THE CONVOLUTIONS. IOI 



the weight of the brain and intelligence is a very close 

 one, provided that the weight of the mesencephalon be 

 taken as the standard of comparison, not that of the 

 whole body. 



The inference that is suggested by the anatomical rela- 

 tions of the corpora striata, viz., that they serve to bring 

 into connection the cortex of the hemispheres, particularly 

 of the " motor region," with the reflex motor centres of 

 the mid-brain, is supported by clinical observations, for 

 the almost constant result of lesions of these organs is 

 loss of voluntary control over the muscles of the opposite 

 side of the body (hemiplegia), a condition which is not, 

 as a rule, permanent. By experiment, we learn that the 

 part of the corpus striatum on which this effect depends, 

 is the nucleus lenticular -is ; for while destruction of this 

 part produces as complete hemiplegia as if the whole 

 hemisphere were removed, interference with the nucleus 

 caudatus is almost without effect. 



There is, at present, no evidence either that the corpora 

 striata contain motor centres, or the contrary. As regards 

 the functions of the optic thalamus, no general statement 

 can at present be made. 



The Convolutions. It was formerly believed that all 

 parts of the cortex of the hemispheres have the same 

 function. It is now known that in certain regions each 

 part has physiological relations which are peculiar to 

 itself. The proof that this is so, lies in the observation 

 of two classes of phenomena, viz., (i) the effects of 

 electrical excitation of particular parts of the surface of 

 the hemispheres, and (2) the results of ablation of certain 

 parts. 



As regards the effects of excitation, the best ascer- 

 tained facts are those which relate to the excitation of 

 the prse-frontal, post-frontal, and super-sylvian convolutions 

 of the dog ; corresponding in position to the convolutions 

 which surround the fissure of Rolando in the human 



