DEVELOPMENT OF THE NERVOUS SYSTEM 477 



chronous with the appearance of their function. Thus those neu- 

 rones, e. g., the peripheral sensory neurone, which in the fetus are 

 first acted upon by stimuli from without, are the first to obtain a 



roe. post. 



FIG. 376. TRANSECTION OF THE SPINAL COED OF AN EMBRYO CHICK. 



c. rad. ant., neuraxes to the ventral roots ; c. rad. post., neuraxes to the dorsal roots ; 

 col, collateral from a neuraxis back to the grey matter ; gg, dorsal root ganglion ; rac. ant., 

 ventral root ; rac. post., dorsal root. (After van Gehuchten.) 



myelin sheath. Following these, medullary sheaths are formed 

 in the peripheral motor neurones, and reflex movements begin ; 

 still later, myelin sheaths appear in the intrinsic neurones of the 

 paths of the spinal and cranial nerves which conduct sensory im- 

 pulses toward the brain ; and finally, the cerebral sensory and 

 motor neurones receive their myelin sheaths, and consciousness 

 and voluntary movements are manifested. 



To this peculiarity in the formation of the medullary sheaths, 

 as well as to the phenomena of Wallerian degeneration, we are in- 

 debted for much of our knowledge of the course of the fibre tracts 

 in the central nervous system. If, for example, sections from the 

 spinal cord of embryos of suitable age be stained to show their 

 myelin sheaths, certain tracts or fibre bundles will be found to 

 have already acquired their medullary coat, while in other bundles 

 the myelin has not yet made its appearance. Thus in a fetus at 

 the seventh or eighth month the sensory paths of the spinal cord 

 will be found to be well medullated, while the adjoining motor 

 paths show scarcely any myelinization. 



This method of study, the myelinization method, together with 

 the degeneration method and the staining methods of Golgi, have 

 supplied most of our knowledge concerning the course of the 

 intricate fibre paths of the brain and spinal cord. 



