270 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of blinking when the eye is stimulated by the flash of a strong light. 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 intermediate between 

 the sense-organ and the cortex is progressively diminished. Thus, as the 

 pathway to the cortex becomes more permeable, the impulses arriving at the 

 primary optic centres arc in a less and less degree reflected toward the cord. 

 \\ hen, therefore, the cortex has been removed, the reactions taking place by 

 way of it are disturbed in proportion to their normal importance. 



In the first instance, when the reflection 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 of the dog 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 por- 

 tions with the inferior, while the area in one hemisphere corresponded with 

 the nasal portion of the retina of the opposite eye, 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 portions of the retina are delivered at definite 

 parts of the cortex, and that when the parts are injured in the dog or higher 

 mammals these impulses are blocked. By stimulation, it will lie remem- 

 bered, Schafer determined similar relations in the monkey. 



Before leaving the cerebral hemispheres, mention of the fact should be 

 made that still other functions, control of the sphincter ani (Fig. 103), secre- 

 tion of saliva, and micturition, can be roused by the stimulation of the cortex 

 in the appropriate region — namely, in the region where the muscles and glands 

 concerned might be expected to have representation if they followed the gen- 

 eral law of arrangement. Changes in the production a .d elimination of heat 

 from the body follow interference with the motor region of the cerebrum, and 

 the removal of portions of the cortex in this region is followed by a rise in 

 the temperature of the muscles affected and an increased blood-supply to them. 



In the encephalon, the cerebrum, and especially its outer surface, is the 

 portion the functions of which have been studied. The significance of the 

 other portions of the encephalon can lie far less well determined. The dis- 

 turbances caused by the section and stimulation of the callosum have been 

 studied by Koi'anyi • and by Schafer. 3 It was found that complete section of 

 the corpus callosum was not followed by any perceptible loss of function. 

 On the other hand, stimulation of the uninjured callosum from above gave 

 symmetrica] bilateral movements, while if the cortex on one side was removed 

 stimulation of the callosum gave unilateral movements on the side controlled 



1 Ueber die Functionen der Orosshirnrinde, Berlin. 1881. 



2 Archiv fur Anatomic und Physiologic, lid. zlvii. 

 Brain, 1890. 



