GENERAL ANATOMY OF THE NERVOUS SYSTEM. 317 



In addition to the preceding structure, the nerves present a 

 satin-like undulated surface, with small bands that pass some- 

 what spirally and in a zigzag direction. The latter appear- 

 ance is illusory, and depends upon the contraction or sbo/ten- 

 ing of the nerve when not stretched; its seat is in the neurileme, 

 and it accordingly disappears upon extension. 



Some years ago M. M. Prevost and Dumas asserted that they 

 had found the ultimate filaments of nerves* distributed to mus- 

 cles, terminating in loops, either by anastomosis with other fila- 

 ments or in a loop of the individual filament. The observations 

 of Valentine and Emmert corroborate those of the preceding 

 gentlemen. Valentine has found the same arrangement in the 

 iris and in the ciliary ligament, in the cochlea of birds, in the 

 sacs of the teeth, and in the skin of the frog. This arrange- 

 ment does not, however, exist as a universal one in the nerves 

 of sensation as in the retina of the vertebrated animals and of 

 insects.f 



The nerves abound in blood vessels ; when a vascular trunk 

 reaches them, one of its branches ascends and another descends, 

 and, if successfully injected, the neurileme is covered by its ca- 

 pillary ramifications. As in the brain, the lymphatics have not 

 yet been injected. 



There are three modes by which the nervous fasciculi unite 

 with one another; anastomosis, plexus, and ganglion. Anasto- 

 mosis is the junction of the filaments, either of the same nerve 

 or of different nerves, and the examples of it are very abun- 

 dant. Plexus is an anastomosis on a larger scale, and occurs 

 between the larger fasciculi of the same nerve, <.or of different 

 nerves, whereby a very complete intertexture of their fibres oc- 

 curs. 



Kronenburg asserts that the primitive fibres of the- cerebral 

 nerves continue separate up to their ultimate distribution, and 

 that, in their apparent union, they only change from one fasci- 

 culus to another, and that this arrangement prevails not only in 

 the plexus, but in every part of the nervous trunk and its 

 branches. This conclusion he founds upon his- observations of 



* See General. Anatomy of Muscles. t Mailer's Report on Nervous System. 



28* 



