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The nerve supplies the following branches :
1. Muscular branches to the muscles of the second intercostal space.
2. Cutaneous branches. (a) Anterior terminal branches to the skin over the second intercostal space (Fig. 623). (b) A large lateral cutaneous branch, the intercostobrachial (O.T. intercosto-humeral) nerve (Fig. 614, p. 701). This nerve pierces the intercostal and serratus anterior muscles, and, crossing the axilla, extends to the arm. It pierces the deep fascia just beyond the posterior fold of the axilla, and can be traced as far as the interval between the medial epicondyle of the humerus and the olecranon. It supplies an area of skin stretching across the axilla and along the posterior surface of the arm on the medial side as far as the elbow (Fig. 617, p. 706). It may supply the axillary arches, when present.
The intercosto-brachial nerve varies in size. It may pierce the first intercostal space, and it | is often divisible into anterior and posterior branches, like the lateral branch of an ordinary intercostal nerve.
Communications.-(1) The intercosto-brachial nerve communicates with two adjacent nerves. Either before or after piercing the fascia of the axilla it is joined by the medial cutaneous nerve of the arm. It also communicates with the posterior part of the lateral branch of the third intercostal nerve by means of the branches distributed to the floor and boundaries of the axilla. (2) Besides the branches referred to, the second thoracic nerve in many cases transmits a nerve to the brachial plexus, which becomes incorporated with the first thoracic nerve after passing over the neck of the second rib. This branch is i inconstant. As already mentioned, it may join only the intercostal part of the first thoracic nerve, it may join the brachial plexus only, or it may send branches to both parts of the first thoracic nerve. (3) Besides the communications effected by branches of the second thoracic nerve in its course, it also receives a gray ramus communicans from the second thoracic ganglion of the sympathetic trunk in the thorax. It also sends to the sympathetic a white ramus communicans, probably the first, though this is not known with certainty.
The anterior ramus of the third thoracic nerve differs from a typical thoracic nerve only in one respect. Its lateral branch divides in the usual way into anterior and posterior parts, of which the latter is carried to the arm and supplies an area of skin on the medial side near the root of the limb. It effects a junction with the intercosto-brachial nerve (Fig. 614, p. 701).
The anterior rami of the fourth, fifth, and sixth thoracic nerves have a course and distribution which is simple and typical. Except for the peculiarities above mentioned, the second and third thoracic nerves have a similar distribution.
The nerves lie on the posterior wall of the thorax, in the costal groove of the corresponding rib. They extend forwards between the intercostal muscles as far as the middle of the chest wall, lying at a lower level than the intercostal vessels. At the side of the chest each nerve passes obliquely through the internal intercostal muscle, and comes to lie upon the pleura, transversus thoracis muscle, and internal mammary artery. Thereafter, piercing the fibres of the internal intercostal muscle, the aponeurosis of the external muscle, and the pectoralis major, each nerve ends by supplying the skin of the front of the chest, over an area corresponding to the medial or anterior part of the intercostal space to which it belongs.
Branches. Each intercostal nerve supplies, in addition to the anterior terminal cutaneous branches, muscular branches to the intercostal muscles and a lateral cutaneous ramus, which, piercing the intercostal and serratus anterior muscles, divides into anterior and posterior branches for the innervation of the skin over the side of the chest. Each area of skin thus innervated is continuous anteriorly with the area innervated by the anterior rami of the same nerves, and posteriorly with the areas supplied by their posterior rami.
The upper six intercostal nerves supply the muscles of the first six intercostal spaces and the transversus thoracis (3, 4, 5, 6). The second, third, fourth, fifth, and sixth nerves supply the skin of the front of the chest: the second, opposite the sternal synchondrosis; the sixth, opposite the base of the xiphoid process. Their lateral branches supply branches to the intercostal muscles and the skin of
the side of the chest, the second (intercosto-brachial) and the third, in part, being drawn out to the arm. The fourth supplies the nipple (Fig. 623).
Communications. Each of these intercostal nerves communicates with the sympathetic trunk and ganglia by two branches—a white ramus communicans to the corresponding sympathetic ganglion or the adjacent part of the sympathetic trunk; and a gray ramus communicans, which passes to each nerve from the corresponding ganglion.
The anterior rami of the seventh, eighth, ninth, tenth, and eleventh thoracic nerves differ from the preceding nerves only in regard to a part of their course and distribution. Each has the same course and communications as the preceding nerves in the thoracic wall. In addition, these nerves have a further course and distribution in the abdominal wall. Each nerve traverses an intercostal space in the way described. At the anterior end of the space, the nerve pierces the attachment of the diaphragm and the transversus abdominis muscle to the costal cartilages, and courses forwards in the abdominal wall between the transversus and obliquus internus muscles. The nerve then passes between the rectus abdominis muscle and the posterior layer of its sheath, and eventually reaches the anterior abdominal wall and becomes cutaneous by piercing the rectus abdominis itself and the anterior layer of its sheath.
Muscular Branches. The lower intercostal nerves supply the intercostal muscles of the spaces in which they lie; and in the abdominal wall they innervate the transversus, obliquus externus and internus, and rectus abdominis. The branches arise from the main trunk as well as from its lateral and anterior branches. (The ninth, tenth, and eleventh nerves are described as assisting in the innervation of the diaphragm by communications with the phrenic nerve.)
Cutaneous Branches.-These are lateral and anterior. The lateral branches divide into anterior and posterior parts, and, becoming superficial along the line of inter-digitation of the obliquus externus muscle with the serratus anterior and latissimus dorsi, they are directed more obliquely downwards than the lateral branches of the higher intercostal nerves, and are distributed to the skin of the loin as low down as the buttock. The lateral branch of the eleventh nerve can be traced over the iliac crest (Fig. 625).
The anterior branches are small. That of the seventh nerve innervates the skin at the level of the xiphoid process. The eighth and ninth appear between the xiphoid process and the umbilicus; the tenth nerve supplies the region of the umbilicus; and the eleventh, the area immediately below the umbilicus.
The cutaneous branches of these nerves, including those of the posterior rami, thus supply continuous belts of skin, which can be mapped out on the body from the vertebral column behind to the median plane in front. These areas are not placed horizontally, but tend to be drawn more downwards anteriorly as the series is followed from the upper to the lower nerves.
The anterior ramus of the twelfth thoracic nerve is peculiar in its course and distribution. It emerges below the last rib (Fig. 625), and passes laterally and downwards in the posterior abdominal wall under cover of the psoas muscle, and between the lateral lumbo-costal arch and the quadratus lumborum muscle; it pierces the transversus abdominis muscle, and courses forwards in the interval between it and the obliquus internus as far as the sheath of the rectus muscle. After piercing the posterior layer of the sheath, the rectus muscle, and the anterior layer of the sheath, it terminates by supplying the skin of the anterior abdominal wall midway between the umbilicus and the os pubis. The branches of the nerve are muscular to the transversus, obliqui, rectus, and pyramidalis muscles of the abdominal wall; and cutaneous branches, two in number-an anterior terminal branch, which supplies the skin of the anterior abdominal wall midway between the umbilicus and the pubis, and a large lateral cutaneous branch, which, passing obliquely downwards through the lateral muscles of the abdominal wall, becomes superficial above the iliac crest, a couple of inches behind the anterior superior spine. It supplies the skin of the buttock as far down as a point below and anterior to the greater trochanter of the femur (Fig. 623, p. 715).
The twelfth thoracic nerve, in many cases, receives a communicating branch from the eleventh, near its origin, and still more frequently sends a fine branch to join the origin of the first
lumbar nerve in the psoas muscle. It may communicate also with the ilio-hypogastric nerve, a 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.
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. 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.
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 (O.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.
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 cutaneous 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