CENTRAL CONTROL OF POSTURAL REACTIONS 



929 



be disturbed as the result of cerebellar injury, this is because the mus- 

 cles do not respond properly in the attempt to prevent falling, and not 

 because the sense of equilibrium is in any way impaired. 



Localization of Function in the Cerebellum. The observations on cere- 

 bellar injuries in man which we have described indicate that the two 

 halves of the cerebellar cortex are each concerned with the regulation of 

 tone and movement in the corresponding half of the body. Beyond that 

 they do not afford any evidence of localization of function in the cere- 

 bellum. By studying the correlation between the functional importance 

 of different muscle groups and the development of the different parts of 

 the cerebellum in different animals Bolk has assigned the control of each 

 muscle group to a definite part of the cerebellar cortex, as is indicated 

 in Figure 231. 



Basing his work on these anatomic conclusions, Van Rijnberk has studied 

 the effect of circumscribed extirpation of certain lobules of the cerebellum 



Fig. 231. Schema of the parts of the mammalian cerebellum spread out in one plane. (After Bolk 

 by Van Rijnberk from Luiciani. Op. cit.) On the right side of the figure the relation . of tne 

 different lobules to the functional development of the musculature is indicated according to the 

 theory of Bolk noted in the text. (From Davidson Black.) 



on the muscular control of the different parts of the body, with the following 

 results. Total or partial extirpation of the lobulus simplex produces side to 

 side oscillations of the head, indicating the removal of the influences of the 

 cerebellum that control the movements of the muscles of the neck. Complete 

 extirpation of the crus primum of the lobuli ansiformes causes as an imme- 

 diate irritative effect dynamic disturbances of the fore limb of the same 

 side, replaced later by a condition of atonia, which makes the limb hang 

 limp, and of asthenia, which makes it feeble in its movement when it 

 is excited to contract. Extirpation of the crus secundum has a similar 

 influence on the muscles of the hind limb of the corresponding side. Extir- 

 pation of both crura of the lobulus ansiformis causes marked asthenia and 

 atonia in both fore and hind limb on the same side as the lesion. A char- 



