FUNCTIONS OF THE CEREBRAL HEMISPHERES 445 



RIGHT RETINA 



TACTILE AND MOTOR SENSIBILITY 



A lesion limited to the ascending frontal convolution may produce 

 paralysis of definite movements or groups of muscles without any detectable 

 interference with sensation. When however in man a widespread injury, 

 involving both the Rolandic area and the adjacent portions of the brain, 

 occurs as the result of some morbid condition, such as blockage of the middle 

 cerebral artery, the resulting hemiplegia is almost always associated with a 

 greater or lesser degree of hemiancedhesia. We are therefore justified in 

 locating tactile and muscular sens- 

 ibility somewhere in the region of 

 the central convolutions, and it is 

 probable that, while it may in- 

 clude the motor area, its chief 

 representation is to be found in 

 the post-central gyms, i.e. the 

 ascending parietal convolution. 



The sensory aura which pre- 

 cedes an attack of Jacksonian 

 epilepsy points to the motor area 

 itself having some degree of sen- 

 sory functions, and it has been 

 observed that faradisation of the 

 central convolution in man may 

 produce tingling sensations in the 

 part of the body which is the 

 seat of the muscular contractions 

 induced by stimulation. No pain 

 is however felt as a result of the 

 stimulation.- The impulses which 

 subserve cutaneous and muscular 

 sensibility travel up to the brain in 

 the mesial fillet. This tract comes 

 to an end in the ventro-J.ateral 

 portion of the thalamus and the 



subthalamic region. The new relays of fibres, which carry on impulses to 

 the cortex, arise in the thalamus and pass through the hinder limb of the 

 internal capsule to be distributed to the central convolutions. Their area 

 of distribution is however much wider than the area of origin of the pyra- 

 midal fibres. We may therefore conclude that tactile and muscular sensi- 

 bility are chiefly subserved by the central convolutions, including the motor 

 area, but are especially dependent on the integrity of the post-central gyms. 

 Flechsig has shown that fibres from the thalamus, which may probably be 

 regarded as continuations of the fillet system, are also distributed to other 

 portions of the cortex, i.e. the temporal, the frontal, and the occipital lobes. 

 It is therefore not surprising that the hemiansesthesia produced by lesions 

 in the central convolutions is rarely or never complete. 



FIG. 232. Diagram showing the probable 

 relations between the parts of the retina? 

 and the visual area of the cortex. (SciiAFER. ) 



