Part 108

Cervical Nerves (p. 383).--The ventral rami of the cervical nerves are to be sought as they pass out between the bundles of the scalenus, or between the scalenus and longus capitis, in the neck. This region has already been uncovered in dissecting the vagus and sympathetic (Fig. 156). Dissect first the second cervical (p. 385). Find its ventral ramus as it emerges between the levator scapulæ ventralis and cleidomastoid (Fig. 158, 2), and follow its branches,--the auricularis magnus (5) and cutaneus colli (6). Find its dorsal ramus, the great occipital nerve (p. 384), by reflecting the clavotrapezius muscle; the nerve will be found emerging from the underlying muscles close to the craniomedial angle of the clavotrapezius, near its origin. Trace the nerve in both directions.

The ventral ramus of the first cervical (p. 385) will be found emerging from beneath the wing of the atlas, a little distance craniad of the second (Fig. 156, j). Trace it. To find its short dorsal ramus, the suboccipital nerve (p. 384), it is necessary to dissect apart the muscles on the dorsal side of the atlas till the nerve is found passing from the atlantal foramen.

Dissect the third, fourth, and fifth nerves (p. 385, and Fig. 158).

Brachial Plexus (p. 386, and Figs. 159 and 160).--The brachial plexus has been partly uncovered in dissecting the vagus and sympathetic. (If a new specimen or the opposite side is used, reflect the skin from the ventral surface of the thorax and arm, and cut the pectoral muscles, thus uncovering the vessels and nerves of the axilla.) Reflect the skin from the ventral surface of the upper arm. (Do not use scalpel, but tear the skin from the muscles. In this way the nerves will be seen passing to the skin, while if the scalpel were used the nerves might be cut.)

Tie the axillary vein or its two branches in two places, and cut the vein between the tied regions. Leave the arteries as guides for dissection, but remove the veins. Now clean thoroughly the nerves forming the brachial plexus as they pass from the thorax or neck. Be careful not to injure any of the fine nerves or the interconnections of the nerves in doing this. Find and distinguish clearly the fifth, sixth, seventh, and eighth cervical nerves and the first thoracic, as they emerge from the neck or thorax. (Compare Fig. 159 and Fig. 157, V-VIII and I′.)

Follow out the branches of the plexus, noting the origin and distribution of each branch, in order to determine its name. To follow the phrenic (p. 388, and Fig. 157, f), remove a portion of the thoracic wall. In following the other branches of the plexus, pull back the skin wherever an exposure is to be made, and separate the muscles. The epitrochlearis may be cut near the elbow, and the clavobrachial near the shoulder. In following the interosseous branches of the median nerve the fifth head of the flexor profundus, and the extensor brevis pollicis, may be cut. As a rule it will not be necessary to cut other muscles.

Thoracic and Lumbar Nerves.--One or two of the thoracic nerves (p. 393) should be dissected from the outside by finding the intercostal nerve along the caudal border of one of the ribs and tracing it in both directions. The nerve may be exposed by removing the external muscles covering the rib, and cutting the external intercostal muscles. The dorsal ramus should be traced after the ventral ramus has been studied.

The first lumbar nerve (p. 395) should be dissected in the same way.

The other lumbar and sacral nerves (pp. 395-400) are best dissected from within.

The alimentary canal and its appendages should be removed from the abdomen, leaving only five or six centimeters of the caudal end of the rectum. The kidneys and urogenital organs may be left, to be removed during dissection. (Compare Fig. 162.)

Turn one of the kidneys to the other side, and find the second lumbar nerve (Fig. 162, a) appearing at the lateral border of the iliopsoas muscle. Trace it to its origin; trace it also distad, following both branches. It will be necessary to trace the nerves through the abdominal wall, then find them from the outside, and follow them to their distribution.

The third nerve (Fig. 163, b) may be found by dissecting apart the fibre-bundles of the iliopsoas and psoas minor, and following in the same way. The kidneys, ureters, vena cava, and aorta may be removed as occasion arises.

The remainder of the lumbar nerves may be found in order, in a similar manner. Follow the saphenous nerve and its branches (p. 397) by removing the integument from the medial side of the leg (see Fig. 127, p. 310). To dissect the sacral nerves (p. 399) separate the innominate bones at the pubic symphysis and divaricate them. Find the nerves arising from the sacral plexus, by cutting the levator ani muscle. After N. hemorrhoidalis inferior and N. pudendus have been traced, the rectum and urogenital organs may be removed, taking great care not to remove more than is necessary. To dissect the great sciatic nerve (p. 400, and Fig. 163), separate the biceps and caudofemoralis near their proximal ends and find the large nerve-trunk (a). Then lift the biceps away from the nerve, cut that muscle near its middle, and reflect it. The nerve may now be followed to its terminal branches; during the process cut the muscles only when absolutely necessary. The inferior gluteal nerve (i) will be found on the dorsal surface of the great sciatic; by cutting the caudofemoralis and gluteus maximus muscles near their insertions and turning them back the distribution of the nerve may be followed. The superior gluteal (j) will be found at the cranial margin of the pyriformis by reflecting the gluteus medius in the same way; the tensor fasciæ latæ may also be cut.

SENSE-ORGANS.

I. THE EYE (p. 410).

Use any specimen on which one side of the head has been left intact. Remove the head from the body by cutting through the neck a little craniad of the first rib.

Study the eye externally. Observe the eyelids, the conjunctiva, nictitating membrane with its cartilage, and the Harderian gland (Fig. 165); the two openings of the lachrymal duct, the Meibomian glands if possible.

Remove the zygomatic arch and expose the orbit and the structures which it contains. Study the periorbita (p. 409), and find the lachrymal gland (p. 410).

Study the muscles of the eyeball (p. 411, and Fig. 166). The lateral rectus on the lateral surface will perhaps be first found, and its tendon traced beneath the inferior oblique. Use great care not to injure the levator palpebræ superioris.

After studying the muscles, find the optic nerve. Cut it and the muscles, and remove the eyeball for farther study.

The Eyeball (p. 412, and Fig. 167).--For an examination of the eye it is well to have a fresh specimen and one hardened in formalin or alcohol. The hardened specimen is more essential, however. An eye from one of the specimens used in dissection is usually satisfactory. All accessory portions should be trimmed from the eyeball, leaving only the spherical ball with a short stalk formed by the optic nerve.

Observe such features of the eye as can be seen externally: the optic nerves, sclerotic, cornea, iris, and pupil. This should be done on a fresh specimen, if one is at hand. The changes in size and form of the pupil can be observed in the living cat by changing it from a light to a dark place and vice versa.

Dissection.--Examine the internal structures on a preparation. Then with fine scissors and forceps remove from the eye about one-fourth of the wall, in the form of a quadrant having one point at the optic nerve, the other at the centre of the cornea. The coats of the eye can then be studied on the piece removed, while the other structures will be visible within the eyeball. No special directions are necessary for observation of the structures described, unless it be the capsule of the lens and the zonula ciliaris. The capsule of the lens may be demonstrated by tearing a bit of it off with fine forceps. The zonula ciliaris is easily seen by divaricating the edges of the cut made in removing the quadrant, so as to stretch the fine fibres of which the zonula is formed.

II. THE EAR (p. 415).

(The muscles of the external ear are of little practical importance, and will doubtless usually be omitted.)

(1) The External Ear. Read the description of the external ear (p. 415), verifying it by examination and comparison of an ear still covered with integument, in the natural condition, and of a preparation of the isolated cartilages of the ear (Fig. 168). The latter may be obtained by dissecting the skin and muscles from a fresh ear.

(2) The scutiform cartilage (p. 418) should be observed in the natural position, and as isolated.

(3) The muscles of the external ear (p. 418, and Fig. 169). Those connecting the external ear with other parts of the head have been studied in connection with the facial muscles. The remainder will be studied on an external ear removed according to the directions given on page 435.