648 SPINAL NERVES. 



Branches. Numerous slender muscular filaments supply the Intercostal and 

 Triangularis Sterni muscles. Some of these branches, at the front of the chest, 

 cross the costal cartilages from one to another intercostal space. 



Lateral cutaneous nerves. These are derived from the intercostal nerves, 

 midway between the vertebrae and sternum; they pierce the External Inter- 

 costal and Serratus Magnus muscles, and divide into two branches, anterior 

 and posterior. 



The anterior branches are reflected forwards to the side and the fore part of 

 the chest, supplying the integument of the. chest and rnamma, and the upper 

 digitations of the External Oblique. 



The posterior branches are reflected backward to supply the integument over 

 the scapula and over the Latissimus Dorsi. 



The first intercostal nerve has no lateral cutaneous branch. The lateral 

 cutaneous branch of the second intercostal nerve is of large size, and named, 

 from its origin and distribution, the intercosto-humeral nerve (Fig. 352). It pierces 

 the External Intercostal muscle, crosses the axilla to the inner side of the arm, 

 and joins with a filament from the nerve of Wrisberg. It then pierces the 

 fascia, and supplies the skin of the upper half of the inner and back part of 

 the arm, communicating with the internal cutaneous branch of the musculo- 

 spiral nerve. The size of this nerve is in inverse proportion to the size of the 

 other cutaneous nerves, especially the nerve of Wrisberg. A second intercosto- 

 humeral nerve is frequently given off from the third intercostal. It supplies 

 filaments to the armpit and inner side of the arm. 



The Lower Intercostal Nerves (excepting the last) have the same arrangement 

 as the upper ones as far as the anterior extremities of the intercostal spaces, 

 where they pass behind the costal cartilages, and between the Internal Oblique 

 and Transversalis muscles, to the sheath of the Eectus, which they perforate. 

 They supply the Eectus muscle, and terminate in branches which become sub- 

 cutaneous near the linea alba. These branches, which are named the anterior 

 cutaneous nerves of the abdomen, supply the integument of the front of the 

 belly ; they are directed outwards as far as the lateral cutaneous nerves. The 

 lower intercostal nerves supply the Intercostal and abdominal muscles, and, 

 about the middle of their course, give off lateral cutaneous branches, which 

 pierce the External Intercostal and External Oblique muscles, and are dis- 

 tributed to the integument of the abdomen, the anterior branches passing 

 nearly as far forwards as the margin of the Eectus ; the posterior branches 

 passing to supply the skin over the Latissimus Dorsi, where they join the 

 dorsal cutaneous nerves. 



PECULIAR DORSAL NERVES. 

 / 



First dorsal nerve. Its roots of origin are similar to those of a cervical nerve. 

 Its posterior or dorsal branch resembles, in its mode of distribution, the dorsal 

 branches of the cervical nerves. Its anterior branch enters almost wholly into 

 the formation of the brachial plexus, giving off, before it leaves the thorax, a 

 small intercostal branch, which runs along the first intercostal space, and ter- 

 minates on the front of the chest, by forming the first anterior cutaneous 

 nerve of the thorax. The first intercostal nerve gives off no lateral cutaneous 

 branch. 



The last dorsal is larger than the other dorsal nerves. Its anterior branch 

 runs along the lower border of the last rib in front of the Quadratus Lumbo- 

 rum, perforates the aponeurosis of the Transversalis, and passes forwards be- 

 tween it and the Internal Oblique, to be distributed in the same manner as the 

 preceding nerves. It communicates with the ilio-hypogastric branch of the 

 lumbar plexus, and is occasionally connected with the first lumbar nerve by a 

 slender branch, the dorsi-lumbar nerve, which descends in the substance of the 

 Quadratus Lumborum. 



