THE SENSORY PATHS 525 



Primary neurone fibres leave the motor tract at a level a little 

 above that of the cranial nerve root, pass dorsalward along the 

 median raphe, decussate, and pass directly to the motor nucleus of 

 the cranial nerve about whose cells they form terminal arborizations. 



The motor path is thus seen to consist of two distinct sets of 

 neurones, a central and a peripheral. The cell body of the central 

 or primary motor neurone lies in the motor area of the cerebral 

 cortex ; the terminal arborizations of its fibres and collaterals sur- 

 round the nerve cells of the motor nuclei of the cranial and spinal 

 nerves. A second neurone, the peripheral or secondary motor 

 neurone, arises from these cells, enters the motor nerve root as 

 a medullated nerve fibre, and passes by means of the cranial or 

 spinal nerve trunk to its peripheral nerve ending in muscle, epi- 

 thelium, etc. 



It is a noteworthy fact that there is one and only one decussa- 

 tion in any given fibre tract of the motor path ; this decussation, 

 for the spinal nerves, occurs in the central neurone either at the 

 great motor decussation of the medulla oblongata or through the 

 ventral commissure of the cervical region of the spinal cord. 

 There is no decussation in the peripheral neurone of the motor 

 path. Since every central motor neurone almost without excep- 

 tion decussates somewhere in its course, the peripheral nerve end- 

 ing of each fibre tract of the motor path lies on the opposite side 

 of the body from its origin in the motor area of the cerebrum. 



The motor tracts may be traced in the adult by means of his- 

 tological sections taken after destruction by operation, injury, or 

 disease of the motor area in the cerebrum, which thus causes de- 

 generation of the entire central neurone, or by a similar destruc- 

 tion of the motor nuclei of the cranial nerves or ventral horns of 

 the spinal cord, which produces degeneration of the peripheral 

 neurones arising from these motor nuclei. 



In the fetus the central motor neurones acquire their myelin 

 sheaths at a very late period (in great part after birth), and are 

 thus readily distinguished from the centripetal paths, which be- 

 come medullated at a much earlier stage. 



B. The Sensory Paths 



The centripetal or " sensory " paths are more complicated than 

 the motor in that their central neurones which are multiple in- 

 stead of single follow one of four different central paths, whereas 

 the central motor neurones, as we have seen, follow a common 



