604 



THE NERVOUS SYSTEM 



especially the post-central convolution, is intimately connected with the 

 perception of general body sensations. Physiological and pathological ob- 

 servations supported this view, and recently Flechsig has much strengthened 

 the view by his method of studying the progressive development of the brain. 

 In figure 405 we produce Flechsig' s diagram showing the body sensory 

 area, aptly designated by Barker the somesthetic area. The borders of the 

 area are more or less indefinite and less distinct than the main portion. 

 This is indicated in the figure by the lighter shading. Lesions of this area 

 in the cortex lead to loss of sensibility in definite regions of the opposite 

 side of the body. 



Visual or Optic Center. The termination of the optic nerve in each 

 eye, the retina, to the structure of which we shall return when treating of the 

 eye, is so arranged that when we look at an object with both eyes, symmetrical 

 parts of the retinae are used. For example, if we examine an object to the 



AESTHETIC 



FIG. 405. Diagrams to Show Flechsig's Sensory and Association Areas on the Surface of the 

 Cerebral Hemisphere. (From Cunningham, after Flechsig.) 



left of the center of vision, an image of that object is focused upon the right 

 half of both retinae, viz., upon the temporal side of the right retina, and upon 

 the nasal side of the left retina. The optic nerve fibers of these symmetrical 

 parts of the retinae are gathered together behind where the optic nerves de- 

 cussate, viz., in the optic chiasma. The fibers which come from the right 

 side of both eyes are contained in the optic tract of the same side, viz., the 

 right, those from the right eye being outside of the others. In the same way 

 the left optic tract contains internally fibers from the left side of the right eye 

 and externally those from the left side of the left eye. The optic tract thus 

 formed then passes backward and terminates in three distinct nuclei, viz., 

 the pulvinar of the thalamus, the anterior corpus quadrigeminum, and the 

 lateral corpus geniculatum. These nuclei atrophy if the eyes are removed 

 from an adult animal; and if the eyes are removed from a newly-born 

 animal, they do not fully develop. Through the superior corpora quadri- 

 gemina the optic tract establishes synapses that bring it into relation with the 



