_Consideration of the Forces which propel the Wings of Bats and Birds._--The muscular system of birds has been so frequently and faithfully described, that I need not refer to it further than to say that there are muscles which by their action are capable of elevating and depressing the wings, and of causing them to move in a forward and backward direction, and obliquely. They can also extend or straighten and bend, or flex the wings, and cause them to rotate in the direction of their length during the down and up strokes. The muscles principally concerned in the elevation of the wings are the smaller pectoral or breast muscles (_pectorales minor_); those chiefly engaged in depressing the wings are the larger pectorals (_pectorales major_). The pectoral muscles correspond to the fleshy mass found on the breast-bone or sternum, which in flying birds is boat-shaped, and furnished with a keel. These muscles are sometimes so powerful and heavy that they outweigh all the other muscles of the body. The power of the bird is thus concentrated for the purpose of moving the wings and conferring steadiness upon the volant mass. In birds of strong flight the keel is very large, in order to afford ample attachments for the muscles delegated to move the wings. In birds which cannot fly, as the members of the ostrich family, the breast-bone or sternum has no keel.[90] [90] One of the best descriptions of the bones and muscles of the bird is that given by Mr. Macgillivray in his very admirable, voluminous, and entertaining work, entitled History of British Birds. Lond. 1837. The remarks made regarding the muscles of birds, apply with very slight modifications to the muscles of bats. The muscles of bats and birds, particularly those of the wings, are geared to, and act in concert with, elastic ligaments or membranes, to be described presently.