THE MESENCEPHALON AND CEREBELLUM. 639 



special sense impulses is not so definite or well marked since the 

 animal flies or runs against even the most obvious obstacles. 



We may conclude, then, that while the medulla controls the 

 coordinated movements absolutely necessary for the vegetative 

 functions, the mid-brain (including the cerebellum of birds and 

 mammals) controls the still more complex associations of coordi- 

 nated movements necessary for the perfect performance of such 

 acts as balancing our bodies, and enables us at the same time to 

 carry on elaborate coordinated motions with the upper extremi- 

 ties, or vocal and respiratory muscles. 



The enormous number of muscles simultaneously used in some 

 of our commonest daily actions, concerning which we have but 

 little thought, and take no voluntary trouble, shows the great 

 importance of this part of the brain. If we take a simple ex- 

 ample, that of standing in the upright position (equilibration} (see 

 page 478), we find that a great number of muscles have to act 

 together with the most exact nicety to accomplish what, even in 

 man, is a quite thoughtless, if not quite involuntary action. In 

 the frog, as has been seen, equilibration is performed by reflex 

 action alone. In man, the nervous mechanisms are probably 

 more complicated by his erect attitude and the addition of the 

 cerebellum, etc., but they are nevertheless comparable with those 

 of the frog. It may therefore be instructive to examine the 

 details of the mechanisms in a frog deprived of its cerebral hemi- 

 spheres. 



The optic lobes (which correspond to the corpora quadrigemina, 

 and also take the place of the cerebellum of the higher animals) 

 form, in the frog, the great centres of equilibration, locomotion, 

 etc. If these lobes be destroyed, the animal can no longer sit 

 upright, jump, or swim. The first point to determine is, whence 

 do the impulses arrive which bring about these complex coordi- 

 nations. The first set are those coming from the tactile sense of 

 the skin of the parts touching the surface ; another set of impulses 

 arrives from the acting muscles acquainting the centres with the 

 amount of work done. A third set comes from the eyes, by which 

 the position of the surrounding objects is gauged ; and finally, 

 from the semicircular canals of the internal ear come impulses 



