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forearm it is called the dorsal interosseous nerve, and is placed in the proximal part of its course beneath the superficial extensor muscles, and upon the supinator and abductor pollicis longus, along with the dorsal interosseous artery. In the distal half of the forearm it passes under cover of the extensor pollicis longus, and lies upon the interosseous membrane. At the wrist it passes deep to the extensor tendons, on to the back of the carpus, where it terminates in a gangliform enlargement of small size, from which branches pass to the inter-carpal articulations. The nerve supplies the following branches:
(1) Terminal articular branches to the carpal joints.
(2) Muscular branches, in its course through the forearm. Thus, on the lateral side of the radius, it supplies the extensor carpi radialis brevis and the supinator muscles before it enters the fibres of the last-named muscle. After emerging from the supinator it supplies a large bundle of nerves which enter the extensor digitorum communis, extensor digiti quinti proprius, and extensor carpi ulnaris, near their origins. At a more distal level the nerve gives off branches to the abductor pollicis longus, extensor pollicis longus and extensor pollicis brevis, and extensor indicis proprius.
There are two subscapular nerves (Figs. 614 and 616).
The first or short subscapular nerve is generally double, and there may be three trunks present. It arises from the posterior cord of the plexus behind the axillary nerve, and comes from the fifth and sixth cervical nerves. It passes distally behind the axillary artery and enters the subscapularis muscle.
The second or lower subscapular nerve also arises behind the axillary nerve from the posterior cord of the plexus (from the fifth and sixth cervical nerves). Its origin is distal and lateral to that of the first nerve. It courses distally behind the axillary artery and the axillary and radial nerves, to the teres major muscle. It supplies branches to the lateral part of the subscapularis muscle and ends in the teres major.
The thoraco-dorsal nerve (O.T. long subscapular) arises from the back of the posterior cord of the plexus, behind the radial nerve, and from the sixth, seventh, and eighth cervical nerves, or from the seventh and eighth nerves only. It is directed distally and laterally between the two preceding nerves, behind the axillary artery and over the posterior wall of the axilla, in company with the subscapular artery, to the latissimus dorsi muscle, which it supplies on its anterior (deep) surface.
The anterior rami of the thoracic nerves are twelve in number, each nerve emerging below the corresponding vertebra and rib. Eleven of the series are intercostal, the twelfth lying below the last rib. The first, second, third, and twelfth nerves present peculiarities in their course and distribution. The other thoracic nerves, as already stated, are simple, and may be regarded as types both in course and distribution.
The anterior ramus of the first thoracic nerve is the largest of the series. It emerges from the vertebral canal below the neck of the first rib, and divides in the first intercostal space into two very unequal, superior and inferior, parts. The superior and larger part ascends obliquely over the neck of the first rib, lying lateral to the arteria intercostalis suprema, and enters the neck behind the subclavian artery and the pleura. It proceeds laterally upon the scalenus medius muscle and enters into the formation of the brachial plexus, as already described.
The inferior, intercostal part of the nerve is much smaller in size. It courses
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, i.e. 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 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 (O.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.C12, 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.
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 o 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 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