Chapter III: Peripheral Nervous System

(There are some advantages in dissecting the eye with its muscles before dissecting the nerves, as a knowledge of the eye-muscles is presupposed for dissecting some of the cranial nerves. For directions on the eye, see p. 469.)

A new specimen should be used, if possible, for the peripheral nervous system, though that used for the blood-vessels can be employed, at considerable disadvantage.

Prepare as for the blood-vessels. The arteries should be injected with red starch, to aid in tracing the nerves.

1. THE CRANIAL NERVES (p. 369) AND SYMPATHETIC SYSTEM (p. 404).

1. Reflect the skin covering the sternomastoid muscle, and make a longitudinal incision of the muscle so as to expose the carotid artery. Lying along the artery find the combined trunk of the sympathetic and vagus nerves. Follow the vagus (p. 378) first craniad; transect the muscles as necessity arises, and find its ganglion nodosum and at the same time locate the superior cervical ganglion of the sympathetic nerve (p. 404, and Fig. 156). Then find the hypoglossal nerve (Fig. 156, _b_), passing outside of the carotid artery to the tongue, and the accessory (Fig. 156, _c_), passing to the trapezius. Cut and reflect the digastric muscle and find the small glossopharyngeal nerve (Fig. 156, _a_), passing to the surface of the bulla and then beneath the carotid artery.

2. Follow the vagus (p. 378) caudad to its termination. To do this it is necessary to remove one side of the thorax, as in dissecting the blood-vessels. Do not injure the nerves of the axilla, nor the phrenic or sympathetic nerves. For the vagus in the thorax, compare Fig. 157. Find the branches of the nerve; in dissecting them, pull on them to make them tense. They are then more easily visible. To dissect the abdominal portion of the vagus, open the abdominal cavity, and compare Fig. 164 (p. 407).

3. Dissect the sympathetic (p. 404), following it and its branches to the pelvic region (Figs. 156, 157, and 164).

4. The hypoglossal (p. 383, and Fig. 156, _b_).

5. The glossopharyngeal (p. 378, and Fig. 156, _a_).

6. The accessory nerve (p. 382, and Fig. 156, _c_; Fig. 158, 1).

Cut away a portion of the tympanic bulla and the base of the skull, sufficient to follow these nerves in the jugular foramen, to the brain.

7. Locate the stylomastoid foramen and pick away overlying tissue until the facial nerve is found emerging and then follow its branches to their distribution (p. 375, and Fig. 155).

8. Expose the ventral surface of the pterygoid muscles just mediad of the angle of the jaw. Divide and reflect them, and the mandibular division of the fifth nerve (p. 373, and Fig. 154) will be found dorsad of them and of the internal maxillary artery. The chorda tympani (p. 375) passes ventrad of the artery to join the lingual. Follow out (1) the lingual branch (p. 375) (with the chorda tympani), and (2) the inferior alveolar (p. 375) by cutting away the ventral border of the mandible. Then cut the mandible near the canine tooth, and pull it to one side, and follow out the muscular branches of the mandibular nerve.

9. Remove the mandible and find the maxillary nerve (p. 371) emerging from the foramen rotundum. Follow its branches and find the sphenopalatine ganglion (p. 372).

10. Remove the zygoma so as to expose the whole ventral aspect of the orbit. Carefully pick away the fat in the orbit without injuring any nerves, so as to expose the four recti muscles and the inferior oblique (see p. 411, and Fig. 166). Find the abducens nerve (p. 375, and Fig. 154), entering the dorsal edge of the lateral rectus, and follow it back. Look on the inner surface of the inferior rectus for the branch of the third nerve (p. 369) which supplies it. Find the branch of this nerve which runs to the inferior oblique muscle, and on it the ciliary ganglion; find the branches to the ciliary ganglion from the ophthalmic nerve and follow them (p. 371). Follow also the short ciliary nerves (p. 370) to the eyeball.

11. Trace the third nerve (p. 369) to its foramen of exit and find its branches. Where it passes between the superior and lateral recti, find the ophthalmic nerve (p. 370) by its side and trace its branches.

12. Find the fourth nerve (p. 370), passing outside of the lateral rectus at its origin and entering the superior oblique.

13. Follow the third, fourth, fifth, and sixth nerves into the skull by chipping away the bone and removing the dura. Note the semilunar or Gasserian ganglion (p. 370, and Fig. 138, _k_) and the origin of the fifth nerves from it, and the relation of the ventral root of the fifth nerve to the mandibular nerve.

2. SPINAL NERVES.

The spinal nerves may be dissected on the same side used for dissecting the cranial nerves. (If an undissected specimen is used, remove the skin from the side of the neck, and cut the sternomastoid, sternohyoid, and sternothyroid muscles, as directed for the vagus and sympathetic.)

_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.