FUNCTIONS OF THE BRAIN 



917 



We see, then, that homologous organs are not necessarily, nor 

 indeed usually, of the same physiological value in different kinds of 

 animals. A loss which perhaps hardly narrows the range of the 

 psychical, and certainly restricts only to a slight extent the physical 

 powers of a fish, impairs in a marked degree the voluntary move- 

 ments of a dog, in addition to cutting off from it a great part of its 

 intellectual life, and is in man incompatible with life altogether. 



The results of the removal of the entire cerebral hemispheres help 

 us to fix their position as a whole in the physiological hierarchy. A 

 more minute analysis shows us that the cerebral cortex itself is not 

 homogeneous in function, that certain regions of it have been set 

 aside for special labours. 

 Our knowledge of this 

 localization of function in 

 the cerebral cortex has been 

 derived partly from clinical, 

 coupled with pathological 

 observations on man, and 

 partly from the results of 

 the removal or stimulation 

 of definite areas in animals. 

 In addition, the study of 

 the development of the 

 myelin sheath, and especi- 

 ally in recent years the 

 minute study of the hist- 

 ology of the various regions, 

 have aided materially in 

 mapping out the cortex. 



Fig. 367. Motor Areas of Dog's Brain. , neck ; 

 /./., fore-limb; h.l., hind-limb; t, tail;/, fac; 

 c.s., crucial sulcus; e.nt., eye movements; p. 

 dilatation of the pupil in both eyes, but espe- 

 cially in the opposite eye. All the areas are 

 marked in the figure only on the left side 

 except the eye rreas, whose position, to avoid 

 confusion, is indicated on the right hemi- 

 sphere. 



It is a fact which might 

 appear strange and almost 

 inexplicable did the history 

 of science not constantly 

 present us with the like, that 

 fifty years ago the universal 

 opinion among physiologists, 

 pathologists, and physicians 



was that the cerebral cortex is inexcitable to artificial stimuli, that 

 no visible response can be obtained from it. The great names of 

 Flourens and Magendie stood sponsors for this error, and repressed 

 research. In 1870, however, Hitzig and Fritsch showed that not only 

 was it possible to elicit muscular contractions by stimulation of the 

 cortex of the brain in the dog with voltaic currents, but that the 

 excitable area occupied a definite region in the neighbourhood of the 

 crucial sulcus or sulcus centralis, which runs out over the convexity of 

 the hemispheres nearly at right angles to the longitudinal fissure. In 

 this region they were further able to isolate several distinct areas, 

 stimulation of which was followed by movements- respectively of the 

 head, face, neck, hind-leg, and fore-leg (Fig. 367). This was the 

 starting-point of a long series of researches by Ferrier, Munk, Horsley, 



