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 (p[p. 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.