318 MANUAL OF HISTOLOGY. 



Internally this perineurium extends between the bundles of nerve-fibres, 

 which, as in muscle, may be divided into primary and secondary, and in 

 which the tubes are already arranged in the series in which they even- 

 tually leave the trunk. In some cases the connective-tissue preserves its 

 fibrous character, especially when it binds together larger collections of 

 nervous tubes than usual, while around the primary fasciculi it appears 

 rather as a homogeneous nucleated substance. Again, having become 

 modified to a homogeneous material, it enters into the formation of the 

 primitive sheaths around the medullary portion of the nerve-tubes con- 

 tained in the trunk. Finally, the latter is traversed by a scanty network 

 of capillaries, consisting of fine tubes measuring about 0'0056 mm. 



From the fact that the primitive fibres run along side by side unchanged 

 in the nerve, without giving any indications as to their functions by their 

 appearance, all branchings, anastomoses, and formations of plexuses, may 

 be regarded by the physiologist with tolerable indifference. 



As is well known, there occurs a progressive splitting up of the nervous 

 trunks at very acute angles in their course towards the periphery. Thus, 

 the primitive tubes leave the stem or common path in bundles, and, bend- 

 ing off sideways, pursue their way separately towards the organs they 

 supply. The actions of the various nerves is, however, by no means 

 determined by this arrangement ; but in one made up of sensitive and 

 motor fibres, this formation of branches may bring about a separation of 

 the latter. 



Anastomoses, for the interchange of different kinds of fibres for anato- 

 mical ends, are connections between neighbouring nerves or branches of 

 the latter. We may distinguish between simple and double anastomoses 

 also. In the first, a number of nerve-tubes pass through a connecting 

 branch into another trunk, pursuing in this their further course ; in the 

 second, both nerves exchange fibres with one another. 



This interchange of fibres between adjacent nerves, when it takes place 

 to any great extent, gives rise to the so-called plexus. 



These branchings, anastomoses, and formations of plexuses, are con- 

 tinued down to nerves of microscopic minuteness, and even in the 

 organs within which the latter are to terminate. In them especially the 

 formation of plexuses is a very general occurrence just before the final 

 radiation of the fibrillae, and has been alike described in earlier and more 

 recent times. In the larger and more massive plexuses an interchange of 

 single primitive fibres alone is to be observed ; while in the finer, or, as 

 they have been called, the terminal plexuses, repeated divisions of the 

 nervous tubes arid retiform intercommunications between their branches 

 have been met with. 



In its whole course, from its central origin until towards its peripheral 

 distribution, the nerve fibre does not change its character in the least, and 

 its diameter but to a slight degree. 



With the progressive division of the nervous trunk, however, certain 

 modifications of the connective-tissue sheathing make their appearance, 

 This latter, namely, decreases in amount from the stem towards the 

 branches, and appears no longer fibrillated on the finer twigs, but only 

 streaky, until, finally, in the terminal filaments, we find nothing but a 

 homogeneous nucleated mass. This simplest form of perineurium may be 

 seen on little twigs which only contain a few primitive fibres. Even 

 single nervous tubes may course through a tissue for a considerable dis- 

 tance in a clothing of this kind, until they terminate finally with loss of 



