GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM 165 



complicated on the sensory side than the most elaborate spinal 

 cord reflexes, the latter being based on stimulation of single groups 

 of receptors. On the motor side, also they are more complicated. 

 The degree of muscular co-ordination involved is greater than in 

 any spinal cord reflex. For the translation of the complex stream 

 of sensory impulses into an equally complicated stream of motor 

 impulses a more elaborate arrangement of interconnecting 

 neurons is required than the spinal cord affords. For this pur- 

 pose a special portion of the brain, the cerebellum, is set apart, 

 and our next concern is with the structure and connections 

 of this organ, and its functioning in the mediation of reflexes of 

 locomotion. 



Structure and Connections of the Cerebellum. This organ, 

 as shown in Figs. 58-60 is a distinct portion of the brain, lying 

 underneath the posterior part of the cerebrum, and behind and 

 above the midbrain and medulla. It consists of a thin layer of 

 gray matter superposed upon white matter, and having embedded 

 within the white matter at its base gray masses, the nuclei of the 

 cerebellum. The thin outer gray layer, known as the cortex, is the 

 region in which the incoming streams of sensory impulses are con- 

 verted into outgoing streams of co-ordinated motor impulses. The 

 cerebellum communicates with the brain stem, as, for convenience, 

 the midbrain and medulla together are often called, by three pairs 

 of stalks or peduncles. These consist of bundles of axons. The 

 upper and lower stalks (Fig. 66) lead directly into the brain stem. 

 The middle peduncles form the backward extension of the pons 

 varolii (Fig. 58). 



The senses which are concerned with locomotor reflexes all have 

 connection, either directly or by means of association neurons, 

 with the brain stem, and thence, by neurons whose axons extend 

 through the peduncles, with the cerebellum itself. The detailed 

 anatomy of these paths will be presented later in connection with 

 the study of the cerebrum (p. 173). 



The outgoing paths from the cerebellum, the paths over which 

 pass the co-ordinated streams of impulses which carry on the acts 

 of locomotion, consist of chains of association neurons. These 

 begin in the cerebellar cortex and pass thence over the peduncles to 

 the brain stem. Here communication is made with others which 

 pass down the spinal cord to final terminations in the ventral horn 



