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lumbar nerve in the psoas muscle. It may communicate also with the ilio-hypogastric nerve, as they lie together in the abdominal wall.

Inter-communications of the Thoracic Nerves.-It has been noted already that the belts or areas of skin supplied by the branches of the thoracic nerves are also innervated by adjacent nerves on either side which invade the area supplied by a given nerve. Communications also take place between the branches of the nerves supplying the intercostal muscles, whereby the muscles of a given space derive their innervation from more than one intercostal nerve.

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The lumbo-sacral plexus is formed by the union of the anterior rami of the remaining spinal nerves—five lumbar, five sacral, and one coccygeal. Frequently

a fine communicating branch of the twelfth thoracic nerve joins the first lumbar nerve near its origin.

Of the nerves in question the first sacral is generally the largest in size, the nerves diminishing gradually above and rapidly below this nerve. The plexus, for the most part, forms the nerves destined for the supply of the lower limb. In addition, however, nerves arise at its superior limit which are distributed to the trunk above the level of the limb, and at the inferior end of the plexus nerves arise for the supply of the perineum.

Partly for convenience of description, and partly on account of the differences in position and course of some of the nerves emanating from it, the plexus is subdivided into three subordinate parts --- lumbar, sacral, and pudendal plexuses. There is, however, no strict line of demarcation between the three parts.

Plexus Lumbalis.—The lumbar plexus is formed by the first four lumbar nerves, and is often joined by a branch from the twelfth thoracic nerve as well. It is limited below by the fourth lumbar nerve (n. furcalis), which enters also into the composition of the sacral plexus. The nerves of the lumbar plexus are formed in the loin, and supply that region as well as part of the lower limb. They are separated from the nerves of the sacral portion of the plexus by the articulation of the hip bone with the sacrum.

Plexus Sacralis. — The sacral plexus is formed by the fourth and fifth lumbar, and the first two or three sacral nerves. It is generally limited below by the third sacral nerve (n. bigeminus), which assists also in forming the pudendal plexus. The nerves of the sacral plexus are placed on the posterior wall of the pelvis, and are destined almost entirely for the lower limb.

Plexus Pudendus.— The pudendal plexus is formed by the second, third, fourth, and fifth sacral nerves, and the minute coccygeal nerve. It is placed on the posterior wall of the pelvis and supplies branches mainly to the perineum.

Communications with the Sympathetic.-Each of these nerves has communications with the gangliated trunk of the sympathetic in the abdomen and pelvis.

Gray Rami Communicantes.-From the lumbar and sacral ganglia long slender gray rami communicantes are directed backwards and laterally over the bodies of the vertebræ, and in the lumbar region) beneath the origins of the psoas muscle, to reach the spinal nerves. These branches are irregular in their arrangement. A given nerve may receive branches from two ganglia, or one ganglion may send branches to two nerves. The rami are longer in the loin than in the pelvis, owing to the projection of the lumbar portion of the vertebral column.

White Rami Communicantes.—Certain lumbar and sacral nerves are also connected with the abdominal and pelvic sympathetic by means of white rami communicantes. From the first two, and possibly in some cases also the third and fourth lumbar nerves, white rami communicantes are directed forwards, either independently or incorporated with the corresponding gray rami, to join the upper part of the lumbar sympathetic trunk. The fifth lumbar nerve and the first sacral nerves are unprovided with white rami communicantes. From the anterior rami of the second and third, or third and fourth sacral nerves, white rami (visceral or splanchnic branches) pass medially, and, crossing over (without joining) the sympathetic trunk, enter the pelvic plexus of the sympathetic. The fifth sacral and coccygeal nerves possess no white rami communicantes.

PLEXUS LUMBALIS.

The lumbar plexus is formed by the anterior rami of the first three and a part of the fourth lumbar nerves, with the addition, in some cases, of a small branch from the twelfth thoracic nerve. The nerves increase in size from above downwards (Fig. 624).

Position and Constitution. The plexus is formed in the substance of the psoas muscle, in front of the transverse processes of the lumbar vertebræ.

The nerves, on emerging from the intervertebral foramina, are connected as above described with the sympathetic system, and then divide in the following manner in the substance of the psoas major muscle. The first and second nerves divide into superior and inferior branches. The superior branch of the first nerve (which may be joined by the branch from the twelfth thoracic nerve) forms two nerves, the ilio-hypogastric and ilio-inguinal. The inferior branch of the first joins the superior branch of the second nerve, to produce the genito-femoral nerve (O.T. genito-crural). The inferior branch of the second nerve, the whole of the third, and that part of the fourth nerve engaged in the constitution of the plexus divide each into two unequal parts -smaller anterior and larger posterior parts. The smaller anterior portions combine together to form the obturator nerve, which is thus formed by the second, third, and fourth lumbar nerves. The root from the second nerve is not always present. The larger posterior portions of the same nerves combine together to form the femoral nerve (0.T. anterior crural). From the posterior aspect of the posterior parts of the second and third nerves the lateral cutaneous nerve of the thigh (O.T. external cutaneous) arises. The nerves also provide, near their origins, irregular muscular branches for the psoas and quadratus lumborum muscles. The following is a list of the nerves which spring from the lumbar plexus (Figs. 624 and 625):

(1) Muscular branches to the quadratus

lumborum and psoas muscles.
(2) Ilio-hypogastric.
(3) Ilio-inguinal.

(4) Genito-femoral.
(5) Lateral cutaneous.
(6) Obturator.
(7) Femoral.

Muscular Branches.--The nerves to the quadratus lumborum muscle arise independently from the first three or four lumbar nerves (and sometimes also from the twelfth thoracic nerve). The nerves to the psoas muscles arise from the second and third lumbar nerves, with additions, in some cases, from the first or fourth. They are often associated in their origin with the nerve to the iliacus from the femoral nerve. The psoas minor, when present, is innervated by the first or second lumbar nerve.

The ilio-hypogastric and ilio-inguinal nerves closely resemble, in their course and distribution, the lower thoracic nerves, with which they are in series.

N. Iliohypogastricus.—The ilio-hypogastric nerve is the highest branch of the first lumbar nerve. It receives fibres also from the twelfth thoracic, when that nerve communicates with the first lumbar nerve. After traversing the psoas muscle obliquely, it appears at its lateral border, on the surface of the quadratus lumborum and behind the kidney. It courses through the loin, lying between the transversus and obliquus abdominis internus muscles, above the crest of the ilium. About an inch in front of the anterior superior spine it pierces the obliquus internus, and continues its course in the groin beneath the aponeurosis of the obliquus externus. It finally becomes cutaneoụs in the anterior abdominal wall, by piercing the aponeurosis of the obliquus externus about an inch and a half above the subcutaneous inguinal ring (Fig. 623, p. 715).

Its branches are—(1) muscular to the muscles of the abdominal wall; and (2) cutaneous branches, two in number. The lateral cutaneous branch corresponds with the lateral branch of an intercostal nerve, and, after piercing the obliquus internus and obliquus externus, becomes cutaneous just above the iliac crest, below and behind the iliac branch of the last thoracic nerve. It is small, and may be absent. It is distributed to the skin over the superior part of the lateral side of the buttock, in continuity with the cutaneous branch of the posterior ramus of the first lumbar nerve. The anterior cutaneous branch is the anterior terminal branch of the nerve. It supplies the skin of the anterior abdominal wall below the level of the last thoracic nerve and above the os pubis.

N. Ilioinguinalis.- The ilio-inguinal nerve is the second branch given off from the first lumbar nerve. It also may receive fibres from the last thoracic

Not infrequently the ilio-hypogastric and ilio-inguinal nerves are represented for a longer or shorter part of their course by a single trunk. When separate the nerve takes a course similar to that of the ilio-hypogastric nerve, but at a lower level, as far as the anterior abdominal wall. It then pierces the obliquus internus farther forward and lower down than the ilio-hypogastric; and coursing forwards beneath the aponeurosis of the obliquus externus, just

nerve.

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

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FIG. 625.-THE MUSCLES AND NERVES ON THE POSTERIOR ABDOMINAL Wall.

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 (0.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).

NERVUS OBTURATORIUS.

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 :

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