2^6 HUMAN BIOLOGY 



on the left side, so that if the animal chooses to seize an 

 object to the left of him the hmbs on that side are ready 

 to support his weight when he takes off with his right foot. 

 The same reactions are exhibited by human beings when 

 in a decerebrate condition. Indeed, to the neurologist the 

 occurrence of neck and labyrinthine reflexes in man is an 

 important diagnostic sign. These reactions off"er still another 

 instance of the more comphcated field of integration which 

 one encounters on examining the higher levels of the brain 

 stem. 



The Cerebellum and Bulb. Lying over the bulb in intimate 

 anatomical association with it is the large convoluted 

 organ known as the cerebellum. If this is removed, the 

 rest of the brain stem being allowed to remain intact, 

 orderly locomotion and dehcately adjusted skilled move- 

 ments become forever impossible. Extirpation of the cere- 

 bellum, however, produces no eff"ect upon the mind. The 

 wildly incoordinated character of the movements which 

 result from a cerebellar lesion is usually referred to as 

 "cerebellar ataxia." The way in which the cerebellum 

 operates to secure delicate adjustments of the voluntary 

 muscles is still to some extent a mystery. No reflex, for 

 instance, is known to occur in normal animals which does 

 not also take place after the cerebellum is removed. The 

 anatomical relations of the cerebellum, however, provide 

 important information as to its probable mode of action. 

 It is known that the great proprioceptive system of sensory 

 nerves arising within the skeletal muscles (tendon organs 

 and muscle spindles) send large fiber tracts which pass 

 up the spinal cord and into the cerebellum, eventually 

 terminating within the cerebellar cortex as do other large 

 groups of fibers which descend from the cerebral hemispheres. 

 Emerging from the cerebellum are other fiber tracts which 

 descend via the red nucleus to the motor neurones of the 

 spinal cord, immediately subserving the skeletal muscle 

 fibers. Electrical stimulation of the cerebellum sometimes 

 causes excitation, and sometimes inhibition of the voluntary 

 musculature, but more often a mixture of both in the mus- 

 culature as a whole. Consequently, one may conclude 

 that the cerebellum can bring to bear both excitatory and 



