446 



PHYSIOLOGY 



LE.FT RETINA 



RIGHT RETINA 



(a) 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 

 hemiancesthesia. We are therefore justified in locating tactile and muscular 



sensibility somewhere in the region 

 of the central convolutions, and 

 it is probable that while it may 

 include the motor area its chief 

 representation is to be found in 

 the post-central gyrus, i.e. the 

 ascending parietal convolution. 



The sensory aura which pre- 

 cedes an attack of Jacksonian 

 epilepsy points to the motor area 

 itseif 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-lateral 

 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 gyrus. 

 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 cetral convolutions is rarely or never complete. 



/PlTAt- 



FIG. 232. Diagram showing the probable 

 relations between the parts of the retinae 

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



