166 THE HUMAN BODY 



of gray matter in immediate synaptic connection with the cell 

 bodies of the motor neurons. 



We can trace reflex arcs for locomotor reflexes as for the simpler 

 spinal reflexes. In both cases the paths begin with sensory neurons 

 and terminate with motor neurons. Many more association neu- 

 rons are always involved in locomotor reflexes than in spinal, and 

 they always include the cerebellum in their course. As indicated 

 above, however (p. 164), not one, but several senses co-operate in 

 locomotor reflexes, and many muscles are concerned in their per- 

 formance, so that no single reflex arc suffices to carry them on, but 

 several paths into the cerebellum and a number out of it must be 

 thought of as involved. 



Functions of the Cerebellum. In a previous paragraph the 

 general function of the cerebellum was stated, namely, to translate 

 the streams of impulses from the receptors of muscle sense, equi- 

 librium, touch, and sight into co-ordinated motor impulses by which 

 are carried on the important reflexes of locomotion. We need to 

 bear in mind, in this connection, that our muscles will not work 

 spontaneously. We can cause them to contract by an act of the 

 will (p. 184) or they can be operated reflexly by means of stimuli 

 conducted to them from receptors. We know from our own 

 experience that our common locomotor acts, such as walking, are 

 not volitional in the sense that every muscular movement is volun- 

 tary. We can see, also, that in such an act as walking there are 

 abundant sources of sensory stimulation. The pressure of the 

 feet upon the ground, the muscular movements themselves, the 

 disturbances of equilibrium, the appearance of the footing, all give 

 rise to streams of sensory impulses which, if properly co-ordinated 

 can be made to operate complicated muscular movements. This 

 co-ordination is the function of the cerebellum. 



All the reflexes which the cerebellum mediates are reflexes of 

 skeletal muscles. They are all such as the higher parts of the brain 

 through the property of volition are competent to carry on. If 

 their performance depended on the higher brain regions, however, 

 these would have little time left for other, and more important 

 activities. We may view the cerebellum, therefore, as an organ 

 which by taking up complicated but not highly intellectual tasks 

 leaves the higher parts of the brain free for higher forms of activity. 



An important difference between cerebellar and spinal reflexes 



