746 THE NERVOUS SYSTEM IN GENERAL. 



To sum up, the nerves possess a siugle physiological ]^vo]^Tij—neunlif!/. 

 This' is manifested by excif ability and by centripetal conductibilitt/ in the nerves, 

 the roots of which are uppermost, centrifugal conductihility in the nerves with 

 inferior roots. 



The spinal cord is inpxcitabh in its grey snl)stan(X', but is excitable on the 

 surface of its superior fasciculi, and with difficulty 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 reflex 'power. 



The brain is endowed with a special activity, to which is due sensibiUtg, 

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



It remains to examine the nature of the influence the nervous system exer- 

 cises on the other apparatuses, 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 which maintain 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 induced by the impulsion 

 originating in the brain, and transmitted to the muscles by the nerve-fibres with 

 centrifugal conductihility ; 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 con- 

 ductibihty, which transmits to the brain the stimulus developed at their 

 terminations 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 physio-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 movements which produce mixture of 

 the food, and propel it towards the pyloric orifice ; while from the surface of the 

 mucous membrane is poured an abundant solvent secretion. This change is due 

 to the stimulus exercised by the alimentary particles on the extremity of the 

 centripetal nerve-fibres, and which has been transmitted by them to the medullary 

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

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



It is worthy of remark that the properties of the nervous system, which act 

 in so important a manner on the organs of vegetative life, have no direct influence 

 on nutrition itself. Destruction of the nerves in a certain region will certainly 

 derange the nutrition of its tissues, in consequence of the paralysis of the vessels, 

 but it is not destroyed. There is a very large category of organized beings — 

 vegetables, for instance — in which nutrition is very active, and in which there 

 is no nervous system. So that the property which determines the essential 

 phenomena of nutrition is an attribute of living matter. 



