MORPHOLOGY OF NERVES. 



381 



T 



MORPHOLOGY OF THE PERIPHEKAL NERVES. 



SPINAL NERVES. Segmentation. The spinal nerves in their arrangement as they 

 leave the vertebral column exhibit in a marked degree the character of segmentation. This is 

 not the case however at their attachment to the spinal cord, where the filaments of both the 

 anterior and posterior roots form each a nearly continuous series along the whole length of 

 the cord. In their connection with the grey matter of the spinal cord there is again evidence 

 of a segmental arrangement, which appears not to correspond exactly to the segmentation 

 seen in the nerve-trunks, for it seems probable that the fibres composing one posterior root, 

 for example, may be connected with more than one segment of the spinal cord (cf. p. 350). 



Mode of division. A typical segmental nerve, on leaving the vertebral canal, divides at 

 once into two parts, which are known as the anterior or central, and the posterior or dorsal 



Fig. 238. PLAN OF THE MODE OF BRANCHING OF A 



SEGMENTAL NERVE. 



primary f/icixio/i*. In addition to these, the trunk also 



gives off a small recurrent or meningeal branch to the 



interior of the spinal canal, and the anterior division 



furnishes a branch to the sympathetic. The primary 



branching of the nerve may thus be looked upon as a 



separation into four parts, of which the anterior, posterior, 



and recurrent divisions are said to be somatic, while the 



offset to the sympathetic is usually called visceral or 



splanchnic (rainu* vtieeralis). The latter term, although often convenient, is, however not 



strictly accurate, since the sympathetic division contains, in addition to visceral fibres, other 



fibres which influence the unstriped muscles of the vessels and skin of the trunk and limbs, 



parts which are of somatic origin. 



Posterior primary divisions. These supply the dorso-lateral muscle (see Vol. II, p. 347) 

 and the overlying integument. As a rule, each is divided into internal and external branches 



Fig. 239. PLAN OF AN UPPER DORSAL NERVE, 



SHOWING THE TYPICAL MANNER OF BRANCH- 

 ING OF THE ANTERIOR AND POSTERIOR 

 PRIMARY DIVISIONS OF A SEGMENTAL NERVE. 



(GKD. T.) 



corresponding to the cleavage of the dorso- 

 lateral muscle, and from one only of these 

 an offset passes to the skin. The first cer- 

 vical nerve has no external branch or 

 cutaneous offset ; while the last two sacral 

 and the coccygeal nerves give off no muscu- 

 lar branches, as the dorso-lateral muscle 

 terminates at the fourth sacral vertebra, 

 and therefore they do not divide into the 

 usual two branches. As has been previously 

 pointed out (p. 341), cutaneous branches are 

 not usually furnished by those nerves, the 

 anterior divisions of which occupy a central 

 position in the limb-plexuses (lower cervical 

 and lower lumbar nerves). 



Anterior primary divisions. In the 

 primitive condition the anterior primary 

 division of a segmental nerve supplies the 

 ventro-lateral muscle of the segment and a 

 corresponding zone of skin. The cutaneous 

 or perforating branches are two in number, 



a lateral, which again divides into anterior and posterior branches, and an anterior. This 

 arrangement is however retained only in the nerves from the second dorsal to the first lumbar 

 inclusive. Above and below the region thus indicated the primitive condition is much 

 modified by the union of the nerves in plexuses, in great part for the supply of the limbs. 

 But the cutaneous distribution of each of these nerves, as has already been shown (p. 345), 

 is to a definite continuous area of skin, altered in shape and position in consequence of 

 growth-changes, and in the case of the central limb-nerves shifted so much that all connec- 

 tion with the trunk is lost. 



VOL. III., PT. 2. B 



