SPECIAL PHYSIOLOGY. 59 



contract one muscle apart from others, but contraction of 

 muscles usually occurs in groups ; this association is wholly 

 performed in the spinal cord by certain groupings of globules 

 and fibres, and the brain serves only to excite this group of 

 globules ; this association is found in those movements which 

 are purely of a reflex character, as those of defence which 

 are observed in experiments on decapitated animals (physi- 

 ology of the spinal-cord, p. 53). 



Special functions of certain cerebral centres^ or what is 

 called encephalic centres. 



We shall not enter into the detail of the numerous hypo- 

 theses which, even in the experimental investigations of the 

 modern school, have founded the physiology of the encepha- 

 lic organs. The system founded upon the union of spec- 

 ified faculties of the mind to particular circumscribed 

 portions of the brain is regarded as an illusion (Phrenology^ 

 system of Gall), especially when an attempt is made to de- 

 fine these faculties, otherwise arbitrarily classified, according 

 to the development of certain external portions of the skull 

 (Craniology). 



However, very recently it has been believed according to 

 certain pathological observations that the centre of faculty 

 of language or at least the memory for words is seated in the 

 third convolution of the left (Broca) or right (Bouillaud) 

 hemisphere. It is evident that perception and thought form 

 an undefined phenomenon, which depends upon a peculiar 

 activity of the cerebral cells and of an association of these 

 cells connected by numerous commissures ; yet our knowl- 

 edge is too uncertain to localize these functions. 



The tubercula quadrigemina (corpora quadrigemina) are 

 the centre of visual perceptions, and of reflex movements 

 which cause the dilatation or contraction of the iris (Herbert, 

 Mayo, and Flourens) ; yet when the cerebral hemispheres 

 are removed, luminous impressions, though perfectly per- 

 ceived (the animal follows with his eyes and head the move- 

 ments of a lighted taper), are not preserved and cannot give 

 rise to an intellectual effort ; in this aspect of the case the 

 sensation must be imperfect, the animal looks but does not 

 see. The tubercular quadrigemina are to visual sensations 

 what the protuberance generally is to sensations of touch, 

 pain, etc. 



It is probable that these tubercles, moreover, preside over 

 Other functions, not now known, since they appear considera- 

 bly developed in animals completely deprived of the power 



