206 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



quality of the central reactions to the cortex, there is no proot 

 that the lower centers are entirely lacking in this property. 

 In Goltz's dog without cerebral cortex, for instance, the animal 

 responded to various sensory stimuli, and when hungry gave 

 evidence, so far as his actions were concerned, of experiencing 

 the sensations of hunger; but whether or not these actions were 

 associated with conscious sensations is hidden from us, and we can 

 hope to arrive at positive conclusions upon this point only by obser- 

 vations upon man himself. 



The Center for Vision.— The location in the cortex of the 

 general area for vision has been estabUshed by anatomical, physio- 

 logical, and clinical evidence. The physiologists have experimented 

 chiefly by the method of ablation. Munk, Ferrier, and later ob- 

 servers have found that removal of both occipital lobes is followed 

 by defects in vision. According to Munk, removal of both occip- 

 ital lobes is followed by complete loss of visual sensations, or, as he 

 expresses it, by cortical blindness. All the experiments upon 

 the higher mammals (monkeys) and clinical experience upon 

 man tend 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 the 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. 



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 



