xvi. 8 WING MUSCLES 449 



away from the sternum, with which it makes a joint. The furcula, 

 probably consisting of the combined clavicles and interclavicles, is 

 loosely attached to the sternum and carries the origin of muscles that 

 rotate the humerus about its long axis. 



8. Wing muscles 



Depression of the wing is produced mainly by a single mass of 

 muscle, the huge pectoralis major, making up as much as one-fifth of 

 the whole weight of the body. It runs from the sternum and furcula to 

 the under side of the humerus, to which it is attached, at some distance 

 from the joint, by a complicated tendon of insertion. The fibres of this 

 muscle are very red in strongly flying birds and often contain numerous 

 lipoid inclusions. In the fowl the fibres are white and contain glycogen, 

 but little lipoid. Elevation of the wing is produced by a muscle also 

 attached to the sternum, lying deep to the pectoralis major and often 

 called the pectoralis minor, but more properly supracoracoideus. Its 

 tendon passes through the foramen triosseum, between the furcula, 

 scapula, and coracoid, to be inserted on the upper side of the humerus. 

 It is assisted by latissimus dorsi and deltoid muscles. 



The chief muscles of the shoulder are thus a massive set serving to 

 raise and lower the wing. There is little development of the other 

 muscles such as are present in other vertebrates for the purpose of 

 balance and drawing the limb backwards and forwards for standing 

 and locomotion. Such a system of braces all round the joint is unneces- 

 sary; the bird balances on its wings mainly by the action of the pec- 

 toralis major as the chief brace, between the sternum and the humerus, 

 with the coracoid as a compression member between. Stresses must of 

 course arise in other directions besides those tending to produce a 

 vertical fall and these are met by the muscles that produce rotation of 

 the humerus and various other movements of the wing, especially a 

 pronation, depressing the leading edge. The muscles used in other 

 tetrapods to sling the weight of the body to the pectoral girdle and 

 fore-limb are little developed. The scapula is held to the vertebral 

 column by small rhomboid muscles and there is a short series of slips 

 attached to the ribs, the serratus anterior. 



Other muscles running from the body to the humerus produce 

 rotation of the humerus at the glenoid and adjustments of the patagia, 

 movements that are very important in flight. From the outer surface 

 of the scapula arises a scapulo-humeral muscle, inserted in such a way 

 as to produce adduction and lateral rotation of the humerus, raising 

 the hinder edge of the wing. The coraco-humeral muscle is a compact 



