168 Muscular Variations 
Figs. 9 and 10 represent the superficial and deep pectoral muscles in 
a fresh dissection of an adult male orang—Simia satyrus. 
RECONSTRUCTION OF THE COMMON PRIMATE PECTORAL GROUP. 
In view of the disposition found in Nyecticebus we may assume that 
the ancestral common Primate type of the Pectoral group presented a 
very shghtly differentiated ventro-appendicular muscular sheet, con- 
nected at the insertion with an extensive thoraco-humeral pannicular 
muscle. 
The skeletal relations of this primitive muscle at the origin give us 
the indications of the lines along which subsequent differentiation took 
place. 
The pre- and mesosternum evidently afforded the most advantageous 
line for the mesal attachment of the muscle, and consequently we see in 
Nycticebus, and still more in Hapale, that both the superficial and the 
deep segment of the Pectoral plane in the sternal portion is strongly 
developed and in contrast, as a massive condensed muscular mass, with 
the thin expanded abdominal sheet. 
It isnot difficult to recognize in the latter a derivative of the pan- 
nicular layer which has obtained a secondary line of attachment to the 
abdominal aponeurosis, but which, lacking the advantage of the firm 
connection afforded by the sternum, has failed to develop in a correspond- 
ing degree. Jor the original pannicular derivation of the abdominal 
pectoral we have evidence in the connection of this muscle at the inser- 
tion both in Nycticebus and Cynocephalus with the axillary arch, which 
appears as,a distinct pannicular element in Nycticebus, and only begins 
to obtain its secondary connection with the Latissimus in Cynocephalus. 
Of course the question of the original derivation of the entire Pectoral 
mass. as a differentiated product of the deeper layers of a primitive 
extensive tegumentary muscular sheet demands consideration at this 
point. We may assume, however, that in the common Primate type the 
skeletal attachment to the sternum had already sufficed to produce the 
differentiation of the cephalic portion as a distinct muscular integer. 
Probably, in this segment, the further differentiation into Ecto- and 
Entopectoral (Pect. major and minor) had likewise begun in the com- 
mon ancestral prototype. 
On the other hand very probably the abdominal portion was still incor- 
porated in the general thoraco-humeral pannicular sheet, closing the 
floor of the axilla without definite mesal attachment, but beginning to 
join laterally the intrinsic appendicular muscles, along the lines indi- 
cated by the abdominal pectoral and axillary arch in Nycticebus at the 
