i;8 VETERINARY PHYSIOLOGY 



also cause flexion of the elbow of the same side and entension 

 of the opposite elbow with entension of the trunk and lower 

 limbs. This may be associated with the maintenance of the 

 body in the erect position and the alternate movements of the 

 limbs in the act of progression. 



Taking into consideration the fact that lesions of the 

 posterior nerve roots cause loss of muscular co-ordination, 

 while destruction of the ascending cerebellar tracts produces 

 decrease of muscular tone on the same side, it may be 

 concluded that both the kinsesthetic mechanism, which plays so 

 important a part in maintaining the balance of the body, and 

 the labyrinthine mechanism, have an important central station 

 in the cerebellum. 



It would thus appear that the cerebellum is to be regarded 

 as a mechanism supplementary to the great cerebro-spinal mechan- 

 ism, and that it has for its purpose more especially the muscular 

 co-ordination and 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. Channel for such 

 impulses exists (1) 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. 41, p. 88) and (2) in the superior peduncles. 



To maintain the constant muscular adjustments involved 

 in balancing the body an arrangement whereby any dis- 

 turbance of the equilibrium can produce an appropriate 

 reaction is required. 



The ingoing impulses which are more especially of service 

 in this way are derived from (1) the kincesthetic mechanism 

 (see p. 170) ; (2) the tactile mechanism from the soles of the 

 feet ; (3) the eye ; and (4) the labyrinthine mechanism 

 (p. 152). 



When the information as to the relationship of the animal 

 with its surroundings derived from these various sources is 

 not concordant e.g. when the semicircular canals indicate 

 movement and the eyes an absence of movement balancing 



