102 THE CONVOLUTIONS. 



brain. These excitations severally induce co-ordinated 

 motions (varying in their character according to the pre- 

 cise part excited) of the head and neck, of the extremi- 

 ties, and of the muscles of the face, on the side of the 

 body opposite that on which the cerebral surface is ex- 

 cited. After removal or destruction of any of these parts 

 of the cortex, excitation of the white substance sub- 

 jacent to it produces the same effect as excitation of the 

 part of the cortex with which its fibres were previously 

 in relation, so that the cortical substance is not essential. 

 In all cases, a perceptible interval of time intervenes 

 between the excitation and the muscular action which 

 results from it. 



The effects of ablation can only be studied after the 

 animal has entirely recovered from the immediate patho- 

 logical effects of the injury. From observations so made, 

 we learn (i) that absence of parts of the cortex, by the 

 excitation of which particular groups of muscles are 

 brought into action, is not associated with any impair- 

 ment of the function of the muscles, but only with loss or 

 impairment of the power of the animal to employ them 

 n the performance of certain combined motions ; (2) that 

 destruction of the whole or the greater part of the cortex 

 is attended with impairment of memory and perception. 

 If the lesion is on one side only, this impairment mani- 

 fests itself chiefly in relation to impressions received from 

 the opposite side of the body. According to recent 

 observations of Prof. H. Munk, visual perception is 

 localized in the occipital lobe. Thus (in the dog) ablation 

 of the posterior extremity of this lobe on one side, pro- 

 duces blindness of the middle and inner part of the retina 

 of the opposite side, and of the outer part of the retina 

 of the same side. In the ape, the same lesion produces 

 blindness of the inner half of the opposite retina, and of 

 the outer half of the retina of the same side. It has 

 been long known clinically, that destruction (by disease) 



