498 NERVOUS SYSTEM. 



to the active organs of locomotion, the muscles. The sympathetic or ganglionic 

 nerves are distributed chiefly to the viscera and bloodvessels, and are termed the 

 nerves of organic life. 



The Cerebro-spinal nerves consist of numerous nerve fibres, collected together and 

 inclosed in a membranous sheath. A small bundle of primitive fibres, inclosed 

 in a tubular sheath, is called a funiculus : if the nerve is of small size, it may 

 consist only of a single funiculus, but, if large, the funiculi are collected together 

 into larger bundles or fasciculi; and are bound together in a common membranous 

 investment, termed the sheath. In structure, the common sheath investing the 

 whole nerve, as well as the septa given off from it, which separate the fasciculi, 

 consist of areolar tissue, composed of white and yellow elastic fibres, the latter 

 existing in greatest abundance. The tubular sheath of the funiculi, the neurilemma, 

 consists of a fine, smooth, transparent membrane, which may be easily separated, 

 in the form of a tube, from the fibres it incloses; in structure, it is, for the 

 most part, a simple and homogeneous transparent film, occasionally composed of 

 numerous minute reticular fibres. 



The cerebro-spinal nerves consist almost exclusively of the tubular nerve fibres, 

 the gelatinous fibres existing in very small proportion. 



The bloodvessels supplying a nerve terminate in a minute capillary plexus, 

 the vessels composing which run, for the most part, parallel with the funiculi ; 

 they are connected together by short transverse vessels, forming narrow oblong 

 meshes, similar to the capillary system of muscle. 



The nerve fibres, as far as is at present known, do not coalesce, but pursue an 

 uninterrupted course from the centre to the periphery. In dissecting a nerve, 

 however, into its component funiculi, it may be seen that they do not pursue a 

 perfectly insulated course, but occasionally join at a very acute angle with other 

 funiculi proceeding in the same direction ; from these, again, branches are given 

 off, which join again in like manner with other funiculi. It must be remembered, 

 however, that in these communications the nerve fibres do not coalesce, but merely 

 pass into the sheath of the adjacent nerve, become intermixed with the nerve 

 fibres, and again pass on to become blended with the nerve fibres in some adjoining 

 fasciculus. 



Nerves, in their course, subdivide into branches, and these frequently commu- 

 nicate with branches of a neighboring nerve. In the subdivision of a nerve, the 

 filaments of which it is composed are continued from the trunk into the branches, 

 and at their junction with the branches of neighboring nerves the filaments pass 

 to become intermixed with those of the other nerve in their further progress ; in 

 no instance, however, do the separate nerve fibres inosculate. 



The communications which take place between two or more nerves form what 

 is called a plexus. Sometimes a plexus is formed by the primary branches of the 

 trunks of the nerves, as the cervical, brachial, lumbar, and sacral plexuses, and 

 occasionally by the terminal fasciculi, as in the plexuses formed at the periphery 

 of the body. In the formation of a plexus, the component nerves divide, then 

 join, and again subdivide in such a complex manner that the individual fasciculi 

 become interlaced most intricately ; so that each branch leaving a plexus may 

 contain filaments from each of the primary nervous trunks which form it. In the 

 formation also of the smaller plexuses at the periphery of the body, there is a free 

 interchange of the fasciculi and primitive fibrils. In each case, however, the 

 individual filaments remain separate and distinct, and do not inosculate with each 

 other. 



It is probable, that, through this interchange of fibres, the different branches 

 passing off from a plexus have a more extensive connection with the spinal cord 

 than if each of them had proceeded to be distributed without such connection with 

 other nerves. Consequently, the parts supplied by these nerves have more ex- 

 tended relations with the nervous centres ; by this means, also, groups of muscles 

 may be associated for combined action. 



The termination of nerve fibres signifies their mode of distribution and con- 



