474 PHYSIOLOGY CHAP. 



head and neck must, according to Bolk, be influenced by the two 

 separate median segments of the cerebellum. 



In the upper and lower limbs the case is different. We know 

 that each limb is able in man to execute a great variety of more 

 or less complex movements, independently of the limb on the 

 opposite side. But this independence is not always complete and 

 absolute. Learners of the piano and violin have by practice to 

 overcome great difficulties in order to render the muscles of the 

 two sides independent, and to avoid the simultaneous contraction 

 of the homologous muscles of the upper limbs. Bolk infers from 

 this that in order to regulate the movements of the limbs there 

 must be three distinct centres in the cerebellum : one unpaired 

 for synergic bilateral movements ; two paired for the dissociated 

 movements of each limb. 



Finally the trunk muscles specially employed in the respiratory 

 movements, and in equilibration during the erect posture and in 

 locomotion, must, according to Bolk, be represented in the cere- 

 bellum by one unpaired median, and two lateral centres. 



Which cerebellar lobes represent these hypothetical centres 

 that can be distinguished, according to Bolk, in the cerebellar 

 cortex ? He begins by pointing out that the lobes and lobules of 

 the cerebellum, as above indicated, are really arranged one above 

 the other, like the corresponding muscular areas of the body. On 

 the basis of this correspondence we may assume that : 



(a) The lobulus anterior contains the centres for the muscles 

 of the eye, jaw, face, tongue, pharynx, larynx, that is, all the 

 muscles of the head region ; 



(b) The lobulus simplex contains the centre for the muscles 

 of the neck ; 



(c) The upper part of the lobulus medianus posterior represents 

 the median centre for the associated movements of the two 

 extremities ; 



(d) Each of the lobuli ansiformes or paramediani contains the 

 lateral centres for the dissociated movements of the two limbs, 

 the crus primum being more exactly the centre for the fore or 

 upper limbs, the crus secundum and lobulus paramedianus that for 

 the hind or lower limbs ; 



(e) The lower part of the lobulus medianus posterior includes 

 the centres for the respiratory and perineal musculature ; the 

 formatio vermicularis the centres for the trunk muscles ; and the 

 lobulus petrosus the centre for the muscles of the tail. 



This arrangement is represented in Bolk's diagram of the 

 mammalian cerebellum, which is reproduced in Fig. 2-S9. 



According to Bolk this hypothetical functional localisation in 

 the cerebellum is confirmed by correlation of the development of . 

 the lobes and lobules, respectively, in different mammals with the 

 functional development of the corresponding groups of muscles. 



