294 



AVES. 



scapula, and is inserted along the middle of the 

 outer side of the humerus ; it brings the wing 

 upward and backward. 



Birds have the Pcctoralis muscle divided, as 

 in many of the mammalia, into three portions, 

 which are so distinct as to be regarded as sepa- 

 rate muscles ; they all arise from the enormous 

 sternum, and act upon the proximal extremity 

 of the humerus. 



Thejirst or great Pectoral muscle (25) is ex- 

 traordinarily developed, 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 inner side of 

 the anterior 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 Nata- 

 tores generally, but in many of these birds, as 

 the Penguin, its origin is limited to the external 

 margin of the subjacent pectoral muscle, which 

 is here remarkably developed. The great pec- 

 toral 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 into 

 the humerus. 



In the Ostrich its origin is limited to the an- 

 terior and external eighth part of the sternum, 

 and it is inserted by a feeble tendon into the 

 commencement of the crest of the humerus, to 

 which it gives a strong rotatory motion for- 

 wards. 



The second Pectoral muscle is situated be- 

 neath the preceding; 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 in- 

 creases in size as it ascends, then again be- 

 comes suddenly contracted, passes upwards 

 and backwards round the coracoideum, between 

 that bone and the clavicle, then turns down- 

 wards and outwards, and is inserted, fleshy, 

 above and in front of the great pectoral, into 

 the upper extremity of the humeral crest. 



The interspace between the clavicle, cora- 

 coid, and scapula, through which its tendon 

 passes, serves as a pulley, by means of which 

 the direction of the force of the carneous fibres 

 is changed, and although these fibres ascend 

 from below towards their insertion, yet they 

 forcibly raise the humerus, and thus a levator 

 of the wing is placed without inconvenience 

 on the lower part of the trunk, and the centre 

 of gravity proportionally depressed. 



In the Penguins, Guillemots, and Gulls, 

 this muscle is almost the largest of the three, 

 occupying the whole length of the sternum. 

 It is remarkable for the length and strength 

 of its tendon, which is inserted so as to draw 



forwards the humerus with great force. It is 

 proportionally the smallest in the Raptores; 

 and is very small and slender in the Struthious 

 birds. 



We have already alluded to the use which 

 the Penguin makes of its diminutive anterior 

 extremities as water-wings, or fins ; to raise 

 these after making the down-stroke obvi- 

 ously requires a greater effort in water than a 

 bird of flight makes in raising its wings in air : 

 hence the necessity for a stronger development 

 of the second pectoral muscle in this and other 

 Diving Birds, in all of which the wings are 

 the chief organs of locomotion, in that action, 

 and consequently require as powerful a deve- 

 lopment of the pectoral muscles as the gene- 

 rality of Birds of Flight. 



The third Pectoral muscle, which is in ge- 

 neral the smallest of the three, arises from the 

 anterior part of the inferior surface of the ster- 

 num, and also by a more extended origin, from 

 the posterior moiety of the inferior surface of 

 the coracoid ; it is directed forwards, and is 

 inserted by a short and strong tendon into the 

 internal tuberosity of the humerus, which it 

 depresses. 



It is proportionally large in the Penguins 

 and Gulls, but attains its greatest development 

 in the Gallinaceous order. 



Above the preceding muscle there is another 

 longer and more slender one, analogous to the 

 Coraco-brachialis, which arises from the middle 

 of the posterior surface of the coracoid ; its 

 direction upwards is less vertical than that of 

 the third pectoral, along the outer side of 

 which it is attached to the anterior tuberosity 

 of the humerus. This muscle is wanting in 

 the Struthionidtf, is of small size in the 

 Heron and Goose, is much more developed 

 in the Raptores and many Natatores, espe- 

 cially the Penguins, and attains its greatest 

 relative size in the Rasores, where it arises 

 from almost the whole of the coracoideum. 



Birds in general possess Iwojlexors and one 

 extensor (27) of the fore-arm, analogous to those 

 which are found in the mammalia. They have 

 also the muscles corresponding to the pronators 

 and supinators of this higher class, but their 

 action is limited in the feathered tribes to in- 

 flexion and extension of the fore-arm, and to 

 adduction and abduction of the hand. 



A remarkable muscle, partly analogous in its 

 origin to the clavicular portion of the deltoid, 

 but differently inserted, is called by Carus 

 Extensor plica alaris (30, a b) and forms 

 one of the most powerful flexors of the 

 cubit. It is divided into two portions, of 

 which the anterior and shorter arises from 

 the internal tuberosity of the humerus ; the 

 posterior and longer from the clavicular ex- 

 tremity of the coracoid bone. In the Ostrich 

 and Rhea, however, both portions arise from 

 the coracoid. The posterior muscle (b) sends 

 down a long and thin tendon which runs pa- 

 rallel with the humerus, and is inserted, gene- 

 rally by a bifurcate extremity, into both the 

 radius and ulna. The anterior muscle (a) 

 terminates in a small tendon, which runs 



