712 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



arm, paralysis of shoulder movements as the result of cortical lesion 'is least 

 complete, while as we travel toward the extremity of the arm the liability to 

 disturbance of its function as the result of cortical injury increases steadily. 



Turning, now, to the " sensory " areas of the cortex, the principles under- 

 lying their physiological significance and connections appear to be similar. 

 The lower the animal in the vertebrate series the more probable that its reac- 

 tions can be controlled by the afferent impulses which have not passed through 

 the cerebral cortex. 



None of the senses except vision can be analyzed sufficiently to bring out 

 the significance of subdivisions of the cortical area ; hence the illustrations are 

 taken from that sense alone. 



It has already been shown that without cerebral hemispheres a bony fish can 

 distinguish the colors of wafers thrown on the water and discriminate between 

 a bit of string and a worm. In the same case a frog is able to direct its move- 

 ments and to catch flies i. e. to detect objects in motion and react to them 

 normally. A pigeon can direct its movements in some measure, and even select 

 a special object as a perch, but it is not able to respond to the sight of food or 

 its fellows or those objects which might be supposed to excite the bird to 

 flight. In the dog the vision which remains permits only the response of 

 blinking when the eye is stimulated by the flash of a bull's-eye lantern. 

 The progressive diminution in the response which follows visual stimuli 

 in these animals is open to the interpretation that the path by which the 

 impulses may pass over to the cells forming the primary centres interme- 

 diate between the sense-organ and the cortex is progressively diminished. 

 Thus the impulses arriving at the primary optic centres are in a less and 

 less degree reflected toward the cord, as the pathway to the cortex becomes 

 more permeable. When therefore, the cortex has been removed the reac- 

 tions taking place by way of it are disturbed in proportion to their normal 

 importance. 



In the first instance, when the reflexion occurs in the primary centres, the 

 incoming impulses are distributed toward the cord by paths not known, 

 while in the second, they pass from the cortex along the pyramidal tracts. 



In the cortex subdivisions of the visual area have been made by Munk. 1 

 He found that the more anterior portions of the visual area were associated 

 with the superior parts of the retina, and the more posterior portions with the 

 inferior, while the area in one hemisphere corresponded with the nasal portion 

 of the contralateral retina, and to a less degree with the temporal portion of the 

 retina of the same side. The determination of these relations was made by 

 the removal of parts of the visual area (dogs) and the subsequent examination 

 of the field of vision. It appears, therefore, that the incoming impulses from 

 certain parts of the retina are delivered at definite points in the cortex, and 

 that when the paths are interrupted in the dog or higher mammals these 

 impulses are blocked. By stimulation, it will be remembered, Schafer deter- 

 mined similar relations in the monkey. 



1 Ueber die Functionen der Grosshirnrinde, Berlin, 1881. 



