86 ESSENTIALS OF PHYSIOLOGY. 



THE LOCALISATION OF FUNCTION IN THE 

 CEREBRAL HEMISPHERES. 



It has already been pointed out that histological and embryological 

 researches have indicated that different areas of the cortex subserve 

 different functions, and both observation of the results of disease in 

 man and experimental studies in connection with animals have con- 

 firmed and extended these conclusions. Injuries and tumours of 

 different parts of the human cortex give rise to muscular paralysis, 

 blindness, deafness, aphasia, or mental deficiency, according to the site 

 of the lesion. Experiments carried out on animals, either of the nature 

 of stimulation of various parts of the surface of the cerebral hemi- 

 spheres or of removal of localised portions, have given results parallel 

 with those derived from the study of diseases and injury in human 

 beings ; and as a consequence it has been possible to map out the 

 surface of the hemispheres into areas, each of which possesses a definite 

 function. The pre-central convolution is motor in function, the post- 

 central is sensory and is especially concerned with the reception 'of 

 kinsesthetic impulses, that is, impulses from muscles, tendons, and 

 joints, and with tactile discrimination. The mesial aspect of the 

 occipital lobe, or that part of it which lies on the borders of the 

 calcarine fissure, is visuo-sensory, while the convolutions immediately 

 adjacent to the visuo-sensory area are visuo-psychic. The audito- 

 sensory area and the audito-psychic areas are situated in the superior 

 temporal convolution. The area for taste and smell is in the hippo- 

 campal convolution. No special area has been discovered for the senses 

 of heat, cold, pain, and tactile localisation. The parie to- temporal region, 

 the island of Reil, and the pre-f rental region form the three special 

 association areas (fig. 25). . 



The Motor Area. Stimulation of either the grey matter or of the 

 underlying white matter of the pre-central convolution gives rise to 

 muscular movements on the opposite side of the body. The latent 

 period is longer for stimulation of the grey matter than when the white 

 fibres are excited, but a stronger stimulus is required to elicit movement 

 from the white matter than from the grey. In either case the resulting 

 movements are co-ordinated, groups of muscles being affected, and 

 contraction of a particular group being accompanied by reciprocal 

 relaxation of the corresponding antagonistic group. In other words, 

 " movements, not muscles " are represented in the cortex. Further, 

 stimulation of a particular point in the pre-central convolution is invari- 

 ably followed by the same movement, so that, for example, excitation 

 of one point will result in extension of the thigh, of another in flexion 



