MYELON IN MAMMALIA. 



77 



white neurine vary in different parts of the myelon. In fig. 41, 

 i is a section at the fore (upper) part of the pectoral enlargement, 

 the head of the comma is small, the tail narrow : in the middle of 



the enlargement, section 2. the head is larger, 



. * i 



with more distinct processes, the tail is thicker. 



In the dorsal region, sections 3, the grey matter 



is more reduced than in the neck. In the lum- 

 bar region, sections 4, it again expands, the 



head shows the stellar character, is fenced off 



from the ventral periphery by a smaller extent 



of white neurine ; the tail is thicker, but here 



becomes shorter and seems not to reach the 



dorsal surface. Near the termination of the 



myelon the comma-shape is lost, and the grey 



neurine reduced to a subcylindrical tract, 



slightly notched laterally and surrounded, save 



at the commissure, by the white neurine. Of 



this tissue the largest proportion exists in the 



cervical part of the myelon and its enlarge- 

 ment, where the small columns called ( posterior 



pyramids ' are continued from the dorsal part 



of the medulla oblongata, contracting to a point, 



near the end of the brachial enlargements, and 



there allowing the proper dorsal (posterior) 



columns of the myelon to come into contact at 



the posterior fissure. The difference in the 



proportions of white and grey neurine in the 



ventral and dorsal tracts of the myelon coin- 

 cides with the different nervous endowments 



of the pectoral and pelvic limbs : in the former 

 volition and sensation are greatest ; in the latter 

 reflex actions with diminished sensibility : the 

 exercise of the arms and hands induces more 

 calls upon cerebral action, that of the legs 

 and feet operates more exclusively through 

 physical changes of the lumbar part of the mye- 

 lon itself: hence, therefore, the need of a greater proportion of 

 the reproductive or grey tissue. Numerous multi-caudate vesicles 

 are present in the grey neurine, and linear tracts are continued 

 from the major part of its periphery, as seen in transverse section, 

 towards that of the myelon, accompanied by capillary vessels 

 which enter the pia mater. 



The proportion of the neural canal to the myelon varies in 



Transverse sections of the 



human Myelon. 

 A. Anterior or ' ve ntra\. 

 P. Posterior or ' dorsal.' 



