434 BIOLOGY. [BOOK vi. 



each half of the body, first conduct to a cellular nucleus, tJie 

 optic thalamus ; that thence they radiate towards the cortical 

 cells of the circumvolutions ; that other fibres start from these 

 cortical cells, and converge towards another cellular nucleus, 

 the striated body, whence finally irradiates the whole system 

 of peripheric motory fibres, connected, in addition, with the 

 cells of the cerebellum, which seems to be simply a co- 

 ordinating centre of the movements. As the cortical cells of 

 the two cerebral hemispheres are united by transverse fibres, the 

 result is that the hemispheres, the two optic layers, the two ; 

 striated bodies, form a complete system, the different parts of 

 which are anatomically and physiologically connected. If the 

 man is a healthy adult, then the instrument is harmonic, and 

 vibrates accurately : the circumvolutions are swollen, and 

 dilated ; their summits rise equally to the surface of the hemi- 

 spheres ; a cortical, cellular layer, several millimetres in 

 thickness, covers them. On the contrary, in senility, insanity, 

 and mental maladies, certain circumvolutions give way and break 

 down; they are notes which no longer sound. 1 



On the whole, every nervous system, however little developed, 

 in the invertebrated animals as well as the vertebrated, may be 

 traced to a conscious cellular part, in continuous relation with 

 two fibrous systems, the one afferent, through which sensitive 

 excitation is conveyed, the other efferent, by which motory 

 incitation is transmitted. The schema of such a system would 

 be a conscious cell furnished with a single afferent fibre and a 

 single efferent fibre. 



But the mode of operation of a system thus arranged is 

 evidently reflex action, and, in effect, there is not a central 

 nervous act, from the protozoon to man, which cannot be traced 

 to reflex acts ; thus it is infinitely interesting to follow 

 through all its principal phases the gradual transformation and 

 complication of the reflex nervous act. 



1 J. Luys, Le Cerveau (Revue Scientifique, No. 35, 1875). 



