106 ANTHROPOLOGY. 



may be divided into secondary bundles composed of the ultimate or primi- 

 tive fibres. The composition of these has been already referred to. They 

 present the appearance of a series of transparent tubes placed in simple 

 juxtaposition without any intercommunication. 



The main trunk of a nerve breaks up into its component bundles, as 

 it passes from centre to periphery, yielding up branches to the various parts 

 it is destined to connect with the nervous centre. These branches gene- 

 rally come off at acute angles, and soon plunge into the muscles and other 

 parts to which they tend, dividing and subdividing as they proceed. An 

 exception to this mode of branching is where a branch separates from the 

 parent trunk at an acute angle, and then turns to run in an opposite direc- 

 tion, forming an arch, from the convexity of which several branches are 

 given off; such a nerve is said to be recurrent 



In their branchings, nerves subdivide, not only to pass immediately to 

 their muscles or other distant parts, but also to connect themselves by cer- 

 tain of these filaments with other nerves, and to follow the course of the 

 latter, instead of adhering completely to that of the parent trunk. By these 

 means nervous filaments, connected with very different parts of the brain 

 and spinal cord, become bound together in the same fasciculus, and a nerve 

 is formed, compounded of tubes possessing very opposite functions. The 

 anastomosis of nerves formed in this manner difi'ers essentially from the anas- 

 tomosis of blood-vessels, in there being not the slightest communication of 

 the contents of the nervous tubes, as there is in the vascular. 



The 2>?ea;w.se.s are nervous anastomoses of the most complicated and 

 extensive kind. Those which are connected with the spinal nerves are 

 found in the neck, the axilhi?, the loins, and the sacral regions. There are 

 also plexuses connected with the fifth nerve, the portio dura of the seventh, 

 the glosso-pharyngeal, and the par vagum. Each plexus is formed by the 

 breaking up of a certain number of nervous trunks, the subdivisions of 

 which unite to form secondary nerves, and these again, by further inter- 

 change of fibres, give rise to nerves which emerge from the plexus, and 

 consequently in their construction may derive their fibres from several of 

 the trunks that enter the plexus. 



The object of the anastomosis of nerves appears to be to associate 

 together nervous fibres connected with different parts of the brain or spinal 

 cord. In this manner, nerve-tubes of different properties or endowments 

 become united together in one sheath, forming compound nerves ; and cer- 

 tain sets of muscles, instead of receiving their nerves from a very limited 

 portion of the cerebro-spinal centre, are supplied from a considerable extent 

 ofthat centre, and each muscle may probably receive nerves which arise in 

 different and distant parts of the spinal cord or brain ; an arrangement 

 whereby remote parts of those centres may be brought into connexion with 

 neighboring muscles or other parts, or even with a single muscle. 



The nerves serve to conduct impressions from the external world to the 



nervous centres, or to transmit volitions from these centres to the structure 



at large, and especially to the muscular system. The former are called 



afferent or sensory nerves^ the latter efferent or motor^ and the two are 



812 



