MYOLOGY 139 



are not marked on their exterior by transverse grooves, 

 such as we find in the human species in individuals with 

 dehcate skin and whose adipose tissue is not very much 

 developed. 



The rectus abdominis is covered, in its anterior portion, 

 by the sterno-trochinian muscle (posterior segment of the 

 small pectoral). In contracting, this muscle brings the 

 chest nearer the pelvis, and as a result flexes the vertebral 

 column. It also contributes to the compression of the 

 abdominal viscera. 



Pyramidalis Abdominis. — This unimportant little 

 muscle, which in man is situated at the lower part of tlie 

 abdomen, extends from the pubis to the linea alba. It is 

 not present in the domestic animals. 



We consider it interesting, however, to point out, although 

 the fact is not a very useful one as regards external form, 

 that this muscle is distinctly developed in marsupials. 



We know that in the opossum, the kangaroo, and the 

 phalanger fox, the young are brought forth in an entirely 

 incomplete state of development, and that, during a certain 

 period, they are obliged to lodge in a pouch which is placed 

 at the lower part of the abdomen of the mother. Now, 

 this pouch contains the mammary glands ; but the young, 

 being too feeble to exercise the requisite suction, the 

 pyramidal muscles come to their assistance. These muscles, 

 in contracting, approximate to one another two bones which 

 are placed above the pubis, the (so-called) marsupial bones 

 (see Fig. 80) ; by their approximation the bones in question, 

 which are placed behind and on the outer side of the 

 mammary glands, compress the latter, and thus is brought 

 about the result which the little ones, on account of their 

 feebleness, would, without that intervention, be incapable 

 of obtaining for themselves. 



