens. It contains a jelly-like substance, the =vitreous humor= (or vitreous body, =corpus vitreum=).

II. THE EAR. ORGANON AUDITUS.

The ear of the cat, like that of man, is composed of three parts, the external, middle, and internal ear.

The =external= ear is limited internally by the tympanic membrane.

The =middle= ear extends from the tympanic membrane to the fenestra cochleæ and fenestra vestibuli. It contains the bones of the ear, and is connected with the pharynx by the Eustachian tube.

The =internal= ear is wholly contained within the petrous bone, and on the surface of the membrane lining its cavities are distributed the branches of the auditory (eighth) nerve.

=The External Ear.=

The external ear consists of an expanded portion, the =auricle= (=auricula=) or pinna, and of a canal leading from the auricle to the tympanic membrane, known as the =external auditory meatus=. The concave surface of the auricle is directed craniolaterad, and its edges form the dorsal and lateral boundaries of the auditory opening. From the bottom of the cavity which it partly encloses, the external auditory meatus passes medioventrad.

The auricle (Fig. 168) is composed of a thin curved plate of cartilage (=cartilago auriculæ=), to which numerous muscles are attached and which is covered on both sides by integument intimately attached to the cartilage. Distad the edges of the cartilage form a free border (b); proximad the plate is rolled together in such a way that its edges (g and h) overlap on the craniomedial side, thus enclosing a tube or funnel (e) (concha) which forms the enlarged outer portion of the auditory passage.

Within the cavity of the auricle, on its medial surface, somewhat below the level of the lowest portion of the margin of the auditory opening is seen a very prominent pedunculated process (d). This is really part of a transverse ridge, extending craniocaudad on the medial wall of the auricular cavity. This ridge is the =antihelix= (d); it divides the auricula into a distal and a proximal portion. The portion distad of the antihelix is the =scapha= (a); its surface is smooth except for two slight longitudinal ridges extending distad from near the two ends of the antihelix. That portion of the auricle that lies proximad of the antihelix forms a deep irregular cavity known as the =concha= (e). A deep furrow, corresponding to the antihelix, separates scapha from concha externally. The concha extends caudad as a deep, rounded pocket; externally this pocket forms a prominent convexity, the =eminentia conchæ= (j), on the caudal surface of the proximal portion of the external ear. Farther proximad the concha narrows like a funnel and becomes much compressed laterally, so that a section of its cavity forms a narrow craniocaudal slit.

The inner surface of the concha is marked by a number of very prominent ridges and projections. These are usually due to folds in the cartilage, so that external furrows and depressions correspond to the internal projections. The edges of these depressions serve for attachment of muscles (Fig. 169), by contraction of which the form of the concha is changed,--the folds of the cartilage thus serving as regions of greater flexibility.

Two or three centimeters proximad of its external opening the cartilage of the concha ends in a free edge which receives within itself the distal end of the cartilaginous external auditory meatus (e′). The two are united together by fibrous tissue. The cartilaginous auditory meatus (e′) extends as a nearly cylindrical tube mediad and slightly craniad to the tympanic membrane in the opening of the bulla tympani. This tube is lined with integument which continues over the tympanic membrane. The subcutaneous tissue of the tube contains the ceruminous glands, which secrete the ear-wax and open on the surface of the skin.

To understand the relations of the muscles of the external ear it is necessary to consider a little more fully the structure of the auricular cartilage (Fig. 168), especially as compared with the human ear. The ear of the cat differs from the human ear in the fact that the edges of the auricular cartilages are rolled together proximad so as to overlap. There is thus no gap between the tragus (g) and antitragus (h), as in man, but these two structures partly overlap.

a, scapha or pinna; b, helix; c, spine of the helix; d, antihelix; e, concha; e′, cartilaginous auditory meatus; f, crista helicis; g, tragus; h, antitragus; i, processus uncinatus; j, eminentia conchæ.]

The free edge (b) of the auricular cartilage corresponds partly to the =helix= of man. At its mediocranial angle the proximal part of the auricle is composed of muscle and integument only, so that the cartilage when isolated presents here a deep notch distad of which is a prominent projecting angle, the =spine= (c) of the helix, which serves for attachment of the adductor auris superior muscle (Fig. 169, 1). Proximad of the spine, separated from it by the deep notch, the cranial edge (g) of the auricular cartilage approaches the caudal edge (h), finally overlapping it, so that the cavity of the auricle is now completely surrounded. This overlapping portion of the cartilage is on the medial side of the cavity, and in the natural state is covered by muscles and integument, so as not to be apparent.

That portion of the cranial edge of the cartilage that overlaps the caudal edge forms a cartilaginous plate which projects proximad as a blunt point; this plate is the =tragus= (g). Along the inner edge of the tragus is a ridge which forms a continuation of the helix; this is the =crista helicis= (f). The portion of the caudal edge of the cartilage that is partly overlapped by the tragus is an irregular flat plate known as the =antitragus= (h). At its caudolateral margin the antitragus extends distad as a thin, pointed, cartilaginous spine, the =processus uncinatus= (i). This supports a sheet of integument which bears two longitudinal ridges projecting into the cavity of the ear, with a broad groove between them.

=The Scutiform Cartilage.=--Closely connected with the external ear, but not forming a part of it, is the scutiform cartilage. This lies in the temporal fossa, between the integument and the temporal muscle, just behind the orbit and beneath the craniomedial portion of the auricle (Fig. 63, 1, page 97). It is a narrow cartilage about two centimeters long, with its long axis craniocaudal. For the greater part of its length it forms a slender rod, from which a thin cartilaginous sheet extends a short distance laterad; at its caudal end the rod forks, the two arms extending nearly at right angles to the main portion. The scutiform cartilage serves for the origin or insertion of a number of the muscles of the ear.

=Muscles of the External Ear= (Fig. 169).--(a) The muscles connecting the cartilages of the external ear with other parts of the head have been described (page 96), excepting the tragicus lateralis (4). These muscles are the following: M. intermedius scutulorum (Fig. 63, a), M. frontoauricularis, M. levator auris longus (including the cervicoauricular) (Fig. 63, g, g′), M. auricularis superior (Fig. 63, k), M. abductor auris longus (Fig. 63, m), M. abductor auris brevis (Fig. 63, l), M. zygomaticus (Fig. 64, d), M. submentalis (Fig. 64, c), M. depressor conchæ (Fig. 64, b), M. frontoscutularis, M. adductor auris inferior (Fig. 63, f), and M. tragicus lateralis (Fig. 169, 4).

=M. tragicus lateralis= (Fig. 169, 4).--A band about seven millimeters wide, lying beneath the mass of fat about the ear and running ventrad lengthwise of the concha and close against it.

Origin (Fig. 96, a, page 229) on the caudal end of the mandibula, in the cavity found between the condyloid process and the angular process. The muscle passes dorsad and is inserted (Fig. 169, 4) on the caudal margin of the tragus and in the depression on the concha just caudad of the tragus.