II. THE CEREBRAL AND CEREBELLAR ARCS. 



In reflex action involving the spinal arcs the ingoing 

 fibres from the receptors in the skin and subjacent parts of 

 the body when stimulated produce inevitable results vary- 

 ing only with the condition of the spinal neurons and 

 synapses. 



When the cerebral and cerebellar arcs are brought into 

 play, the resulting actions are much more complex, and the 

 possibility of differences in the resulting action is enormously 

 increased. 



The object of the development of these higher arcs is 

 simply to secure a more appropriate response to external 

 conditions, to enable the different external conditions acting 

 through the various receptors (p. 52) not only to produce 

 each its own special effect, but also to ensure that the effects 

 may be brought together so that the resulting action may be 

 determined by their combined effect. 



In studying the action of these arcs it will be convenient 

 to take the ingoing side first and the outgoing side later, and 

 in the study of the cerebral arc to intercalate between 

 these a consideration of the relationship of brain action to 

 consciousness and to mental activity. 



A. INGOING SIDE OF THE ARCS. 



Special parts of the central nervous system are connected 

 and associated with the peripheral structures, so that a 

 definite reaction to each of the various kinds of stimuli may 

 occur. These reactions may be accompanied by changes in 

 consciousness — by sensations ; and since the lower animals 

 have not the power of communicating the character of these 

 sensations by speech, the possibility of studying them is 

 restricted. 



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