A. Brazier Howell

Morphogenesis of the Shoulder Architecture. Part VI. Therian Mammalia

IN SO FAR as concerns the architecture of the appendages, the step from living reptiles to therian mammals is a great one; and in most respects the latter differ almost as widely from prototherians. In fact monotremes are more properly comparable with reptiles than with therian mammals, and they will receive no consideration in the present contribution. Accordingly, in this paper the term Mammalia refers to Theria. The chief reason for the dissimilarity in the pectoral appendages of these groups is attributable to the difference in the manner in which the limbs are used. In the prone position of the reptilian body, the largely horizontal position of the humerus, with divergent elbows, requires a very different muscular arrangement and bony scaffolding from that suited to the needs of mammals. The typically reptilian architecture of the shoulder is unsuited to speedy or long sustained action. In order to change to the mammalian plan it was necessary for reptiles to bring the elbow beneath the body, involving an alteration of the shoulder joint and its controlling musculature, which Reptilia found so difficult of accomplishment hat only a single group (the Theriodontia) ever succeeded in its accomplishment. The resulting improvement in function doubtless played no inconsiderable part in the development of the class Mammalia. The result, in improved form, is that mammals may operate the limbs for purposes of locomotion in a single plane, for extended periods, with the expenditure of a minimum of energy.