218 



INNERVATION. 



[CHAP. vni. 



exceptions to this rule, however, are occasionally met with, in 

 which the branch forms a right or an obtuse angle with the trunk. 

 Before a branch separates, the parent nerve seems wider for some 

 distance above the point of visible separation. This is owing to a 

 divergence of the fibres within the trunk before they actually leave 

 it, and not to any increase in the number of the nervous elements. 

 A good example may be seen in the auricular nerve of the neck, as 

 it winds upwards over the sterno-mastoid muscle. 



Anastomosis. In their branchings nerves subdivide, not only to 

 pass immediately to their distribution in muscles or other parts, but 

 also to form a connexion, by some of their filaments, with other 

 nerves, and to follow the course of the latter, whether to the peri- 

 phery, or back again to the centre, instead of passing to the desti- 

 nation of the primary trunk. By these means nervous filaments 

 connected with very different parts of the brain and spinal cord 

 become bound together in the same neurilemma, and a nerve is 

 formed compounded of nerve-tubes possessing different functions. 

 The anastomosis of nerves thus formed differs from the more cor- 

 rectly-named anastomosis of blood-vessels ; for in the latter case the 

 canals of the anastomosing vessels communicate, and their contents 

 are mingled; but in the former the nerve- tubes simply lie in juxta- 

 position, without any coalescence of their walls, or any admixture 

 of the material contained within them. 



The simplest kind of anastomosis is that which occurs in almost 

 every spinal nerve. The anterior and the posterior roots of these 

 nerves, emerging from different parts of the spinal 

 cord, and possessing, as is now proved, very dif- 

 ferent endowments, are united after passing through 

 the dura mater, and are bound together as one 

 nerve ; the respective tubules being so completely 

 intermixed that the ramifications, which pass off 

 in the subsequent course of the nerve, for the most 

 part contain tubules from both roots, and therefore 

 possess the functions of both. 



And even in a nervous trunk, thus formed, there 

 is an interchange of place between the component 

 filaments, so that those which were at first on the 

 surface of the nerve pass into its centre, and are 

 replaced by others which had been deep-seated ; a 

 decussation of the fibres occurring as they change 

 places (fig. 59). Hence it is often difficult to fol- 

 a low a bundle for any distance in a nervous trunk. 



Fig. 59. 



