SENSE AREAS AND ASSOCIATION AREAS. 193 



animal in this condition the pupil becomes constricted when light 

 is thrown upon the eye ; but this reaction we may regard as a reflex 

 through the midbrain, and there is no reason to believe that it is 

 accompanied by a visual sensation. When the injury to the occip- 

 ital cortex is unilateral the blindness affects symmetrical halves of 

 the two eyes, a condition known as hemiopia. Destruction of the 

 right occipital lobe causes blindness in the two right halves of the 

 eyes, or in accordance with the law of projection of retinal 

 stimuli in the two left halves of the normal visual field when 

 the eyes are fixed upon any object. Destruction of the left oc- 

 cipital lobe is followed by blindness in the two left halves of the 

 retinas or the right halves of the visual field. This result of 

 physiological experiments is borne out by clinical experience. 

 Any unilateral injury to the occipital lobes is followed by a con- 

 dition of hemiopia more or less complete according to the extent 

 of the lesion. Observation, however, has shown that this general 

 symmetrical relation has one interesting and peculiar exception. 

 The most important part of the retina in vision is the region of the 

 foveae centrales, whose projection into the visual field constitutes 

 the field of direct or central vision. It is said that the hemiopia 

 from unilateral lesions of the cortex does not involve this part of the 

 retina. 



The Histological Evidence. The histological results supple- 

 ment in a very satisfactory way the findings from physiology and 

 pathology. The retina itself, considered from an embryological 

 standpoint, is an outgrowth from the brain vesicles, and is there- 

 fore an outlying portion of the central nervous system. The optic 

 fibers, in terms of the neuron doctrine, must be considered as 

 axons of the nerve cells in the retina. If, therefore, an eye is enu- 

 cleated or an optic nerve is cut the fibers connected with the 

 brain undergo secondary degeneration and their course can be 

 traced microscopically to the brain. By this means it has been 

 shown that in man and the mammalia there is a partial decus- 

 sation of the optic fibers in the chiasma. The fibers from the 

 inner side of each retina cross at this point to the opposite optic 

 tract; those from the outer side of the retina do not decussate, 

 but pass into the optic tract of the same side. The fibers of the 

 optic tract end mainly in the gray matter of the external genicu- 

 late body, but some pass also to the optic thalamus (pulvinar) and 

 some to the anterior colliculus of the corpora quadrigemina. These 

 locations, therefore, particularly the external geniculates, must be 

 considered as the primary optic centers. From these points the path 

 is continued toward the cortex by new neurons whose axons consti- 

 tute a special bundle, the optic radiation, lying in the posterior limb 

 of the internal capsule (see Fig. 77, D). A schema representing 

 13 



