440 BIOLOGY. , [BOOK vi. 



modern physiology, to have reduced to such a simple formula the 

 apparently entangled skein of the psychical phenomena ; but the 

 labour of analysis has been carried still farther, and after 

 having demonstrated that every conscious movement is the result 

 of a reflex nervous action, the fact has been established, that 

 there is in the brain a localisation of the reflex properties. 



The division of labour is already made manifest in the optic 

 layer, where, in man, we can isolate four clusters of nervous 

 cells, four receptive centres. 1 The first of these nuclei, the 

 anterior, is the olfactory centre, highly developed in the animal 

 species, which possess large olfactory nerves. Behind is the 

 optical nucleus, the most voluminous of all in man, and, on the 

 contrary, very little developed in the species which live habitually 

 in darkness, as the mole. The third nucleus, or median centre, 

 from front to back, receives the non-special tactile fibres. 

 Finally, the fourth is the receptive centre of the acoustic fibres, 

 the acoustic centre. The special office of these nervous centres is 

 established : 1st, by comparative anatomy, which shows each of 

 them to be more voluminous when the corresponding sense is 

 more developed in such or such an animal species ; 2nd, by patho- 

 logical anatomy, which proves the coincidence of their isolated 

 atrophy with the disappearance of the senses whose sensitive 

 agitations they receive ; 3rd, by experimentation, since M. E. 

 Fournier has succeeded, by making irritant injections in the web 

 of the optic layer, in destroying at will this or that category of 

 sensitive impressions. 



In the cortical substance of the hemispheres, perceptive 

 departments of sensation and impression answer to these optical 

 receptive centres. On the brain of individuals who had suffered 

 amputation long before, Dr. Luys has proved local atrophies of 

 circumvolutions. 2 In these later times, Dr. Ferrier 3 has observed 



1 Arnold, Jcanes Cerebri et Medullce Spinalis, Turici, 1858. Luys, Revue 

 Scientifique, No. 37, 1875. 2 Luys, ibid. 



3 Terrier (Progres Medical, 1873, No. 28, and 1874, No. 1) Reclierches Ex- 

 perimentales sur la Physiologic ct la Pathologic Cerebrates. 



