HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 145 



cells is situated ; the cells of Clarke's column are also seen, having been 

 displaced below the base of the posterior horn by mechanical pressure, 

 (c) A broad anterior horn possessing several groups of large nerve-cells. 



The white matter in the posterior region shows but slight indication of a division 

 into postero-mesial and postero-lateral columns : the lateral columns are of relatively 

 wide extent. 



71 



n. 



L./l 



FIG. 2. A transverse section of the spinal cord (mid-dorsal region). Lettering as in fig. 1, except : 



a.m. Arachnoid mater. 

 c.c. Central canal of cord. 



n. Posterior roots. 



Anterior spinal vessels. 



The posterior nerve-roots are narrower than in the cervical region, but are very 

 well denned : the root-bundles lie just outside, and are seen cut across in the section. 

 The anterior roots pass out in three or four narrow strands. 



The proportion of grey matter to white is as 9 to 20 approx. 



Compared with higher types, the dorsal cord shows but few divergences : the 

 posterior horn of grey matter is rather shorter than in the human cord, and Clarke's 

 column of nerve-cells is but feebly developed. On the other hand, the motor cells in 

 the anterior horn are very well defined. 



(ROY. soc. EDIN. TRANS., VOL. XLVIII., 853.) 



