THE ANATOMY OF BIRDS. — MYOLOGY. 199 



The subcutaneous sheet of muscle (of which the human '' muscles of expression " and 

 platysma myoides are segregations) is broken up in birds into a countless number of little slips 

 which agitate the feathers collectively, and especially the great quills of the wings and tail. 

 There are estimated to be 12,000 in a goose. The prime peculiarity of birds' musculation is the 

 enormous development of the pectorales, or breast muscles, which operate the wings. The 

 great pectoral, p. major or 2>- primus, arises from the sternal keel, when that special bony sep- 

 tum between the fellow-pectorals exists, and from more or less of the body of the sternum, pass- 

 inc directly to the great pectoral or outer ridge of the humerus, near the upper end of that bone. 

 Its origin may even exceed the limits of the sternum, invading the clavicle, etc. ; it may unite 

 with its fellow. It is the depressor of the humerus, giving the doivnward stroke of the wing. 

 The next pectoral, p. secundus or p. medius, arises from much or most of the sternum not occu- 

 pied by the first, under cover of which it lies ; it passes also the humerus, but by an interesting 

 way it has of running through a pulley at the shoulder it elevates that bone, giving the upward 

 wing-stroke. A third pectoral, p. tertius or p. minimus, arising from sternum, and often con- 

 tiguous parts of the coracoid bone, passes directly to the humerus, supplementing the action of 

 the first. A fourth muscle in many birds acts upon the humerus from the sternum or coracoid, 

 particularly the latter. These four differ greatly in their relative development. Such extent of 

 the sternum and pectoral muscles correspondingly reduces that of the belly-walls, and the 

 abdominal muscles are consequently scanty. Fixity of the spinal column in the dorsal region 

 diminishes the musculation of that part, the spinal muscles being much better developed in 

 the cervical region ; where, in cases of some of the long- necked birds, there are curious con- 

 trivances for the mechanical advantage of the muscle in Hexing and extending this mobile part 

 of the body. Muscles of the hyoidean apparatus acquire a singular development in woodpeckers. 

 The lower jaw is depressed particularly by muscle inserted into the end of the mandible ; the 

 ujiper is elevated by particular muscles operating the pterygoid and quadrate bones. Temporal, 

 masseteric, and ordinary pterygoid muscles close the jaws. They are unsymmetrical in Loxia. 



The diaphragm, the musculo-membranous partition which in mammals divides the thoracic 

 from the abdominal cavity, is only represented in birds in a rudimentary condition. Macgillivray 

 has figured that of the rook as consisting of three fleshy slips, v, v, r, passing from as many 

 ribs, 4, 5, 6, to the pleural sac of the lungs, t, t, in fig. 101, p. 212. It is best developed in the 

 Aj^teryx. 



The remarkable specialization of both limbs, — the former for flight, the latter for the 

 perfectly bipedal locomotion which only birds besides man enjoy, — results in corresponding 

 peculiarities of the muscular mechanism. Muscles beyond the shoulder are greatly reduced in 

 number and complexity from an ordinary quadrupedal standard ; those of the legs are rather 

 increased, and their configuration, relative size, and to some extent their relations are so much 

 changed, that great difficulty is experienced in identifying them with the corresponding muscles of 

 quadrupeds. The result is, great confusion in their nomenclature, which is still shifting, though 

 inucli has been done of late to give it precision. Attention has recently been called by Garrod 

 to the classificatory value of certain muscles of the limbs. The tensor patagii, that muscle or 

 muscles which may have elastic tendons, and by which the folds of skin in the angles of the 

 wing bones are regulated, may have difterent characters in different groups of birds. It has 

 loug been known that particular muscles of the hind limb are in direct and important relation 

 to the prehensile power of the toes, and consequently co-ordinated with the insessorial or the 

 reverse character of the foot. In the highest birds, Passeres, the foot grasps with great 

 facility, owing to the distinctness or individuality of the flexor longus hallucis, or bender of the 

 hind toe. The ambiens (Lat. atnbiens, going around) is a muscle of which Garrod has even 

 made so much as to divide all birds into two primary groups according to whether they possess 

 it or not. The ambiens arises from the ))elvis about the acetabulum, and passes along the inner 

 side of the thigh ; its tendon runs over the convexity of the knee to the outer side, and ends by 



