'.Hi THK XKKYois SYSTEM AM.) ITS CONSERVATION 



of contraction (tonus) in other muscles, which must 

 yield readily thai they may not hinder the movement. 

 In all but the simplest instance's another element come- 

 in, that of sequence or succession in the responses of indi- 

 vidual muscles. Most movements occur in stages and 

 many have an alternating character. When this is true 

 we mu-t suppose that each step establishes conditions of 

 pressure or tension in the moving parts of such a nature 

 as to excite certain receptors, and that the next shift in 

 the musculature follows as a reflex. In the higher animals 

 and in man only a little of this coordinating power resides 



Fig. 18. The principle of reciprocal innervation. Tlic cortical 

 nounm is represented a- exciting the upper of the two subordinate 



neurons, causing the contraction of one group of muscle-fibers, 

 while it inhibits the second neuron, allowing the associated muscle- 

 fibers to relax. The + sign suggests .stimulation; the sign, in- 

 hibition. Doubtless the actual arrangement i- less simple than this. 



in the spinal cord. Most of it is represented in the brain. 

 At this place il is convenient to speak briefly of the 

 physiology of the cerebellum. 



The Cerebellum.^ It will be recalled that this division 

 of the brain is a dorsal outgrowth of the nerve-axis. It is 

 connected by a rich supply of fibers with the medulla 

 behind, the ventral region known as the pons. and. less 

 directly, with the cerebrum. Its prominence in different 

 -peeies of animals is most unequal. It is large in the fish, 

 greatly reduced in the frog, very large again in the bird. 

 and fairly large but overshadowed by the cerebrum in 

 most mammals. A moment'- reflection will show that the 



