SENSE AREAS AND ASSOCIATION AREAS. 195 



the dog at least removal of the entire cerebral cortex leaves the 

 animal with some degree of vision, since he will close his eyes if a 

 strong light is thrown upon them. All the experiments upon the 

 higher mammals (monkeys) and clinical experience upon man 

 tends, however, to support the view of Munk. Complete removal 

 of the occipital lobes is followed by apparently total blindness. 

 If any degree of vision remains it is not sufficient for recogni- 

 tion of familiar objects or for directing the movements. In an 

 animal in this condition tjje pupil is 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 occipital 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 ex- 

 periments is borne out by clinical experience. Any unilateral 

 injury to the occipital lobes is followed by a condition of hemiopia 

 more or less complete according to the extent of the lesion. Obser- 

 vation, 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 fovea centralis, 

 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 Histoloqical 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 tne gray matter of the external genicu- 



