658 THE NEBVOUS SYSTEM IN GENERAL. 



masticating and swallowing it. After this demonstration, it is no longer 

 possible to doubt that, if an animal /eeZ«, it is by the brain, and if it wills, it is 

 also by the brain. 



But sensibility and volition do not constitute the only attributes of the 

 brain tissue ; for it is the seat of other manifestations not less interesting — 

 those of the instincts and intelligence. 



If the encephalon is to be considered as the immediate instrument of all 

 these phenomena, it would be impossible — it is impossible — to attribute the 

 cause, properly speaking, to the activity of its physical matter ; above this 

 hovers a mysterious power that can only be demonstrated by a methodical 

 analysis of the manifestations produced by that activity. But we dare not 

 venture to touch upon the nature of this power ; the first word would be 

 useless without the last, and this would carry us too far. 



To sum up, the nerves possess a single vital property — neurility, which 

 is manifested by excitability and by centripetal conductihility in the nerves 

 whose roots are uppermost, centrifugal conductihility in the nerves whose 

 roots are inferior. 



The spinal cord is mexcitdble in its grey substance, but is excitable on 

 the surface of its superior fasciculi, though scarcely so in the remainder of its 

 white substance. It serves as the organ of transmission between the brain 

 and the nerve-roots ; and is, in addition, endowed with the reflex property or 

 power. 



The brain has for its appanage a special activity, to which is due sensi- 

 bility, volition, and the manifestations of instinct and intelligence. 



It remains to make known the nature of the influence the nervous system 

 exercises on the other apparatus through the properties we know it to 

 possess. But here again we must limit ourselves to principles. 



Since Bichat's time, it has been agreed to divide into two great classes 

 those functions whose operation maintains the life proper of the individual : 

 those of animal life or relation, and those of organic or vegetative life. 



The first, which are exercised with consciousness, comprise the sensorial 

 functions and voluntary movements; the latter are provoked by the spon- 

 taneous excitation originating in the brain, and transmitted to the muscles 

 by the nerve-fibres whose conductihility is centrifugal ; the former have 

 for their object the appreciation, by the brain, of tactile sensations — of heat, 

 light, taste, and smell, by means, or through the instrumentality, of the 

 nerve-fibres possessed of centripetal conductihility, which transmits to the 

 encephalic mass the stimulus developed at their terminal extremity by these 

 diverse physical agents. 



The functions of vegetative life— those which are executed unconsciously, 

 we may say, in animals, and which are not the result of physico-chemical 

 forces— are placed under the influence of the reflex power of the spinal cord. 

 For example, the stomach is empty and its mucous and muscular membranes 

 remain altogether passive; there being no contractions in the first, nor 

 secretion of gastric fluid in the second. Food arrives in its interior, and 

 immediately its activity is developed ; the muscular tunic executes move- 

 ments which cause the mixture of the food, and propel it towards the pyloric 

 orifice; while the free surface of the internal membrane pours out an 

 abundant solvent secretion. This change is due to the stimulus exercised 

 by the presence of the alimentary particles on the extremity of the centri- 

 petal nerve-fibres, and which has been transmitted by them to the medul- 

 lary axis, there reflected on the centrifugal fibres, and carried by these to the 

 tunics of the stomach, whose special functions are thus brought into play. 



