164 Muscular Variations 
Pectoralis minor arises from the ventral surface of the sternum, under 
cover of the superficial layer, from the level of the 2nd chondro-sternal 
junction caudad to the fusion of both layers in the terminal portion of 
the common origin. This deeper sheet, receiving all the muscular fibres 
from the caudal common origin corresponding to the cartilages of the last 
two sternal ribs, proceeds, under cover of the superficial layer, intersect- 
ing the course of the fibres of the same at an acute angle, across the ven- 
tral axillary wall to expand into a broad tendinous lamina which, after 
receiving the well developed axillary arch along its caudal margin, ex- 
pands under cover of the Deltoid to be inserted into the lateral tubercle 
of the humerus and the adjacent portion of the lateral surface of the 
shaft. 
The axillary arch appears as a derivative of the well developed ventro- 
lateral thoracic panniculus. It crosses the axillary border of the Latis- 
simus, but has no direct connection with that muscle. 
CoMPARISON OF PECTORAL GRouP IN Hapale and Nycticebus. 
Both these forms, selected as representative examples of the primitive 
Pectoralis type in existing Primates, show a comparatively early stage 
in the differentiation of the two main component elements, Pectoralis 
major and minor. In Nycticebus the differentiation is still incomplete 
in the caudal segment of the Pectoral mass. In Hapale the cleavage is 
complete—and with this the ectopectoral layer has gained a distinct 
caudal limit and presents a thick well marked border extending from the 
caudal end of the sternal origin to the humeral insertion. On the other 
hand the entopectoral layer, occupying the same extent of sternal origin 
at this point, is left to become directly continuous with the thin abdomi- 
nal extension of the Pectoralis. In Hapale a fairly distinct inter- 
mediate entopectoral muscular slip (Pl. ILL, Fig. 3, left side) effects the 
junction between the caudal margin of that portion of the deeper layer 
which arises under cover of the ectopectoral entirely from the sternum, 
and the thin expanded caudal sheet arising from the abdominal aponeur- 
osis. This intermediate entopectoral slip arises by an aponeurotie la- 
mella over the cartilages of the lower sternal ribs, and proceeding lat- 
erad, fuses with the main entopectoral plane before the same unites with 
the abdominal portion near the common insertion into the lateral surface 
of the humerus. Comparison with Nycticebus shows this slip to corre- 
spond to the distal united portion of both pectoral planes, arising from 
the 8th and 9th costal cartilages, while the abdominal Pectoralis in 
Nycticebus has not yet differentiated from the general pannicular plane. 
Careful examination of the two forms indicates very clearly the value 
