Part 35

5. =N. cutaneus femoris posterior= (Fig. 162, _k_; Fig. 163, _h_).--This arises from the sacral plexus, its roots coming chiefly from the second and third sacral nerves, and passes caudodorsad, at first in close connection with N. pudendus. It then accompanies the posterior gluteal artery and vein, sends branches (perineal nerves) into the fat at the sides of the anus, and branches onto the lateral surface of the biceps muscle. One of the latter follows the communicating vein (Fig. 163, _k′_) from V. saphena parva, and may be traced as far distad as the popliteal space.

6. =N. hemorrhoidalis inferior= (or =posterior=) (Fig. 162, _m_).--This arises by two roots, from the second and third sacral nerves, and passes with the inferior hemorrhoidal artery ventrad across the lateral surface of the rectum to the urethra. Here it divides into two parts: one passes craniad to the bladder, the other caudad onto the ventrolateral surface of the rectum.

A small nerve passes from the sacral plexus, especially from the third sacral nerve, into M. levator ani (Fig. 162, 11). Another small nerve from the same region passes to M. coccygeus, and a third to the proximal end of M. tenuissimus.

E. NERVES OF THE TAIL. COCCYGEAL NERVES.--From the intervertebral foramina of the first seven or eight caudal vertebræ spinal nerves of the usual type are given off. The dorsal rami innervate the dorsal muscles of the tail. The ventral rami are interconnected with each other and with the last sacral nerve by a longitudinal cord; they innervate the muscles and integument of the ventral side of the tail.

3. The Sympathetic Nervous System. Systema nervorum sympathicum.

The sympathetic system consists essentially of a chain of ganglia on each side of the ventral surface of the vertebral column, interconnected by longitudinal nerve-cords, and stretching from the base of the skull to the tail. The ganglia are connected to the spinal nerves by communicating branches, and numerous branches pass from them to the abdominal and thoracic viscera, and to the walls of the lymph- and blood-vessels, forming complicated plexuses.

=Cervical portion= (Fig. 156, page 379).--The sympathetic system begins just caudad of the tympanic bulla as the =superior cervical ganglion= (=G. cervicale superius=) (Fig. 156, _e_). This is a large ganglion closely applied to the ganglion nodosum (_d_) of the vagus, and lying on its ventrocranial side. From the superior cervical ganglion the sympathetic trunk passes caudad usually closely bound up with the vagus (_d′_), so that the two can scarcely be distinguished. They lie (_i_) at the side of the trachea (6), close against the lateral surface of the carotid artery. A few centimeters before reaching the first rib the two separate (Fig. 157, page 381); the sympathetic (_c_) is here the smaller and more dorsal of the two. Just craniad of the first rib the sympathetic usually forms a small swelling, the =middle cervical ganglion= (Fig. 157, _d_), beyond which it divides into two portions. The larger dorsal division passes dorsocaudad and joins a large ganglion, the =inferior cervical= (_e_), which lies at the head of the first rib, on the lateral surface of the longus colli muscle. The ventral branch (_g′_) passes caudad, turns laterad about the medial and caudal surface of the subclavian artery, and likewise reaches the inferior cervical ganglion.

In some cases the middle cervical ganglion (_d_) is lacking.

=Branches of the Sympathetic in the Cervical Region.=--From the cranial end of the superior cervical ganglion several nerves pass craniad. Some of these become connected immediately with the cranial nerves emerging from the jugular foramen, forming a plexus in this region. Others pass with the internal carotid artery craniad, forming a plexus about that artery; from the plexus twigs arise which pass to the fifth and sixth cranial nerves; doubtless also to the seventh and eighth. A specially large branch, the =deep petrosal nerve= (see page 373), passes from the superior cervical ganglion along with the internal carotid artery to the pterygoid canal, thence through this to the =sphenopalatine= ganglion. Before separating from the vagus the sympathetic gives off one or two =cardiac= nerves (Fig. 157, _i_). These are closely bound up with the vagus (_b_), so that they appear usually to have origin from the vagus at about the level of the first rib. These pass with the vagus to the heart and take part in the formation of the =cardiac plexus= (_k_).

At the middle cervical ganglion (_d_) usually a communicating branch is given to the vagus. The branches of the inferior cervical ganglion (_e_) fall in the thoracic region, and are described in that connection.

=Thoracic Portion of the Sympathetic= (Fig. 157, page 381).--The inferior cervical ganglion (_e_) lies entirely within the thoracic cavity. It has the following branches:

1. One or two small =cardiac= branches (_h_), which pass to the heart (_r_).

2. A =large communicating branch= (_e′_), which passes craniad from the cranial end of the ganglion (_e_) along the lateral surface of the longus colli muscle, and gives communicating branches to the sixth, seventh, and eighth cervical nerves (_VI_, _VII_, and _VIII_).

3. One or two small communicating branches to the first thoracic nerve (_I′_).

4. A communicating branch to the second thoracic nerve.

The arrangement of these communicating branches is somewhat variable; they may arise in various combinations, as at first united, or as more or less completely separated nerves.

The sympathetic trunk (_c_) continues caudad from the caudal end of the inferior cervical ganglion, lying at first on the lateral surface of the longus colli, then on the lateral surface of the centra of the vertebræ. For each vertebra there is a slight enlargement, forming one of the =thoracic ganglia= of the sympathetic, and from each ganglion one or two =communicating branches= are given off to the corresponding spinal nerve. Just caudad of the last rib (but while still in the thoracic cavity) the sympathetic gives off the large =N. splanchnicus major= or great splanchnic nerve (_n_), which passes ventrocaudad and pierces the diaphragm. The sympathetic itself then passes through the diaphragm laterad of the crus.

=Abdominal Portion of the Sympathetic= (Fig. 164).--The great splanchnic nerve (_b_) passes through the diaphragm (2) laterad of the crus, extends two or three centimeters caudoventrad, and joins a large ganglion lying just caudad of the cœliac artery (4), almost on the lateral surface of the superior mesenteric (5). This is the =cœliac= (or semilunar) =ganglion= (_d_). From it nerves branch in all directions, forming the =cœliac plexus= (_c_), a part of the large =solar plexus=. Less than a centimeter ventrocaudad of the cœliac ganglion, on the caudal surface of the superior mesenteric artery (5), is the somewhat smaller =superior mesenteric ganglion= (_f_); the two are connected by two strong nerve-trunks.

[Illustration: FIG. 164.--SYMPATHETIC AND VAGUS IN THE ABDOMEN (SOMEWHAT SCHEMATIC).

1, stomach; 2, crus and cut edge of diaphragm; 3, aorta; 4, cœliac artery; 5, superior mesenteric artery; 6, kidney; 7, inferior mesenteric artery; 8, large intestine. _a_, main trunk of sympathetic; _b_, great splanchnic nerve; _c_, lesser splanchnic nerves; _d_, cœliac (or semilunar) ganglion; _e_, cœliac plexus; _f_, superior mesenteric ganglion; _g_, superior mesenteric plexus, following the artery; _h_, aortic plexus; _i_, inferior mesenteric ganglion and plexus; _j_, suprarenal plexus on suprarenal body; _k_, renal plexus following renal artery; _l_, ventral œsophageal branch of vagus, forming anterior gastric plexus; _m_, dorsal œsophageal branch of vagus, forming posterior gastric plexus.]

The cœliac ganglion (_d_) receives from the sympathetic (_a_), in addition to the great splanchnic nerve, two (or three) =lesser splanchnic= nerves (_c_), which arise from the main trunk of the sympathetic, the first just as it reaches the abdominal cavity, the second one or two centimeters caudad of the diaphragm.

From the cœliac and superior mesenteric ganglia a network of nerves passes in all directions to the abdominal viscera. These nerves follow especially the arteries, forming plexuses over their surfaces, and passing with them to the organs which they supply. The cœliac plexus anastomoses with the =posterior gastric= plexus of the vagus. Parts of the cœliac plexus following the branches of the cœliac artery may be distinguished as =hepatic= plexus, =splenic= plexus, etc. A dense plexus is formed on the surface of the suprarenal body (=suprarenal= plexus, _j_), and a =renal= plexus (_k_) follows the renal artery to the kidneys (6). The =superior mesenteric plexus= (_g_) forms a dense network over the superior mesenteric artery (5) and follows it to the intestine. A plexus passes caudad on the aorta (=aortic plexus=, _h_). This is a continuation of the cœliac and superior mesenteric plexuses; it receives a number of branches from the main sympathetic trunk, lying dorsad of the aorta. The aortic plexus contains a number of small ganglia.

From the aortic plexus the smaller =inferior mesenteric= plexus (_i_) follows the inferior mesenteric artery (7) to the large intestine; in this lies a ganglion of considerable size, the =inferior mesenteric ganglion=.

In the pelvic region small plexuses are formed in a similar manner, from branches given off by the main sympathetic trunk, together with branches from the plexuses of the abdominal cavity. These plexuses supply the various organs in the pelvis.

The main sympathetic trunk (_a_) passes into the abdomen at the side of the crus of the diaphragm, lying dorsad of the great splanchnic nerve (_b_). In the abdomen it lies nearer the middle line than in the thorax, so that the two trunks of right and left side are separated by but a few millimeters as they lie on the ventral surface of the centra of the lumbar vertebræ. There is a ganglion for each vertebra (Fig. 162, _n_), and from each ganglion are given off one or more communicating branches to the spinal nerves, and usually a branch which passes ventrad to join one of the plexuses among the viscera. The first three branches of this kind (Fig. 164, _c_) are larger and pass to the cœliac and superior mesenteric ganglia. The sympathetic may be traced into the pelvis, where it becomes smaller, and is lost as the caudal region is reached.

SENSE-ORGANS AND INTEGUMENT.

I. THE ORGAN OF SIGHT. ORGANON VISUS.

1. =The Orbit.=--The bony framework of the orbital fossa, in which the organ of sight is situated, has already been described (page 53). The orbit is not protected by bone on all sides, being open caudad and ventrad, and partly laterad. The structures within the orbit are further protected and separated from other structures by a very thin, tough, transparent membranous sac, the =periorbita=, which surrounds them almost completely and nearly fills the orbit. The periorbita forms a conical sac with the apex of the cone directed caudoventrad and attached about the optic foramen and orbital fissure. The base of the cone is attached about the margin of the orbital fossa,--to the supraorbital margin of the frontal, to the maxillary and malar bones, and to the orbital ligament, connecting the frontal process of the malar with the zygomatic process of the frontal. The periorbita is partly separated from the walls of the orbital fossa by masses of fat. It comes in contact, aside from the bones, with the temporal and pterygoid muscles, and with nerves and blood-vessels in the orbit.

The sac thus formed encloses the eyeball with its muscles and glands (Fig. 166), as well as a mass of fat in which these are imbedded. The periorbita and its contents are freely movable with relation to surrounding structures.

2. =The Eyelids. Palpebræ.=--The eyelids are two thin folds which protect the eye. Externally they are covered with hair; internally by the thin membranous =conjunctiva=. The eyelids contain the =Meibomian= or =tarsal= glands, arranged in short rows passing perpendicularly inward from the edge of the lids; these may sometimes be seen with the naked eye as broad yellowish lines. They secrete a substance which prevents the adhesion of the edges of the two lids. On the edge of each eyelid, three to four millimeters from the medial angle of the eye, is one of the openings of the lachrymal canals.

At the medial angle of the eye is the large =nictitating membrane= (=membrana nictitans=), or “third eyelid,” corresponding to the plica semilunaris of man. In the cat this is large and may cover the whole surface of the eye. It is supported (Fig. 165) by a broad central strip of cartilage (_a_), passing from its edge to its inner angle, and the inner end of this cartilage is surrounded, on the medial (concave) side of the membrane, by the large lobulated =Harderian= glands (_b_).

[Illustration: FIG. 165.--INNER SURFACE OF MEMBRANA NICTITANS,

showing the supporting cartilage and Harderian glands. _a_, supporting cartilage; _b_, Harderian glands; _c_, outer edge.]

The =conjunctiva= is the thin membrane covering the inner surface of the lids, the outer surface of the eyeball, and both surfaces of the nictitating membrane.

The muscles of the eyelids are =M. orbicularis oculi=, already described (page 98), and =M. levator palpebræ superioris=, described below.

3. =Lachrymal Apparatus.=--The =lachrymal gland= is a large reddish gland, not lobulated externally, which lies on the surface of the eyeball just beneath the lateral angle of the eye. The gland is situated immediately craniad of, and in contact with, the zygomatic process of the frontal bone (Fig. 154, 11, page 374). It is flat and about one centimeter in diameter. The ducts which pass from the gland to the eye are not visible to the naked eye. The fluid secreted by the gland collects at the medial angle of the eye and passes into the two openings of the lachrymal canals, one of which is found on the pigmented edge of each lid, three or four millimeters from the medial angle of the eye. The canals passing from these openings soon unite to form the =nasolachrymal duct=, which passes through the lachrymal bone, along the medial surface of the maxillary, to open into the nasal cavity ventrad of the ventral concha of the nose.

4. =Muscles of the Orbit= (Fig. 166).--Within the orbit are eleven muscles, ten connected with the eyeball, and one, M. levator palpebræ superioris, with the upper eyelid.

Of the ten muscles of the eyeball, eight are straight muscles, passing from an origin about the inner end of the orbit directly distad; the other two have an oblique direction. Four of the straight muscles are larger and are known as the =Mm. recti= (_b_, _c_, _d_); these are distinguished according to their position as =lateral= (_c_), =medial=, =superior= (_d_), and =inferior= (_b_). The four smaller straight muscles constitute together =M. retractor oculi= (_e_). The two oblique muscles are known as =Mm. obliqui= and are distinguished according to their position as =inferior= (_a_) and =superior=.

[Illustration: FIG. 166.--MUSCLES OF THE EYEBALL, LATERAL VIEW.

_a_, M. obliquus inferior; _b_, M. rectus inferior; _c_, M. rectus lateralis; _d_, M. rectus superior; _e_, parts of M. retractor oculi; _f_, the oculomotor nerve.]

=Mm. recti= (_b_, _c_, _d_).--The four recti muscles arise from the bone about the optic foramen and pass toward the eyeball. They are inserted by thin, flat tendons along a line which separates the darker caudal part of the sclerotic from the white zone of the sclerotic which borders the cornea,--the line of insertion of the four tendons forming thus a circle about the eyeball. The rectus superior (_d_) is partly covered by M. levator palpebræ superioris. The tendon of the rectus inferior (_b_) is crossed by M. obliquus inferior (_a_). The recti muscles are all innervated by the oculomotor (third) nerve (_f_), except the lateral (_c_), which is supplied by the abducens (sixth) nerve.

=M. retractor oculi= (_e_).--The retractor oculi arises about the optic foramen and divides into four heads which lie nearer the eyeball than the recti and are therefore partly covered by the latter. They alternate with the recti and are inserted into the eyeball at about its equator, except the inferior division, which is inserted on a line with the recti. They are supplied by the oculomotor nerve.

=M. obliquus inferior= (_a_).--The inferior oblique arises from the maxillary bone just laterad of the lachrymal bone and curves over the ventral side of the eyeball along the lines of insertion of the recti, crossing the tendon of the rectus inferior (_b_) to the ventral edge of the tendon of the lateral rectus (_c_), where it is inserted. It rotates the eyeball and is supplied by the third nerve (_f_).

=M. obliquus superior.=--The superior oblique takes origin from the cranial border of the optic foramen, passes dorsocraniad, and ends in a small rounded tendon which passes through a fibrous pulley-ring. This ring is situated near the rim of the orbit, one or two centimeters craniad of the zygomatic process of the frontal bone, and is attached to the wall of the orbit by two fibrous bands. After passing through the pulley-ring the tendon turns and passes laterocaudad; it becomes much expanded, passes caudad of the superior rectus, and is inserted into the eyeball along the caudal margin of the insertion of the superior rectus tendon. The superior oblique is supplied by the trochlear (fourth) nerve.

=M. levator palpebræ superioris.=--This is a small thin muscle which takes origin on the wall of the optic foramen close to the rectus superior, passes over the outer surface of the rectus superior and beneath the lachrymal gland, and forms a thin tendon which is inserted into the margin of the upper eyelid. It is innervated by the oculomotor.

5. =The Eyeball.=--The eyeball is approximately spherical in form, but the cranial surface has a slightly greater curvature than the caudal portion, so that the eye appears slightly pointed at the cranial (free) surface. The large cylindrical =optic nerve= (Fig. 167, _a_) enters the eyeball at nearly the centre of the caudal half of the eye. On the outer surface is seen the transparent =cornea= (_c_), covering the free surface of the eye, and the opaque =sclerotic= (_b_), covering the remainder of the ball. Through the cornea can be seen the yellow curtain-like =iris=, with an opening in its centre, the =pupil=.

[Illustration: FIG. 167.--DIAGRAM OF A SECTION OF THE EYE.

_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.