162 Muscular Variations 
living representatives of the order, and must contain within its struc- 
tural plan the elements which, by successive modifications, lead to the 
differentiation of the diverse extant types. 
Thus the detailed examination of the Pectoral muscular group in the 
Primates will show the existence of three main types, phylogenetically 
related to each other, which can be illustrated by the following selected 
forms: 
I. PRIMITIVE TYPE. 
1st Example: Hapale jacchus, Common Marmoset, Columbia Univer- 
sity Museum, No. 427 (Pl. III, Fig. 3). The Pectoral mass in this 
animal is composed of two main layers. 
The Superficial Layer (Ectopectoralis, Wilder) is again distinctly 
divisible into two segments: 
A. CrpHatic Portion.—A thick triangular muscle, arising by its 
base from the entire ventral surface of the sternum along the ventral 
midline, and inserted by its truncated apex into the lateral ridge of the 
humerus. This muscle corresponds to the main portion of the sterno- 
costal division of the human Pectoralis major. 
B. Caupat Portion.—This forms an extensive thin sheet of paler 
muscular fibres, prolonging the level of the superficial portion caudad 
of the sternum. It arises from the ventral abdominal aponeurosis, 
passing superficially to the Rectus abdominis, as far as the umbilicus. 
It is inserted, under cover of the cephalic portion, in combination with 
the deeper layer, into the lateral tubercle of the humerus, cephalad of 
the insertion of the cephalic portion (Pectoralis major). 
This segment is the homologue of the reduced abdominal pectoralis 
shp of the human subject. 
The Deep Layer, corresponding to the human Pectoralis minor (1nto- 
pectoralis, Wilder), arises from the ventral surface of the sternum in its 
entire extent, under cover of the sternal or cephalic portion of the super- 
ficial layer. Cephalad this layer is almost directly continuous with the 
broad origin of the subclavius muscle; caudad, beyond the lower margin 
of the superficial sternal portion, it is directly continuous with the thin 
abdominal sheet of the superficial layer. It would, therefore, be quite 
correct to describe the Pectoral muscles of Hapale as forming two layers 
in the entire extent of their sternal attachment—a superficial layer or 
Pectoralis major, and a deep layer or Pectoralis minor; the Pectoralis 
major ceases at the lower border of the sternum, and allows the caudal 
