THE BRATN 727 



The differences in thickness and structure are well seen in 

 Figs. 441-443. In the motor area the second lamina is thick, 

 the third lamina is thin, and the fourth lamina contains the charac- 

 teristic large pyramidal cells of Betz. 



In the " visuo-sensory " area the third lamina is thick, and is 

 divided into two by a strand of fibres the line of Gennari. 



The " visuo -psychic " area, except for the absence of the large 

 Betz cells, resembles more the motor than the sensory area. It is 

 obvious that from a study of the histological structure of areas of 

 which the function is known valuable indications may be obtained as 

 to the function of the parts of the cortex of which the function has not" 

 been clearly ascertained. 



The Evidence from Myelination. It has been found that groups 

 of fibres going to certain regions of the cortex in the embryo acquire 

 their medullary sheaths earlier than other groups. These regions 

 of the cortex have been shown to correspond closely with the " sensory 

 areas " of the cortex. 



The Localization of the Receiving Sensory Mechanism. The 

 receiving station for cutaneous and kinaesthetic impulses has been 

 localized in the ascending parietal convolution, which lies just behind 

 the fissure of Rolando (Fig. 414). Cutaneous sensations have also a 

 receiving station in the callosal gyrus on the mesial aspect of the 

 brain (Fig. 444). It is probable that the various sensations are repre- 

 sented in special parts of these regions. It is known that during their 

 passage from the cord to the thalamus the various sensations become 

 grouped, and it is probable that the final neurons from the thalamus 

 to the cortex establish communications with definite areas. 



The motor and sensory areas around the Rolandic area are in 

 close connection. It is for this reason that epilepsy is preceded by 

 the sensory " aura," and stimulation of the sensory area leads to 

 movements which, with weak stimluation, are localized in that area 

 of the motor cortex which is in most intimate communication. 



The receptive ana for vision is situated in the occipital regions on 

 both the outer and inner aspect of the brain (Fig. 444). The area varies 

 considerably in extent, being larger in the dog than in man. In man, 

 the main centre is around the calcarine fissure on the mesial aspe3t. 

 The occipital part of the brain is connected with the thalamus, external 

 geniculate body, and superior corpora quadrigemina, by means of 

 the optic radiations. For instance, stimulation of the upper surface 

 of the right occipital lobe causes eye movements downwards and to 

 the left; of the posterior part, eye movements upwards and to the 

 left; of the mesial surface, a turning of the eyes laterally to the left. 

 Extirpation of both occipital lobes induces complete blindness; 

 ablation of one occipital lobe causes a crossed hemianopia. The 

 vision of the fovea is not impaired, since it is represented on both 

 sides of the brain. 



In man, the visuo-sensory region is confined to the mesial aspect 

 of the brain. A lesion of the whole of the left cuneate lobe causes 



