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.