038 MANUAL OF PHYSIOLOGY. 



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 481 ), 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 

 hemispheres. 



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 these impulses arrive which bring about these complex coor- 

 dinations. The first set are those coming from the tactile sense 

 of the skin of the parts touching the surface ; another set of im- 

 pulses 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 



