234 SPECIAL PHYSIOLOGY. 



kind of electricity, nor even physically induced by it, as magnetism 

 may be ; but that, in the special action of a living nerve, a force is 

 generated, peculiar to that tissue, which is so correlated with electric- 

 ity, that an equivalent of the one may, in some yet unknown manner, 

 excite, give rise to, or even be converted into, the other. In this 

 concatenation of the several forces of nature, physical and vital, the 

 force acting in a nerve may also be correlated with chemical force, 

 with the heat developed in the muscle, and even with the peculiar 

 molecular motions which produce muscular contraction, and all its 

 accompanying physical or mechanical consequences. Indeed, as it is 

 more acceptable to the human mind to suppose that the quantity of 

 force, like the quantity of matter in the existing order of nature, re- 

 mains the same, and is never lost or annihilated, some such notion of 

 the interchange of inorganic into organic, and of organic into inor- 

 ganic force, must be entertained. On this view, the nerve force is, as 

 it were, nourished from physical force, as the living substance of the 

 nervous tissues is fed from the inorganic materials of the dead world. 

 The nerve force here spoken of, is, however, merely that which is set 

 free in the exercise of the properties of excitability and conductility, 

 in both sensory and motor nerves. But, as already stated, we must 

 leave unsolved the mystery of sensation arid consciousness, as endow- 

 ments of living matter, even when these are manifested in their simplest 

 forms, in animals; and, in regard to the higher mental faculties, we can 

 only recognize the co-operation of the same nerve force, as one neces- 

 sary physiological condition of all such psychical phenomena in us. 



The particular functions performed by the nervous system, are, as 

 already stated, first, those of sensation common and special. Secondly, 

 the regulation of all the movements of the animal body, whether these 

 be reflex, emotional, ideational, or volitional ; or whether they appertain 

 to the animal, or to the vegetative functions, such as the movements 

 of respiration, those of the alimentary canal, and the motions of the 

 heart and bloodvessels. Thirdly, the nervous system is that part of 

 the frame, through the agency of which, all mental manifestations 

 occur. Lastly, the nervous system influences the nutritive functions 

 of the body, either solely by its effects on the minuter bloodvessels, 

 or, perhaps also, by some special control over, or interference with, 

 the chemical processes of nutrition and secretion. 



In the lowest animals, as we shall see, the nervous system disappears ; 

 and hence all their functions, whether animal or vegetative, are per- 

 formed, so far as we can at present discern, independently of nervous 

 substance. 



In man and the higher animals, not only are both the animal and 

 vegetative functions placed, more or less, under the control of the ner- 

 vous system, but the anatomical connections between the different 

 parts of this system are exceedingly numerous and intricate, and the 

 physiological relation between the most distant parts proportionally 

 intimate. Moreover, the paths and centres concerned in the functions 

 of sensation, reflex and other motions, and psychical acts, are so struc- 

 turally associated, that all parts of the body are more or less in sym- 



