128 HUMAN PHYSIOLOGY. 



loss of memory, speech, etc. Congenital defects of organization from im- 

 perfect development are usually accompanied by a corresponding deficiency 

 of intellectual power and the higher instincts. Under these circumstances 

 no great advance in mental development can be possible, and the intelli- 

 gence remains at a low grade. In congenital idiocy not only is the brain 

 of small size, but it is wanting in proper chemical composition ; phosphorus , 

 a characteristic ingredient of the nervous tissue, being largely diminished 

 in amount. 



3. Experimentation upon the lower animals by removing the cerebral 

 hemispheres is attended by results similar to those observed in disease and 

 injury. Removal of the cerebrum in pigeons produces complete abolition 

 of intelligence, and destroys the capability of performing spontaneous 

 movements. The pigeon remains in a condition of profound stupor, which 

 is not accompanied, however, by a loss of sensation, or of the power of pro- 

 ducing reflex or instinctive movements. The pigeon can be temporarily 

 aroused by pinching the feet, loud noises, light placed before the eyes, etc., 

 but soon relapses into a state of quietude, being unable to remember im- 

 pressions and connect them with any train of ideas ; the faculties of 

 memory, reason and judgment being completely abolished. 



CEREBRAL LOCALIZATION OF FUNCTION. 



From experiments made upon animals, and the results of clinical and 

 post-mortem observations upon men, it has been shown that the phe- 

 nomena of organic and psychical life are presided over by anatomically 

 localized centres in the brain. A knowledge of the position of these centres 

 becomes of the highest importance in localizing the seat of lesions, thrombi, 

 hemorrhages, new growths, etc., which show themselves in paralyses, 

 epilepsies, etc. It has not been possible to thus localize all functions, and to 

 many parts of the brain no special use can be assigned. The following are the 

 centres most definitely mapped out and that are of paramount importance : 



Motor Centres. These are in the cortical gray matter, and are ar- 

 ranged along either side of the fissure of Rolando. The upper third of 

 the ascending frontal and parietal convolutions about this fissure preside 

 over the movements of the leg of the opposite side of the body ; the middle 

 third controls the movements of the arm ; the upper part of the inferior 

 third is the facial area. The lowest part of the inferior third governs the 

 motility of the lips and tongue, and this space, with the posterior extremity 

 of the third frontal convolution, constitutes the speech centre. The centres 

 for the ocular muscles lie in the floor of the 4th ventricle. Destruction of 



