90 ESSENTIALS OF PHYSIOLOGY. 



area of the cortex than that from which motor impulses arise. The 

 chief tract of thalamo-cortical fibres, however, terminates in the post- 

 central gyrus. 



The available evidence indicates, therefore, that the post-central 

 gyrus is a sensory area specially connected with the tactile and 

 muscular senses, and that the senses of heat, cold, and pain have a 

 wider distribution on the cortex of the cerebral hemisphere, corresponding 

 with their wide distribution on the surface of the body. 



The Visual Area. The visuo-sensory area occupies the greater part 

 of the mesial aspect of the occipital lobe, the visuo-psychic area sur- 

 rounding it and extending on to the lateral aspect of the lobe. 



Extirpation of both occipital lobes results in total blindness, extirpa- 

 tion of one lobe leading to blindness of the homolateral half of each retina. 

 Stimulation of the visuo-sensory area is followed by movements of the 

 eyes. From the direction in which the eyes are turned in response to 

 stimuli applied to various parts of the area, it may be inferred that 

 the retinal impulses are projected on to the cortex according to a 

 definite plan. Thus stimulation of the upper part of the occipital 

 lobe is followed by a downward movement of the eyes, while stimula- 

 tion of a lower point of, say, the right lobe leads to a deviation of 

 both eyes towards the left. In the former case the movement is that 

 which would normally follow the excitation of the upper part of the 

 retina, in the latter it is that which would occur from excitation of the 

 right side of either retina. The fovea centralis, or part of the retina 

 concerned with distinct vision, is represented bilaterally. 



Impulses are transmitted from the retinae to the occipital lobes 

 by the optic nerves, optic tracts, and optic radiations. Each optic 

 nerve, consisting of the axons of nerve cells in the retina, divides at 

 the chiasma, the fibres from the mesial side of the retina crossing to 

 the opposite side to take part in the formation of the optic tract of 

 that side. Thus each optic tract is made up of fibres from its own side 

 of each retina. The tract fibres terminate by arborisation in the 

 pulvinar of the thalamus, the external geniculate body and the 

 superior colliculus of the corpora quadrigemina. The fibres which 

 enter into the optic radiation arise in the thalamus and external 

 geniculate body, and are distributed to the cortex of the occipital 

 lobe (fig. 23). The relay fibres from the superior colliculus are dis- 

 tributed to the oculo-motor nuclei, and appear to be concerned with 

 the function of equilibration and with reflex contraction of the pupil. 



The Auditory Area. The localisation of the auditory area is less 

 definite than that of the visual area, largely owing to the difficulty of 

 ascertaining the degree of deafness produced in animals by experimental 





