Part 85

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.

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).

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=.

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=.