DEVELOPMENT OF KERVES. clxiii 



former chapter, is towards the recognition of some more direct opera- 

 tion. 



The properties above mentioned, of the nerves and nervous centres, have 

 been commonly ascribe;! to a peculiar force developed in the nervous system, 

 which has received the names of "nervous force," "nervous principle," 

 "nervous influence," and "vis nervcsa" (in the largest sense of that term) ; 

 and whilst some physiologists consider that force as a species of agency alto- 

 gether peculiar to living bodies, others have striven to identify it with some 

 of the forces known to be in operation in inanimate nature, or to show its 

 fundamental relationship to them. 



The greater number of nerves possess both afferent and efferent fibres, and 

 are named compound or mote-sensory, inasmuch as they minister both to 

 sensation and motion. In such compound nerves the two kinds of fibres 

 are mixed together and bound up in the same sheaths ; but in the most 

 numerous and best-known examples of this class, the afferent and efferent 

 fibres, though mixed in the trunk and branches of the nerves, are separated 

 at their roots. This is the case in the spinal nerves : these have two roots, 

 an anterior and posterior, both for the most part consisting of many fuui- 

 culi, and the posterior passing through a ganglion with which the fibres of 

 the anterior root have no connection. Now it has been ascertained by ap- 

 propriate experiments on animals, that the anterior root is efferent and 

 contains the motor fibres, and that the posterior is afferent and contains the 

 sensory fibres. The fifth pair of cranial nerves has a sensory root furnished 

 with a ganglion, and a motor root, like the spinal nerves. The glosso- 

 pharyngeal and pneumo-gastric nerves are also decidedly compound in 

 nature ; they are also provided with ganglia at their roots, which involve a 

 greater or less number of their fasciculi ; but it has not yet been satisfac- 

 torily determined whether in these nerves the fibres which have different 

 properties are collected at the roots into separate bundles, nor how they are 

 respectively related to the ganglia. The sympathetic, as already stated, 

 contains both afferent and efferent fibres. 



Simple nerves are such as contain either afferent or efferent fibres only. 

 The olfactory, auditory, and optic are simple afferent and sensory nerves. 

 The third, fourth, and sixth, the facial, the spinal accessory and hypoglossal 

 nerves are generally regarded as examples of simple motor nerves ; there is 

 reason to believe, at least, that they are simple and motor in their origin, 

 or as far as their proper fibres are concerned, and that the sensibility evinced 

 by some of them in their branches is owing to sensory fibres derived from 

 other nerves which join them in their progress. 



The nerves governing the motions of the blood-vessels are commonly 

 spoken of as the " vaso-motorial nerves ; " but although this term is often 

 of convenient application, there seems no sufficient reason for reckoning 

 these nerves as a distinct system, any more than motorial nerves distributed 

 to other parts or organs whose motions are independent of the will. 



DEVELOPMENT OF NERVES. 



The knowledge as yet acquired respecting this process is not very positive or 

 consistent, so that much room is left for speculation and conjecture. The nerve-cells 

 are generally said to be derived from the common embryo-cells, which, undergoing 

 modification in their substance, send out branches from their circumference and 

 acquire the character of nerve-cells. As they are sometimes found with double or 

 divided nuclei, it is inferred that they increase in number by division, after the 

 manner of cells generally. According to the most generally current descriptions, 

 the fibres are stated to be formed by the linear coalescence of fusiform cells, and to 



