POSTERIOR EAMI OF THE SPINAL NERVES. 687 



The posterior and anterior rami of the nerves contain fibres from both posterior 

 and anterior roots. Indeed, each root can be seen, on removal of its sheath, to 

 divide into two portions, of which one portion enters into the formation of the 

 posterior ramns, the other into the formation of the anterior ramus. The posterior 

 rami, with the exception of the first two, are smaller than the anterior rami. They 

 are responsible for the innervation of the skin and axial muscles of the back. They 

 do not supply the muscles of the limbs, although in their cutaneous; distribution 

 they are prolonged on to the back of the head, the shoulder, and the buttock. 

 They form two small plexuses the posterior cervical and the posterior sacral 

 plexuses. The anterior rami are, with the exception of the first two cervical 

 nerves, much larger than the posterior rami. They supply the sides and anterior 

 parts of the body, the limbs, and the perineum. For the most part they have a 

 complicated arrangement. The thoracic or intercostal nerves alone have a simple 

 mode of distribution; the other nerves give rise to the three great plexuses 

 cervical, brachial, and lumbo-sacral. 



White Rami Communicantes. From the anterior rami of certain nerves 

 (second thoracic to second lumbar inclusive) a series of fine nerves arises, 

 which serves to connect the spinal with the sympathetic system. These visceral 

 or splanchnic branches, or white rami communicantes, through the medium of the 

 gangliated trunk of the sympathetic, serve to innervate the vessels and viscera in 

 the splanchnic area. A second stream of pelvic splanchnic nerves, associated with 

 the second and third, or third and fourth sacral nerves, connects these spinal 

 nerves with the pelvic sympathetic plexuses (p. 766). 



Distribution of the Spinal Nerves. Although the distribution, like the origin 

 of the spinal nerves, presents primarily and essentially a segmental arrangement, this is 

 masked, and in some instances obliterated, by developmental changes in the parts supplied. 

 In no region can an isolated nerve be traced to a complete segment. The nearest 

 approach to a complete girdle of innervation is found in the thoracic region, in such a 

 nerve as the sixth thoracic nerve. Yet even such a nerve is not distributed to any part 

 entirely alone. In its cutaneous distribution it supplies a complete belt of skin, a 

 distinctly segmental area from the median plane posteriorly to the median plane 

 anteriorly; yet at the same time the adjacent nerves overstep, so to speak, the 

 boundaries of the area and assist in the cutaneous nerve supply. Its muscular distribu- 

 tion, also, is segmental ; the anterior ramus supplies the intercostal muscles of the space 

 in which it lies ; but in this it forms communications with adjacent nerves. The posterior 

 ramus supplies axial muscles of the back, not, however, in an obviously segmental 

 manner, on account of the fusion of the segmental myotomes in the formation of complex 

 longitudinal muscles, which are together supplied by the series of muscular branches 

 derived from the posterior rami of contiguous nerves. In other regions still greater 

 changes of structure are accompanied by deviations from a segmental type of distribu- 

 tion, causing the foundation of the nerve-plexuses by which the trunk and limbs are 

 innervated. 



POSTERIOE RAMI OF THE SPINAL NERVES. 



The posterior rami (O.T. posterior primary divisions) of the spinal nerves 

 innervate both skin and muscles ; the skin of the trunk posteriorly, the back of 

 the head, the shoulder and the buttock, and the longitudinal muscles of the back, 

 but not the muscles of the limbs. 



Each posterior ramus divides as a rule anto two parts, a medial and a lateral 

 trunk (Fig. 606, p. 686). In the upper half of the body the medial trunks 

 generally supply .the cutaneous branches, while the lateral trunks are purely 

 muscular nerves. In the lower part of the body the opposite is the case: the 

 lateral trunks provide the cutaneous nerves and the medial trunks are distributed 

 entirely to muscles. The cutaneous branches have a different course in the two 

 cases. In the upper half of the back they course backwards beneath and among 

 the muscles to within a short distance of the spinous processes of the vertebrae, close 

 to which they become superficial. They then extend laterally in the superficial 

 fascia. In the lower half of the back the cutaneous nerves are directed downwards 



