118 DR ALEXANDER BRUCE 



groups on the right side is difficult to determine ; probably there are fourteen. On the 

 left side there are fourteen fairly distinct groups. This transition in the character of 

 the graph is associated with a decrease in the number of cells in the apical and a rise 

 in that of the reticular series. The latter tends in the lower two-thirds of the segment 

 to assume a distinct wedge-shape, with its apex pointing inwards and forwards in front 

 of Clarke's column (fig. 9, D. 5, 238). In some parts the cells of the reticular group 

 appear slightly smaller in size than those of the apical group, but this relationship is 

 not constant. The outlying cells are rather numerous, especially near the tip of the 



<.t.t 



Fig. 9 (D. 5, 238, R. ). — The reticular cells form a large group, of a wedge-shape, with its apex 

 pointing inwards in front of Clarke's column and its base situated on the formatio reticularis. 

 The apical cells are here relatively few in number. 



lateral horn. The oscillations in the number of cells are greater in the reticular than 

 in the apical group. 



Sixth Dorsal Segment. — This segment was divided into 438 sections, the inter- 

 medio-lateral tract consisting on the right of 8030 and on the left of 8105 cells. 

 Fifteen groups were counted in each side. The graphic chart of these shows that the 

 change begun in D. 5 is continued throughout the segment, the lower half bearing a 

 close resemblance to the appearance found in D. 7. There are no high ascents, and no 

 complete intervals between the mound-like elevations. In section 152 on the left side 

 58 cells were counted — the maximum number in the segment. In most of the other 

 sections the maximum number lay between 30 and 40. In this segment is the only 

 instance of an exception to the rule that the numbers of the apical and reticular groups 

 rise and fall simultaneously. In a cluster which lies between sections 270 and 340 the 

 apical group reaches its maximum while the reticular group is at its minimum. 

 Throughout the segment the reticular group as a whole is considerably smaller than 

 it is in D. 5. Its cells are more loosely scattered, with less of a triangular arrangement, 



