On the pectoral muscles of birds

Abstract

M. pectoralis major of birds in general consists of M. pect. m. proprius and lateralis (Kuroda, '60) and in soaring birds such as some hawks and the Tubinares a distinct deep-seated layer, M. pect. m. profundus (white muscle in the Tubinares) (Kuroda, 1. c.), is differentiated. The M. p. m. lateralis, generally ignored, is an important part in flight in pulling backward the wing (humerus) struck down by M. p. m. proprius, thus giving the propelling effect to the wing. Comparison of this part of the pectoral muscles in various groups of birds is shown in Plate 1. The comparative weight of M. p. m. profundus (Fig. 3, H, I, L, N) in some species of the Tubinares is listed; it is best developed in the albatross in which the M. p. minor (M. supracoracoideus) is the smallest. The relative weight of entire pectoral muscles to the body weight and that of small pectoral muscle to large pectoral are listed by Orders of birds. As a rule smaller species of a group of birds generally have more developed small pectoral relative to large pectoral. The former muscle is least developed in some hawks and the Tubinares which are soarers and best developed in the wing-diving sea-birds, the Alcidae. Their relative development is heighly adaptive to the way of flight. In the herons, an anterior superficial layer of the M. p. m. proprius can be distinguished, and this was named, the M. pect. major antero-superficialis.