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above the inguinal ligament, it becomes superficial after passing through the subcutaneous inguinal ring and external spermatic fascia (Fig. 623, p. 715).
Its branches are muscular to the muscles of the abdominal wall, among which it passes, and cutaneous branches (anterior scrotal, or labial nerves), which innervate
the skin (1) of the anterior abdominal wall over the symphysis pubis, (2) of the thigh over the proximal and medial part of the femoral triangle, and (3) of the superior part of the scrotum, and root and dorsum of the penis (of the mons Veneris and labium majus in the female). These last-named branches are contiguous to branches of the perineal and pudendal nerves. No lateral cutaneous branch arises from the ilio-inguinal nerve. It thus corresponds, like the anterior cutaneous part of the ilio-hypogastric nerve, to the anterior trunk of a typical thoracic nerve.
N. Genitofemoralis.-The genito-femoral nerve (O.T. genito-crural) usually arises by two independent roots from the front of the first and second lumbar nerves, which unite in the substance of the psoas major to form a slender trunk. It appears on the posterior abdominal wall, lying on the psoas major, medial to the psoas minor, and, piercing the psoas fascia, it extends downwards on the lateral aspect of the common and external iliac vessels and behind the ureter, to the inguinal ligament (Fig. 625, p. 721). At a variable point above that ligament it divides into two branches. 1. The external spermatic branch is a small nerve. It crosses the terminations of the external iliac vessels, and, along with the ductus deferens and testicular and external spermatic vessels, enters the inguinal canal through the abdominal inguinal ring. It terminates by supplying small branches to the skin of the scrotum and adjacent part of the thigh. In the female it accompanies the round ligament to the labium majus. This nerve gives off in its course the following small branches: (1) to the external iliac artery; (2) to the cremaster muscle; (3) to communicate with the spermatic plexus of the sympathetic. 2. The lumbo-inguinal branch continues the course of the parent nerve into the thigh, lying on the lateral aspect of the femoral artery. It becomes cutaneous by passing through the fossa ovalis or through the iliac portion of the fascia lata, and supplies an area of skin over the femoral triangle, lateral to that supplied by the ilio-inguinal nerve (Fig. 623, p. 715). It communicates in the thigh with the intermediate cutaneous branch of the femoral nerve. Before piercing the deep fascia it gives a minute branch to the femoral artery.
N. Cutaneus Femoris Lateralis.-The lateral cutaneous nerve of the thigh is distributed only to the skin (Fig. 625). It arises from the back of the lumbar plexus, and usually from the second and third lumbar nerves. Emerging from the lateral border of the psoas major muscle, the nerve crosses the iliacus muscle, beneath the fascia iliaca, to reach the anterior superior iliac spine. It enters the thigh beneath the lateral end of the inguinal ligament, and either over, under, or through the origin of the sartorius muscle. It extends distally along the front of the thigh for a few inches, lying at first beneath the fascia lata, and afterwards in a tubular investment of the fascia. It gives off small branches in this part of its course, and finally, piercing the fascia about four inches distal to the anterior superior iliac spine, it separates into anterior and posterior terminal branches. The anterior branch is the larger, and is distributed on the lateral aspect of the front of the thigh almost to the knee. The smaller posterior branch supplies the skin of the lateral side of the buttock, distal to the greater trochanter, and the skin of the proximal twothirds of the lateral aspect of the thigh (Fig. 625, p. 721).
The obturator nerve supplies the muscles and skin on the medial side of the thigh. It arises in the substance of the psoas major muscle by three roots placed in front of those of the femoral nerve, and derived from the second, third, and fourth lumbar nerves (Fig. 624, p. 718). Sometimes the root from the second nerve is absent. Passing vertically downwards, the nerve emerges from the psoas major at its medial border, behind the common iliac, and on the lateral side of the hypogastric vessels. It passes forwards below the pelvic brim in company with the obturator artery to the obturator groove of the obturator foramen, through which it reaches the thigh. While in the obturator groove it separates into its two main branches, named anterior and posterior (Fig. 626, p. 723).
The anterior (O.T. superficial) branch enters the thigh in front of the obturator externus and adductor brevis muscles, and behind the pectineus and adductor longus. In the middle third of the thigh it is found coursing along the medial border of the adductor longus, anterior to the gracilis; and it finally divides into two slender terminal filaments, of which one enters the adductor canal and ends on the femoral artery, while the other supplies the skin for a variable distance on the medial side of the thigh and joins in the obturator plexus.
The branches of the anterior part of the nerve are:
1. An articular branch to the hip-joint, which arises from the nerve as soon as it enters the thigh, and supplies the joint through the acetabular notch.
2. Muscular branches to the adductor longus, gracilis, adductor brevis (usually), pectineus (occasionally). The last-named muscle is not usually supplied from the obturator nerve.
3. A cutaneous branch of very variable size forms one of the terminal branches (Fig. 626). It becomes superficial between the gracilis and adductor longus, in the middle third of the thigh, and may supply the skin of the distal two-thirds of the thigh on its medial side. It is generally of small size, and is connected with branches of the medial cutaneous and saphenous nerves behind the sartorius muscle to form the obturator (O.T. sub-sartorial) plexus. The branch from the saphenous nerve to the plexus passes medially behind the sartorius after piercing the
aponeurotic covering of the adductor canal. The branch from the medial cutaneous nerve is generally superficial at the point of formation of the plexus.
4. The branch to the femoral artery is the other terminal branch of the nerve. It enters the adductor canal along the medial border of the adductor longus, and ramifies over the distal part of the artery.
5. A fine communicating branch sometimes joins the femoral nerve in front of the hip-joint.
The posterior (O.T. deep) branch of the obturator nerve reaches the thigh by piercing the obturator externus muscle. It passes distally between the adductor brevis and adductor magnus muscles. After passing obliquely through the adductor magnus, it appears in the popliteal fossa on the popliteal vessels, and terminates by piercing the oblique ligament of the knee and supplying the kneejoint.
Its branches are:-(1) muscular branches to the obturator externus, adductor magnus, and (when the muscle is not supplied by the superficial part of the nerve)
forwards in the first intercostal space and supplies the intercostal muscles. It usually gives off no anterior branch to the skin of the chest and no lateral cutaneous branch.
In some cases a lateral cutaneous branch emerges from the side of the first intercostal space This may be derived from the first nerve, or it may be the intercosto-brachial nerve, ie. the lateral branch of the second thoracic nerve. In many cases an anterior cutaneous branch perforates the first intercostal space and supplies the skin on the front of the chest. This branch, similarly, is sometimes traceable to the second thoracic nerve.
Communications.-Besides its junction with the eighth cervical to form the brachial plexus, the first thoracic nerve effects the following communications:-(a) The last cervical or first thoracic ganglion of the sympathetic sends a gray ramus communi cans to join the nerve on its appearance in the thorax. (b) The second thoracic nerve in a majority of cases communicates with the first. This communication varies considerably in size and distribution. It may reinforce the intercostal branch of the nerve, it may send one branch to the intercostal portion and another to the part of the nerve joining the
brachial plexus, or it may consist of a nerve proceeding solely to join the brachial plexus by a junction in the first intercostal space with the part of the first thoracic nerve, which is engaged in forming the plexus. (c) It is possible that the first white ramus communicans in the thoracic region connects the first thoracic nerve with the sympathetic trunk, but this is not known with certainty.
The anterior ramus of the second thoracic nerve is of large size, though much smaller than the first. It passes forwards in the second intercostal space, lying at first in the costal groove, between the external and internal intercostal muscles. At the level of the mid-axillary line it gives off a large lateral branch; continuing its course, it pierces the internal intercostal muscle and lies upon the pleura; finally, at the lateral border of the sternum, it passes forwards in front of the internal mammary artery and through the internal intercostal muscle, and
the aponeurosis of the external intercostal muscle and pectoralis major, and ends
FIG. 623. THE DISTRIBUTION OF CUTANEOUS NERVES ON THE FRONT OF THE TRUNK. On one side the distribution of the several nerves is represented, the letters indicating their nomenclature. G.A, Great auricular nerve; S.C, N. cutaneus colli; S.CL, Supra-clavicular nerves; ACR, Posterior; CL, Middle; ST, Anterior; T.2-12, Lateral and anterior branches of thoracic nerves; I.H, Ilio-hypogastric nerve; I.I, Ilio-inguinal nerve; CIRC, Cutaneous branch of axillary nerve; L.I.C, Medial cutaneous nerve of the arm (0.T. lesser internal cutaneous nerve); I.H, Intercosto-brachial; I.C, Medial cutaneous nerve of the forearm (O.T. internal cutaneous); M.S, Cutaneous branch of radial nerve; E.C, Lateral cutaneous nerves; G.C, Lumbo-inguinal nerve; M.C1 2, Intermediate cutaneous nerves; I.C1, Branch of medial cutaneous nerve; P, Branches of pudendal nerve; S.Sc, Branches of posterior cutaneous nerve of the thigh. On the other side a schematic representation is given of the areas supplied by the above nerves, the numerals indicating the spinal origin of the branches of distribution to each area.
by supplying the skin of the front of the chest over the second intercostal space.