1 64 HUMAN PHYSIOLOGY 



tended, and an arching of the body with the convexity 

 towards the side of the lesion, while the animal may be 

 driven round its long axis to the opposite side. (2) These 

 irritative symptoms soon pass off, and the animal then mani- 

 fests inadequacy or weakness in the limbs of the affected 

 side, so that it droops to that side, and if a quadruped may 

 circle to that side. (3) After some weeks these symptoms 

 disappear, and the loss of one side of the cerebellum is 

 apparently completely compensated for. 



From these experiments it would appear that the cere- 

 bellum is to be regarded as a mechanism supplementary to 

 the great cerebro-spinal mechanism, and that it has for its 

 purpose more especially the muscular adjustment required 

 in maintaining the balance. This it may do in one or both 

 of two ways. 



1. By receiving impulses from without, and sending 

 impulses downwards to act upon the spinal mechanism. 



2. By receiving impulses, and sending impulses upwards 

 to the cerebrum to modify its action. A channel for such 

 impulses exists in the fibres of the pons which cross the 

 middle line to connect with cells from which fibres pass 

 upwards to the occipital and frontal lobes of the cerebrum 

 (Fig. 40, p. 89). 



To maintain the constant muscular adjustments involved 

 in balancing the body requires an arrangement whereby any 

 disturbance of the equilibrium can produce an appropriate 

 reaction. 



The ingoing impulses which are more especially of service 

 in this way are (1) the muscular sense (see p. 97); (2) the 

 tactile sense from the soles of the feet ; (3) vision ; and (4) 

 the sense of acceleration or retardation of motion in any 

 plane or planes of the body derived through the semicircular 

 canals. The importance of the muscular, tactile, and visual 

 senses in maintaining the balance is so obvious that it need 

 not be further considered. 



4. Physiology of the Semicircular Canals. That there is 

 no special mechanism making us aware of uniform movement 

 is proved by the fact that we are not conscious of whirling 

 through space with the earth's surface, and that in a smoothly 



