SUMMARY OF STRUCTURES 245 



of the caudal musculature. It is by no means so insistent in its demands as 

 the rapidly differentiatinp; structures of the upper extremity and particularly 

 of the hand. 



In lemur and tarsius, the pyramidal tract appears to be less prominent 

 than in Mycetes seneculus. Such is the case also in marmoset. The hand ot 

 this animal has made a hesitating advance toward manual differentiation, 

 and its pyramidal tract is hence less conspicuous than in the howling 

 monkey. 



PLANIMETRIC COEFFICIENT 



These facts are clearly illustrated by certain coefficients which show that 

 the pyramidal area in proportion to the remainder of the cross section of 

 the oblongata is greater in the howling monkey than in either marmoset, 

 tarsius or lemur. 



The numerical expression of this proportion may be termed a plani- 

 metric coefficient. It is obtained by means of projection drawings of the cross 

 section in which the structure to be measured appears, the projection being 

 produced at a fixed magnification. The area occupied by the structure under 

 consideration is then determined by means of the planimeter, and in a similar 

 manner the area of the hemisection in which the structure lies. The ratio of 

 the structure whose coefficient is sought to the total hemisection of the axis 

 is then computed and the figure taken to represent the planimetric coeffi- 

 cient. The planimetric coefficients of the pyramidal tract in lemur, tarsius, 

 marmoset and mycetes are: 



Planimetric Coefficients of Pyramidal Tract in Lower Primates 



