STRUCTURE AND FUNCTIONS OF THE CEREBRUM 171 



with the possible exception of those of the sense of smell, made 

 in gray matter of the spinal cord, the medulla, or the midbrain. 

 In order for impulses coming in over these sensory neurons to 

 reach the cerebrum there must be communication by association 

 neurons between the terminations of the sensory neurons and the 

 cerebrum. As a matter of fact such connections are richly sup- 

 plied. Some of the most conspicuous tracts of white matter in 

 the central nervous system consist of the myelinated axons of 

 association neurons which form connecting links between sensory 

 neurons and the cerebrum. Since the cerebrum is the crown of 

 the entire nervous system it is used as a landmark in describing 

 other nervous structures. Thus nerve paths which convey im- 

 pulses toward the cerebrum are called afferent paths; those carry- 

 ing impulses away from it are efferent paths. According to this 

 classification all sensory neurons are afferent and all motor ones 

 efferent, while association neurons are either afferent or efferent 

 according as they carry impulses toward the cerebrum or away 

 from it. 



Tracing Nerve Paths. Wallerian Degeneration. One of the 

 very satisfactory achievements of biologists has been the reso- 

 lution of the apparently inextricable tangle of gray and white 

 matter of the central nervous system into a system of fairly definite 

 nerve tracts whose origins, courses, and terminations are known. 

 Our present knowledge is the result of various methods of study. 

 Perhaps the most fruitful has rested upon recognition of three 

 facts: first, that white matter always consists of myelinated axons; 

 second, that axons always are outgrowths of cell-bodies which are 

 to be looked for in gray matter; and third, the fact discovered by 

 the English physiologist, Waller, in 1852, that axons cut off from 

 connection with their cell-bodies undergo degeneration in a few 

 days. Because of this latter fact if a cut be made anywhere in the 

 central nervous system of an animal, and the animal be killed a 

 few days later and its spinal cord and brain examined microscop- 

 ically, the direction and extent of degeneration reveal the relation 

 of the severed axons to the rest of the nervous system. If the 

 degeneration is all toward the head the severed tract must be an 

 afferent one with cell-bodies somewhere below the cut. Backward 

 degeneration would signify an efferent tract with its origin some- 

 where forward of the point of injury. Wallerian degeneration is 



