348 THE NERVOUS SYSTEM. 



its origin all or nearly all the fibres, which it afterward gives off in 

 branches and ramifications; and the inosculation of two nerves is 

 effected by some of the fibres from one passing over to join the other, 

 while some of those belonging to the second may also cross and join 

 the first. In whatever way, therefore, the nerve fibres are associated 

 in the trunks and branches, each may still preserve its specific and inde- 

 pendent action. 



A nerve usually consists of several distinct bundles of fibres, each 

 bundle enveloped in its lamellated sheath ; and when the bundle, after 

 its separation from the trunk, divides into secondary branches, each 

 branch is covered by a thinner lamellated sheath, an offshoot from that 

 of the parent bundle. These sheaths are lined by a layer of flattened 

 polygonal endothelial cells, like those on the inner surface of the blood- 

 vessels. As the branches are reduced in size by repeated subdivision, 

 their sheaths become thinner in the same proportion, by a diminution 

 in the number of lamellae of which they are composed ; and in those 

 containing but few nerve fibres, the sheath consists of a single endo- 

 thelial layer. This transparent envelope, surrounding the smallest rami- 

 fications of the nerves, is known, from the name of its discoverer, as 

 the "sheath of Henle." Each individual nerve fibre, after separating 

 from the rest, to run an independent course, is also accompanied by 

 such a sheath, of about double its own diameter, in which it lies, sur- 

 rounded by lymph or a lymph-like fluid. 



An isolated nerve fibre, passing through the tissues toward its termi- 

 nation, is therefore covered by two envelopes, quite distinct from each 

 other. One is its tubular membrane, or*" sheath of Schwann," which 

 is part of the fibre and closely invests its surface ; the other is the 

 "sheath of Henle," which is an adventitious tube, of larger size, and 

 separated from it by an appreciable space. 



Peripheral Termination of the Nerve Fibres. Near their peripheral 

 termination, the nerve fibres present certain important modifications 

 both in structure and arrangement. 



First, the smaller branches, or bundles of nerve fibres, after penetra- 

 ting the tissues, suddenly divide and subdivide with unusual rapidity ; 

 and these subdivisions, uniting with each other by inosculation, form 

 plexuses, from which are given off individual fibres to supply the ana- 

 tomical elements of the tissues. In the skin there are two such plex- 

 uses, a deeper and a more superficial, of which the latter is the more 

 closely set and composed of smaller bundles, containing only one or 

 two fibres each. As a rule, in all tissues, the second or terminal plexus 

 is the finest, inclosing between its meshes the narrowest interspaces. 

 The nerve fibres, on reaching the terminal plexus, are also reduced in 

 size, being diminished both in the skin and in the muscles from 10 or 

 15 mmm. to 4 or 5 mmm. in diameter. According to Kolliker it is 

 sometimes possible to observe a diminution in size of single nerve 

 fibres in different parts of the muscular tissue. 



Secondly, both in the terminal plexus and in the branches given off 



