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.