CENTRAL NERVOUS SYSTEM. 255 



face to include part of all the frontal gyri, while on the mesial surface the 

 cortex concerned extends forward from the precuneus over more than half 

 of the mesial surface. In this last area are delivered the afferent impulses 

 from the skin, muscles, joints, viscera, and the lining walls of the alimentary 

 tract. Flechsig points out that the projection-fibres, according to which these 

 areas have been denned, are composed of axones bringing impulses to the 

 cortex, and hence are sensory, in the usual terminology. The areas thus 

 bounded are found to coincide with the areas which (in animals) respond to 

 direct stimulation by the contraction of definite groups of muscles. 



The earlier determinations of the sensory areas in man were made from 

 the study of brains modified by destructive lesions or congenital defects. 



The cortical centre for smell, inferred from comparative anatomy and 

 physiology to be closely connected with the hippocampal and fornicate gyri 

 and the uncus, has been similarly located in man on the basis of pathological 

 observations ; but the evidence lacks precision. Concerning the location of 

 taste sensations, very little is known. Both of these senses, it must be 

 remembered, are insignificant in man, and hence their central connections are 

 not easily studied. 



On the other hand, the cortical areas for hearing and sight have been 

 located with much more precision and certainty. 



Damage to the posterior third of the first temporal gyrus and the asso- 

 ciated gyri transversi causes in man deafness in the opposite ear, and con- 

 cordantly conditions of the ear which early in life lead to deafness and deaf- 

 mutism are accompanied by a lack of development in these gyri. 1 Destruc- 

 tion of this area on one side causes slight deafness mainly in the opposite 

 ear, while complete deafness follows a cortical lesion only when it is double. 



In the case of the visual areas in man there is the same sort of evidence, 

 but somewhat more exact. The destruction of the area represented by the 

 cuneus and the surrounding cortex (Figs. 109 and 110) always injures vision, 

 the maximum disturbance following injury to the cortex of tin; calcarine 

 fissure. Conversely, the failure of the eyes to grow, arrests the development 

 of this portion of the hemisphere. 



Hemianopsia. — Tt is found, moreover, that injury to the visual area in 

 one hemisphere usually produces a hemianopsia or partial defect of vision in 

 both retina'. The homonymous halves are affected on the same side as the 

 lesion and the dividing line is usually vertical. The clinical picture corre- 

 sponds to a semi-decussation of the optic tract and the representation of the 

 homonymous halves of each retina in both hemispheres. At the same time 

 the relation is much more complicated than at firsl sight appears, for the 

 point of most acute vision is often unaffected in such cases. This peculiarity 

 depends apparently on the (act that there is a binocular centre for macular 

 vision in the cortex lining the sides ami bottom of the posterior portion of 

 the calcarine fissure. 2 



1 Donaldson: American Journal <>/" Psychology, 1891- 2. vol. iv. 



2 Laqueur and Schmidt : Virchotfs Archiv, 1899, Bd. 158, Heft 3, S. 467. 



