ORIGINS OR ROOTS OF THE NERVES. 



327 



medullated fibres anastomose together, nor are they observed except in rare 

 instances to divide into branches until they approach their termination. But the 

 nerve-trunks themselves continually ramify, and the branches of different nerves 

 not unfrequently join with one another. The branches are of course formed by 

 collections of nerve-fibres, and it follows therefore that when two branches of nerves 

 join, fibres pass from the one nerve-trunk to become associated with the other in 

 their further progress, or the communication may be reciprocal, so that after the 

 junction each nerve-trunk contains fibres derived from two originally distinct 

 sources. In other cases the branches of a nerve, or branches derived from two or 

 from several different nerves, are connected in a more complicated manner, and form 

 what is termed a plexus. In plexuses of which the one named " brachial " or 

 " axillary," formed by the great nerves of the arm, and the " lumbar " and " sacral," 

 formed by those of the lower limb and pelvis, are appropriate examples the nerves 

 or their branches join and divide again and again, interchanging and intermixing 

 their fibres so thoroughly that, by the time a branch leaves the plexus it may con- 

 tain fibres from several or even from all the nerves entering the plexus. Still, as in 

 the more simple communications already spoken of, the fibres, so far as is known, 

 remain individually distinct throughout. 



In some instances of nervous conjunctions certain collections of fibres, after passing from 

 one nerve to another, take a retrograde course in that second nerve, and, in place of being dis- 

 tributed peripherally with its branches, turn back to its root towards the cerebro-spinal centre. 

 Instances of this occur, according to Volkmann, in the connection between the second and 

 third cervical nerves of the cat, in that of the fourth cranial nerve with the first branch of the 

 fifth, and of the cervical nerves with the spinal accessory and the descending branch of the 

 hypoglossal. 



Origins or roots of the nerves. The cerebro-spinal nerves, as already said, 

 are connected by one extremity to the brain or to the spinal cord, and this central 

 extremity of a nerve is, in the language of anatomy, named its origin or root. In 



Fig. 382. ROOTS OF ONE OF THE SPINAL 



NERVES ISSUING FROM THE SPINAL 



CORD. (Allen Thomson.) 



A, from before ; B, from the side ; 

 0, from above ; D, the roots separated ; 

 5, 5, anterior root ; 6, 6, posterior root 

 with ganglion, 6'. The full description 

 of this figure will be found in the chapter 

 on the cerebro-spinal nervous axis. 



some cases the root is single, that 



is, the funiculi or fibres by which 



the nerve arises, are all attached 



at one spot or along one line or 



tract ; in other nerves, on the 



contrary, they form two or more 



separate collections, which arise 



apart from each other and are 



connected with different parts of 



the nervous centre, and such 



nerves are accordingly said to 



have two or more origins or roots. 



In the latter case, moreover, the 



different roots of a nerve may differ not only in their anatomical characters and 



connections, but also in function, as is well exemplified in the spinal nerves, each 



of which arises by two roots, an anterior and a posterior the former containing 



the efferent fibres of the nerve, the latter the afferent. 



