96 ANATOMY OF VERTEBRATES. 



and distinct in Doves ; ^ where the humero-scapular ossicle exists, 

 a fascicuhis therefrom, large in Owls, appears as a distinct origin 

 of the deltoid, the main mass of which muscle is inserted into the 

 pectoral ridge from its angle distad. The deltoid raises and 

 retracts the wing. 



Birds have the iiectoralis muscle divided, as in many Mammals, 

 into three portions, so distinct as to be regarded as separate 

 muscles ; they all arise from the enormous sternum, and act upon 

 the proximal extremity of the humerus. 



The first or great jjectoral muscle, ib. 25, is extraordinarily de- 

 veloped, and is in general the largest muscle of the body. In 

 birds of flight it often equals in weight all the other muscles of 

 the body put together. It arises from the anterior part of the 

 outer surface of the clavicle or furculum, from the keel of the 

 sternum, and from the posterior and external part of the lower 

 surface of that bone ; it is inserted by an extended fleshy margin 

 into the palmar surface of the pectoral crest of the humerus. It 

 forcibly depresses the humerus, and, consequently, forms the 

 principal instrument in flight. 



This muscle is very long and wide in the Natatores generally, 

 but in the Penguin, its origin is limited to the external margin of 

 the subjacent pectoral muscle, which is here remarkably developed. 

 The great pectoral is very long, but not very thick in the Rasores. 

 In the Herons it is shorter, but much stronger and thicker. Its 

 size is most remarkable in the Humming-birds, Swallows, and 

 diurnal Birds of Prey, where it is attached to almost the whole 

 outer surface of the sternum and its crest, and has an extended 

 insertion. In the Ostrich its origin is limited to the anterior and 

 external eighth part of the sternum, and it is inserted by a 

 feeble tendon into the commencement of the pectoral crest of the 

 humerus, to which it gives a strong rotatory motion forwards. 

 In the Apteryx the pectoralis major - is represented by two thin 

 triano-ular layers of muscular fibres attached to the under and 

 lateral part of the sternum, and converging to be inserted into the 

 proximal third of the minute humerus. 



The second pectoral muscle is situated in birds of flight beneath 

 the great pectoral ; it has the form of an elongated triangle : it 

 arises from the base of the crest of the sternum and from the 

 mesial part of the inferior surface of that bone ; it increases in 

 size as it ascends, then again becomes suddenly contracted, passes 

 upward and backward round the coracoid, between that bone and 



• XLXIII-. p. 400. - xr. vol. iii. p. 289, pis. 31, 32, p. 



