96 FUNCTIONS OF THE NERVOUS SYSTEM. 



found, that an increase in local circulation is accompanied by an exaltation of 

 the sensibility of the part. This may be especially noticed in the genital 

 organs of animals during the period of heat ; and in those of man when in a 

 state of venereal excitement. It may be remarked, also, in those affections so 

 closely bordering upon inflammation, to which the term active congestion, or 

 determination of blood, has been applied. The pain which usually accom- 

 panies inflammation may be partly referred to this source ; but it seems prin- 

 cipally dependent upon other causes. 



118. It may be argued against this view of the respective functions of the 

 granular and fibrous structures, that sensation may be produced by pinching 

 an afferent trunk in its course, and that motion may be excited by irritating 

 an efferent nerve ; so that the changes which have been spoken of as occurring 

 at their points of origin in the vascular plexus, are not to be regarded as the 

 means by which such influences are produced. But this argument will have 

 little weight, when it is recollected that on the same ground, we might infer 

 that neither the organs of sensation on the one hand, nor any part of the brain, 

 or spinal cord, on the other, are the sources of the changes in question. The 

 effects are obviously due to the fact that the artificial stimulus imitates the 

 natural one ; and thus it is that if a sensory nerve be compressed, the sensa- 

 tion produced is referred to the part of the surface, to which its branches are 

 distributed. 



119. Our simplest idea, then, of a nervous system, includes a Central organ, 

 of which the gray matter, formed by the intermixture of nervous fibres, blood- 

 vessels, and ganglionic globules, is the essential part ; and an afferent and 

 efferent set of fibres connected with it, one conveying to it the impressions 

 produced by external changes upon the periphery (where also the nervous 

 structure comes into peculiar relation with the vascular system), and the other 

 conducting from it the motor stimulus, originating in itself, to the contractile 

 tissue. This is precisely what we find in the lowest of those animals in which 

 a nervous apparatus can be distinguished, as will be hereafter explained. At 

 present it will be desirable to consider some other questions, which early pre- 

 sent themselves in the study of Neurology. 



IV. Mode of determining the Functions of Nerves. 



120. Various methods of determining the functions of particular nerves 

 present themselves to the physiological inquirer. One source of evidence is 

 drawn from their anatomical distribution. For example, if a nervous trunk is 

 found to lose itself entirely in the substance of muscles, it may be inferred to be 

 chiefly, if not entirely, motor or efferent. In this manner, Willis long ago deter- 

 mined that the third, fourth, sixth, portio dura of the seventh and ninth cranial 

 nerves, are almost entirely subservient to muscular movement ; and the same 

 had been observed of the fibres proceeding from the small root of the fifth pair, 

 before Sir C. Bell experimentally determined the double function of that 

 division of the nerve, into which alone it enters. Again, where a nerve 

 passes through the muscles, with little' or no ramification among them, and 

 proceeds to a cutaneous or mucous surface, on which its branches are minutely 

 distributed, there is equal reason to believe that it is of a sensory, or rather of 

 an afferent, character. In this manner Willis came to the conclusion, that the 

 ijfth pair of cranial nerves differs from those previously mentioned, in being 

 partly sensory. Further, where a nerve is entirely distributed upon a surface 

 adapted to receive impressions of a special kind, as that of the Schneiderian 

 membrane, the retina, or the membrane lining the internal ear, it may be 

 inferred that it is not capable of transmitting any other kind of impressions ; 

 for experiment has shown that the special sensory nerves, do not possess com- 



