880 CENTRAL NERVOUS SYSTEM 



auditory, and visual sensations. Very little of importance can be said 

 of the physiology of the olfactory and gustatory centers, which are 

 thought to lie in the hippocampal convolution of the median aspect of 

 the temporal lobe, the former occupying the more distal position. The 

 auditory center lies in the lateral side of the temporal lobe. Complete 

 destruction of both temporal lobes causes deafness, but if one lobe only 

 is destroyed, hearing is not impaired in either ear. It appears from this 

 that the afferent paths from each ear lead to both temporal lobes, so that 

 if the auditory center in one lobe only is injured, the center in the other 

 lobe can carry on auditory perception for both ears. Consequently it is 

 very unlikely that deafness will result from a cerebral injury. 



The Visual Areas. The visual centers are known with much greater 

 precision. In order to explain the disturbances in vision which result 

 from lesions in the visual centers, a word must be said about the 

 formation of images upon the retina and the course of the afferent fibers 

 which pass from the retina to these centers. The optical mechanism of 

 the eye is such that the image of any object at which one looks is in- 

 verted when it falls upon the retina. Consequently the upper part of 

 the visual field falls on the lower part of the retina, the left half of 

 the visual field falls on the right half of .the retina, etc. The optic nerves 

 from the two eyes meet at the optic chiasm, and there about half the 

 fibers in each nerve cross to the opposite side of the brain and follow 

 a course which leads to the visual center, which is contralateral to the 

 eye in which they arose. The other half of the fibers in each nerve do 

 not cross in the chiasm, but continue through the brain by a path which 

 leads to the visual center on the same side of the body as the eye in which 

 they arose. The remarkable thing about this arrangement is that the 

 fibers which cross in the chiasm are those which arise from the median 

 half of both retinas. As a result, the left visual center receives all im- 

 pulses which arise from the left halves of the retinas of both eyes, and 

 since these impulses are set up by objects whose images are inverted 

 upon the retinas, this visual center is affected by the right half of the 

 visual field. Conversely the right visual center is affected by the left 

 half of the visual field. A consideration of Fig. 216 will make this rather 

 complicated situation more clear. This arrangement is associated with 

 binocular vision, that is, the simultaneous use of both eyes in viewing 

 a single object. In the lower vertebrates, in which the eyes are on op- 

 posite sides of the head and consequently have different visual fields 

 the crossing in the chiasm is complete. The arrangement in man is of 

 obvious importance in causing the images formed by the two eyes to 

 effect simultaneously the same sensory centers in the brain. 



Because of this arrangement certain characteristics appear in injuries to 

 the various parts of the optic tracts. A lesion located distal to the chiasm 



