FUNCTIONS OF THE BRAIN. 825 



co-ordinated movements in distinct groups of muscles of the opposite 

 side of the bod}^. They indicated certain areas of the cerebral cortex as 

 the actual centres for the production of various movements, and their 

 experiments have been confirmed and extended by a large number of 

 subsequent observers. These points are termed the motor centres of 

 the cortex, and have been located in the dog, the monkey, the cat, sheep, 

 and the rabbit, but are absent in lower animals, such as the frog and fish. 

 They are all found in the anterior part of the parietal lobe, the most 

 important being shown in Fig. 358. When any centre which has been 

 determined to govern any special group of muscles is destroyed or 

 removed the corresponding part of the body is not permanently para- 

 lyzed in the dog, but movements which are produced in that part of the 

 body become irregular and variable, and after a time the disturbance of 

 movement may almost completely disappear. 



In the monkey and man, on the other hand, destructive lesions of 

 definite motor areas of the cortex cause permanent paralysis, this 

 difference being, perhaps, explainable as due to the higher importance of 

 the cortex in higher species, where it assumes more and more the 

 functions subserved by the basal ganglia in lower animals. 



A similar series of experiments has led to the localization in the 

 cortex of certain parts which are in close relationship with the organs 

 of sense, for we know that sensory impulses from the opposite side of 

 the body pass upward through the posterior third of the posterior limb 

 of the internal capsule to pass, in all probability, to the cortex of the 

 occipital and temporo-sphenoidal lobes. Excision of these localities 

 leads to disturbances in the appreciation of sensations coming through 

 the sensory organs. Thus, for example, in the dog a locality has been 

 found in the posterior cerebral lobe (outer convex part of the occipital 

 lobe) the destruction of which produces blindness in the opposite eye ; or, 

 if both corresponding parts are removed, total blindness results. After 

 extirpation of this part the channels which connect it with the optic 

 nerves undergo degeneration. Centres for hearing and for smelling have 

 also been located. The centre for hearing in the dog lies in the second 

 prima^ convolution, while in man and the monkey it has been located in 

 the first temporo-sphenoidal convolution. Such disturbances produced 

 by removal of parts of the cortex are, like the motor disturbances, not 

 permanent, but gradually disappear, and dogs rendered deaf or blind by 

 excision of these parts of the cortex again learn to see and to hear, a fact 

 which is explained Toy the substitution of function in some corresponding 

 part of the .brain-cortex. 



8. THE FUNCTIONS OP THE CEREBELLUM. Experiments as to the 

 function of the cerebellum have led to the conclusion that it is the great 

 organ for the co-ordination of muscular movements, a fact which would 



