528 THE NEKVOtJS SYSTEM 



passes cephalad to be continued to the cerebrum by means of neu- 

 rones of the third or even higher orders. 



In the spinal cord the peripheral sensory neurones sensory 

 neurones of the first order enter as dorsal root fibres and divide 

 into their long ascending and short descending branches.* This 

 division is thought to occur in the dorso-lateral column, where 

 many of the short descending branches unite to form the so-called 

 comma tract. This tract consists of a group of nerve fibres, the 

 most of which, after injury, degenerate downward for a short dis- 

 tance. 



The descending branch, after a short course, enters the grey 

 matter of the dorsal horns, and afterward follows a course exactly 

 similar to such of the ascending branches as also reach the grey 

 matter of the spinal cord. The further course of these fibres may 

 be considered as conforming to one of four paths. 



I. They may pass obliquely cephalad through the postero-ex- 

 ternal column (Burdach's tract) to enter the postero-median col- 

 umn or tract of Goll, in which they travel upward to the medulla 

 oblongata. 



II. They may enter the grey matter and pass directly to the 

 ventral horns, with end arborizations in relation to the cells of this 

 region (reflex tracts). 



III. They may enter the grey matter and end directly with ter- 

 minal arborizations about the cells of Clarke's vesicular cell column. 



IV. They may enter the grey matter and promptly end in re- 

 lation to the nerve cells of the dorsal horns and intermediate zone. 



PATH " L" Burdach's tract, nearly identical with the anatomi- 

 cal postero-external column, contains incoming fibres of the pos- 

 terior spinal nerve roots and is divisible into several root zones 

 which were first mapped out by Flechsig f from embryonic tissues. 



A ventral root zone, semilunar in shape, adjoins the grey com- 

 missure and the ventral third of the dorsal horns. It contains en- 

 dogenous fibres which probably connect the posterior horn cells of 

 opposite sides (cornu-commissural tract). This is the last of the 

 root zones to become medullated in the embryo. 



* It is a peculiar and also an important fact that the fibres of the dorsal 

 roots as they pierce the pia mater to enter the spinal cord, lose temporarily 

 their medullary sheaths, thus forming a constricted band at this point, and 

 leaving the almost naked neuraxes without protection from the vicissitudes of 

 vascular pressure. Once through the pia mater their medullary sheath is 

 promptly restored. 



t Leipzig, 1876. 



