MYELON IN MAMMALIA. 



77 



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

 1 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, ai 



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 e 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 



3 



Transverse sections of the 



human Myelon. 



A. Anterior or ' ve ntral. 



P. Posterior or ' dorsal.' 



