SPECIAL PHYSIOLOGY. 35 



III. SPECIAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 



A. PERIPHERAL NERVES. 



The physiology of the nerves which go from the brain to 

 the spinal cord has been a very engrossing and laborious 

 study ; minute dissections, experiments on animals, pathologi- 

 cal observations studied in man, have been alternately used 

 to prove the functions of each nervous filament, and yet, 

 especially for the cranial nerves, science has not yet accom- 

 plished any degree of desirable precision. We can only 

 here briefly indicate the principal results of physiological 

 researches, which, for the cranial nerves, can be understood 

 only by an exact knowledge of the complicated topography 

 of this portion of the nervous system. 



1. Cranial Nerves. The twelve nerves which originate 

 from the encephalic portion of the nerve centres (base of the 

 brain, protuberance, bulb) preside over the general sensibility 

 or the special sensibility, or the movements of those parts to 

 which they are distributed; they may preside over either 

 one of these functions exclusively, or be composed of different 

 fibres (mixed nerves), some of which are sensitive, others 

 motor. 



Olfactory Nerve (1st pair). This nerve appears to preside 

 solely over the special sensibility that produces the sensation 

 of smells ; we say appears, because Cl. Bernard has compiled 

 a number of observations (and specially in the case of Marie 

 Lemens), where the complete absence of the olfactory nerves, 

 determined at the autopsy, was not marked during life by an 

 absence of the sense of smell. Magendie often confounded 

 the special sensibility of the functions of the olfactory nerve 

 with the general sensibility that the trigemini^Iurnish to the 

 olfactory mucous membrane. 



Optic Nerve (2d pair). This is a nerve of special sensi- 

 bility, and carries to the brain the impressions of light received 

 by the retina (vide organs of the special senses) ; also, every 

 excitation (section, compression, etc.) of the optic nerve pro- 

 duces no sensation of pain, but simply an impression of light. 

 The incomplete decussation (chiasma) of the optic nerves 

 seems to explain single vision when both eyes are used. 

 Indeed, this arrangement is such that the left optic tract, for 

 instance, divides at the point of the chiasma, so that both the 

 right and the left optic nerve form the left halves of both 

 retinae (the outer half of the left retina and the inner half of 

 the right retina). 



