GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 349 



FIG. 87, 



from them, the nerve fibres themselves undergo division; so that a 

 single fibre in this situation may give rise to two or more branches, 

 each branch retaining all the original anatomical characters of the nerve 

 fibre. Such a division of nerve fibres, according to Ranvier,* is occa- 

 sionally visible in the smaller trunks and branches, as in those of the 

 spleen and even sometimes in the muscular nerves; 

 but in general it only occurs in the immediate neigh- 

 borhood of their final distribution. Here, on the other 

 hand, it is very frequent. The division always takes 

 place at an annular constriction. The axis cylinder 

 divides, usually at an acute angle, into two or more 

 secondary axis cylinders, each of which becomes at 

 once enveloped by a medullary layer, like that above 

 the constriction ; and each secondary nerve fibre is at 

 first nearly or quite equal in diameter to that from 

 which it was derived. But after several successive 

 divisions the fibres are diminished in average di- 

 ameter ; and at the same time the annular constric- 

 tions are more frequently repeated. In the small 

 nerve fibres, accordingly, near their peripheral termi- 

 nation, the inter-annular segments are shorter and 

 more numerous than in the large fibres of the ner- 

 vous trunks and branches. 



A nerve fibre may thus pass undivided throughout 

 the roots, trunk, and principal branches and ramifica- ^ 



DIVISION OF A NERVE 

 tions of the nerve, and may then, shortly before its 



termination, break up into a number of separate but 

 closely adjacent secondary fibres. It has been esti- 

 mated iiy Rriclu-rt, that, in the subcutaneous muscles of the frog, one 

 primitive fibre may give rise by its division to about 30 terminal 

 extremities. 



Thirdly, the nerve fibre, near its peripheral extremity, loses its medul- 

 lary layer, and, consequently, its double contour. As the sheath of 

 Schwann also disappears, the nerve fibre finally consists only of the 

 axis cylinder, which near its extreme point of termination sometimes 

 exhibits a fine longitudinal striation, indicating the existence of minute 

 fibrillffi united with each other. 



The termination of nerves in the sensitive integument has been 

 most successfully studied in the " Pacinian bodies " of the skin and 

 mesentery, and in the " end-bulbs "' of the conjunctiva. In these 

 bodies, both in man and animals, there is a general resemblance in 

 the arrangement of the parts, together with variations of detail in dif- 

 ferent species and different situations. They all consist of an ovoidal- 

 shaped mass, surrounded by single or multiple capsules, which are 

 expansions of the lamellatod sheath of the nerve branch supplying them, 

 or of the sheath of Henle accompanying its finest ramifications. They 



* Histologie du Systeme Xerveux. Paris, 1878, tome ii., pp. 109, 268. 



FIBRE; from pulmo- 

 nary membrane of 

 frog's lung. 



