406 GENERAL STRUCTURE AND FUNCTIONS 



lation of two nerves, the communication between them is effected by 

 some of the fibres belonging to the first passing over from it to join the 

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

 join the first ; the individual fibres in each instance remaining distinct, 

 and retaining their identit} r throughout. In whatever way, therefore, 

 the nerve fibres are associated in the various trunks and branches of 

 the nerves, they may still act independently and preserve their specific 

 functions in every part of their course. 



Peripheral Termination of the Nerve Fibres. Near the termination 

 of the nerve fibres in the tissues to which they are distributed, they 

 present certain important modifications both in structure and arrange- 

 ment. 



First, the smaller nervous branches, or bundles of nerve fibres, after 

 penetrating the substance of the tissues, suddenly divide and subdivide 

 with unusual rapidity ; and these subdivisions, uniting with each other 

 by inosculation, form abundant plexuses, from which are given off the 

 individual fibres supplying the anatomical elements of the tissues. In 

 the skin there are two such nervous plexuses, a deeper and a more 

 superficial, of which the latter is the more closely set and composed of 

 more slender bundles, containing only one or two fibres each. As a 

 general rule, also, in other tissues, the last or terminal plexus is the 

 finest, and incloses between its meshes the narrowest interspaces. The 

 nerve fibres, on reaching the situation of the terminal plexus, are also 

 considerably reduced in size, being diminished both in the skin and the 

 muscles from 10 or 15 mmm. to 4 or 5 mmm. in diameter. According 

 to Kolliker it is sometimes possible to observe this diminution in the 

 size of a single nerve fibre in different parts of its course through the 

 muscular tissue. 



Secondly, both in the terminal plexus and the branches given off 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 a nerve fibre. There 

 is usually a marked constriction at the point where the nerve fibre 

 divides ; but this is followed by a corresponding enlargement, so that 

 the secondary fibres soon become nearly or quite equal in diameter to 

 that from which they were derived. A nerve fibre may accordingly pass 

 undivided, so far as we know, throughout its course in the roots, trunks, 

 and principal branches and ramifications 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 estimated by Reichert, 

 that, in the subcutaneous muscle of the frog, ten primitive nerve fibres 

 may give rise by their division, to about 300 terminal extremities. 



Thirdly, the nerve fibre, when near its peripheral termination, be- 

 comes altered in structure. This alteration consists in a disappearance 

 of the medullary layer, by which the fibre loses its double contour; and 

 by a similar disappearance or a separation of the tubular sheath. The 

 nerve fibre, thus altered, is reduced, in its constituent parts, to the axis 



