34 



NERVOUS SYSTEM AND GENERAL SENSATION. 



never divide ; and they seem no more to run into one another, 

 or to communicate by anastomoses here, than they do in the 

 peripheral parts of the body. But these fine primary fibres 

 of the central parts enlarge conspicuously and immediately 

 at the entrances of the different nerves into the brain and 

 spinal cord. 



TERMINATION Or THE PRIMARY FIBRES. 



[ 82. A very important question, which naturally presents 

 itself in connexion with the primary fibrils, is this : how do 

 they end? Although generally traced with difficulty, the 

 peripheral terminations of the nervous fibrils are still much 

 more easily demonstrated than those of the centres. United 

 into bundles, and surrounded with cellulo-membranous sheaths 

 (neurilema), the primary fibres penetrate all the organs nearly 



to their peripheral confines, to where 

 they are covered with epithelial or 

 epidermic formations. Here it is that 

 the bundles of primary fibres separate 

 and form plexuses terminal plex- 

 uses, as they have been designated ; 

 at last single primary fibres form 

 loops, or rather two primary fibres 

 meet and form a loop terminal 

 loops. These loops are smaller or 

 larger in different tissues. (Figs. 

 12, 13.) Wherever the primary 

 fibres of nerves have been distinctly 

 traced to their extremities, this mode 

 of termination in loops has been 

 observed, so that it appears to be 

 general, and even to extend to the 

 nerves of special sense, with the sin- 

 gle exception of the olfactory and 

 optic nerves, in the peripheral ex- 

 pansions of which, no loopings have 



been positively ascertained to exist, 



Fig. 13. Terminal primary 1,1 i J > 



fibres from the ciliary liga- Chough no one has yet conde- 



ment of the common duck, scended upon any other mode of 



After Valentin. termination in regard to these two 



