172 THE HUMAN BODY 



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. 



Successive Myelination. Another valuable method of tracing 

 nerve tracts was discovered by Flechsig, who found that during the 

 embryological development of the animal the axons of individual 

 tracts all become myelinated together, while different tracts re- 

 ceive their myelin sheaths at different periods of development. 

 Thus by examining a large series of embryos in all stages the va- 

 rious tracts can be picked out. 



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. 



Tracts of Body Sense. Sensory neurons of body sense enter 

 the spinal cord all along its length. Afferent paths within the 

 cord begin, therefore, at its extreme end. These are to be looked 

 for, as previously stated, in the columns of white matter which 

 make up the greater part of the substance of the cord. Two dis- 

 tinct regions of white matter in each half of the cord have been 

 shown to consist chiefly of afferent neurons leading toward the 

 cerebrum. These are: first, the dorsal columns, each of which con- 

 sists of two rather well-marked bundles of axons, the so-called fas- 

 ciculus gradlis (Column of Goll) next the dorsal fissure, and the 

 fasciculus cuneatus (Column of Burdach) next to the dorsal horn 

 of gray matter; second, the ventrolateral tracts which lie next to 

 the ventral horns of gray matter, surrounding them on the sides 

 and below (Fig. 65). It is thought that the dorsal columns con- 

 sist chiefly if not wholly of the axons of sensory neurons which, 

 entering the cord by the dorsal roots of spinal nerves, extend for- 

 ward within the dorsal columns, giving off collaterals into the gray 

 matter at various levels. Only a part of the sensory axons which 



