HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 855 
In the section figured the postero-median and antero-median fissures are of about 
the same length, but in a normal specimen the posterior would be rather longer 
than the anterior. 
Outside the dura mater may be seen nerve-bundles representing a high origin of 
the cauda equina: this feature is very marked in the seal, the cord ending in a 
filament at a much higher level than in the human. 
The proportion of grey matter to white is as 1 to 3 approx. 
The chief features of comparative value are the relative shortness of the posterior 
horn as compared with the human, and the presence of the surrounding bundles of 
the cauda equina even high in the lumbar region of the cord. The motor cells of 
the anterior cornua are, as in the dorsal and cervical regions, larger relatively than 
those found in the cord of man. 
II. Histotogy oF THE MEDULLA OBLONGATA. 
A. At the middle of the pyramidal decussation. (Pl. I. fig. 1.) 
Several marked points of difference are seen here as compared with the upper cervical 
region: firstly, the transverse diameter of the section is twice the antero-posterior 
diameter, and the anterior fissure is about twice the length of the posterior. The 
central canal is approaching the posterior surface of the cord, but there still remains a 
fairly wide grey commissure. 
- Indications of the beginning of a restiform body may be made out laterally, and the 
intermedio-lateral horn of grey matter now forms a well-defined mass, known as the 
substantia gelatinosa Rolandi (s.g.R.). 
The grey matter generally exists in relatively large proportion: the large motor 
cells of the anterior horn are still very obvious, and in the postero-median and postero- 
lateral columns are to be seen respectively the nucleus gracilis and nucleus cuneatus 
(n.g. and 7.c.). 
The pyramidal decussation is a marked feature, fibres passing across from the 
lateral column of one side to the anterior column of the opposite half, and to a certain 
extent separating the grey matter of the posterior region from that of the lateral and 
anterior regions.* 
The proportion of grey to white matter is as 3 to 4 approx. 
B. At a point just below the calamus scriptorius of the 4th ventricle. 
GE tie: 22) 
In this region the central canal is fast approaching the floor of the 4th ventricle to 
open out into that cavity. The pyramidal decussation is now no longer noticed, the 
upper level having been passed. 
* The decussation of the pyramids is of greater extent longitudinally than in the human medulla, being found 
quite close to the calamus scriptorius at its upper level. 
