2. The second lumbar nerve ([Fig. 162], a) divides, like the others, into two branches. The lateral branch pierces the muscles of the abdominal wall at the lateral border of the longissimus dorsi and passes, lying just beneath the integument, ventrocaudad nearly to the pubis. It supplies the integument of the caudal half of the abdomen and of the fold between the thigh and abdomen. The medial branch passes at first almost directly caudad, lying on the medial surface of the transversus abdominis; it curves gradually ventrad, lying in the substance of the transversus, and reaches the rectus abdominis three or four centimeters craniad of the pubis.
3. The third lumbar nerve (b) is not united with the fourth, as in many animals, but remains distinct. It divides one or two centimeters from its origin. The lateral branch passes caudad to the outer surface of the abdominal muscles, and extends, lying just beneath the integument, to that portion of the abdominal wall which is partly covered by the thigh. The medial branch passes caudad on the lateral surface of the iliopsoas muscle, follows the iliolumbar artery for a short distance, and gives branches to the caudal part of the transversus and rectus abdominis muscles.
The Lumbar Plexus
([Fig. 162]).—The fourth, fifth, sixth, and seventh lumbar nerves are interconnected by short branches, forming thus the lumbar plexus. They are also connected with the sacral plexus, so that the two are often considered together as the lumbosacral plexus ([Fig. 162]). In some other animals the first three lumbar nerves form also a part of the plexus; but they are not connected with it in the cat.
The fourth lumbar nerve forms the genitofemoral (c) and part of the lateral cutaneous (d) nerves, and is connected by a short strand with the fifth. The fifth aids in forming the lateral cutaneous (d) and femoral (f) nerves, and is connected by a short branch with the sixth. The sixth lumbar is large; it forms the major part of the femoral (f) and a large part of the obturator nerve (g), and sends a large connecting branch caudad to join the seventh and thus pass into the sacral plexus. The seventh passes caudad to join the sacral plexus and aid in forming the great sciatic nerve (h); by its connection with the sixth it aids in forming also the obturator nerve (g).
4. N. genitofemoralis (or N. lumboinguinalis) (c, c′, c″).—This is a direct continuation of the fourth lumbar nerve. Its medial branch (c″) passes along the medial surface of the iliopsoas (8) and the psoas minor (9) to the external iliac artery, accompanies this, lying on the ventral surface, from its origin to the point where it gives off the profunda femoris, then accompanies the latter artery and passes onto that branch of it that spreads out under the integument of the ventral pelvic region. It crosses the spermatic cord and ramifies in the skin of the proximal part of the medial side of the thigh. The lateral branch of the genitofemoralis (c′) pierces the psoas minor (9), appearing on its ventral surface opposite the fifth lumbar vertebra. It passes caudad on the ventral surface of this muscle, crosses the iliolumbar artery, then turns caudolaterad, passes through the abdominal wall, and is distributed to the craniomedial surface of the thigh and to the adjacent abdominal wall.
This nerve is variable in origin and in distribution. Its lateral branch is sometimes lacking.
5. N. cutaneus femoris lateralis (d).—The lateral cutaneous nerve arises from the connecting strand between the fourth and fifth nerves, most of its fibres coming from the fifth. It passes caudoventrad between the iliopsoas and psoas minor and accompanies the iliolumbar artery across the ventral surface of the iliopsoas. It pierces the abdominal wall in company with the artery and passes onto the cranial border of the thigh. Its terminal branches are distributed to the integument along with the branches of the iliolumbar artery,—supplying the lateral surface of the femur and extending as far as the knee.
6. N. femoralis (f).—The femoral is a large nerve which rises by strong roots from the fifth and sixth lumbar nerves. It gives branches to the iliopsoas (8) and passes through that muscle to its ventral surface. It leaves the abdominal cavity lying on the ventral surface of the iliopsoas, and at the point where it pierces the abdominal wall it divides into three (or four) branches. One (or two) of these pass to the sartorius muscle. Another passes between the rectus femoris and vastus medialis muscles, and divides into numerous branches which supply these muscles and the vastus intermedius.
The third branch of the femoral is N. saphenus or the long saphenous nerve ([Fig. 127], g, [page 310]). This passes distad along with the femoral artery and vein, giving a few twigs to the integument. Where the long saphenous artery (f) separates from the femoral artery the nerve follows the former and passes with it along the medial side of the lower leg. At the knee and in the lower leg it gives off a number of cutaneous branches, and finally divides just distad of the middle of the lower leg into two main branches. Both of these pass to the concavity of the ankle-joint and divide into fine branches, which form a sort of plexus on the ankle and dorsal surface of the foot; branches from this can be traced almost to the toes.