660 PHYSIOLOGY OF THE CEREBRUM 



i. e., the visual impression gets its proper interpretation through the idea 



already gained by touch. But if the patient has seen in this way an object 

 a single time, he is able to recognize it immediately with the eye the next 

 time. The connections of the optical center with the other parts of the 

 cerebral cortex were therefore already present., and it was only necessary for 

 the patient to compare the visual impression with the tactile impression a 

 single time in order to fix the memory picture of the object permanently. 

 It can scarcely be* maintained therefore that the optical memory pictures are, 

 as has so often been assumed, so to speak imprinted on the optical cortex, 

 for in cases like these just described there would not be time enough to form 

 such an imprint. 



The exhaustive analysis which Exner has made of the visual disorders ob- 

 served by Hitzig following lesions of the motor cortex in the dog (cf. page 655) 

 shows that different cortical fields participate in the perception of things visually. 

 When the cortex within the motor region is destroyed, many fibers which connect 

 the occipital lobe with this region are severed. Hence the perception aroused by 

 appropriate stimulation of the optic nerve will be wanting in those components 

 which relate to motility. Consequently the elaboration of the visual impression 

 after the operation becomes deficient and hemiamblyopia sets in, notwithstand- 

 ing that the optical cortex is intact. Recovery from the visual disorder is pos- 

 sible because the hemisphere of the opposite side takes up the functions of the 

 injured side, connections being gradually established which lead ultimately to 

 complete restitution of function. Extirpation within the occipital lobe of the 

 injured hemisphere now is without effect because this hemisphere has no fur- 

 ther part to play in the elaboration of visual impressions. 



This interpretation receives substantial support from the fact that section 

 of the corpus callosum which in an injured animal is without demonstrable 

 effect (see page 641) in a dog which has already lost the motor zone and has 

 recovered from the resulting hemiamblyopia, immediately produces this disor- 

 der again and leaves no possibility of recovery. Likewise the hemiamblyopia 

 continues permanently if one removes a piece of the cortex and at the same 

 time sections the corpus callosum. 



What has here been said of the visual area is of course true for the cortical 

 areas of the other higher senses, and for those of general cutaneous sensation, 

 touch, etc.; the excitation immediately aroused is conveyed by means of new 

 pathways to other parts of the brain, and by the cooperation of several different 

 cortical fields the conscious process associated with those particular senses is 

 aroused. 



From all that we know of the central organs of the nervous system, it 

 appears very probable that the motor cortical fields do not of themselves 

 originate the impulses which they send to the muscles of the body, but that 

 they must first be acted upon by other cortical regions. We naturally think 

 first of the closely associated sensory cortical region as a source of such 

 excitation, and it cannot be denied on purely a priori ground that in very 

 many simple movements aroused by the cortex, the efferent impulse is dis- 

 charged by an afferent impulse very much after the manner of a reflex. Here 

 belongs the touch reflex, for example, described by H. Munk, which consists 

 of a feeble flexion of the toes and the foot, when the hairs on the back of the 

 foot are lightly stroked the wfong way, and which is permanently lost after 



