STRUCTURE AND FUNCTIONS OF THE CEREBRUM 143 



efferent, while association neurons are either afferent or efferent 

 according as they carry impulses toward the cerebrum or away 

 from it. 



Tracing Nerve Paths. Waller ian 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, but the most fruitful one of all has rested upon recog- 

 nition 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 microscopically, the direction and extent of de- 

 generation 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 some- 

 where below the cut. Backward degeneration would signify an, 

 efferent tract with its origin somewhere forward of the point of k 

 injury. Wallerian degeneration is not difficult to follow because 

 it is fatty and the drops of fat in the degenerated region can be 

 plainly revealed by the application of osmic acid, which turns 

 them black. 



Paths of the Various Senses. For convenience in describing 

 the paths by which- information is conveyed from the various re- 

 ceptors to the cerebrum, the receptors will be classified as body 

 sense receptors and head sense receptors. The group of body 

 senses- includes all those senses such as touch, pain, muscle sense, 

 etc., whose receptors are for the most part in parts of the Body 

 other than the head, and which therefore communicate with the 

 central nervous system by way of spinal nerves. The head senses, 

 sight, hearing, taste, and smell, are those from which stimuli are 

 carried over cranial nerves to the medulla or midbrain, or in the 

 case of the sense of smell directly into the cerebrum. 



