THE ANATOMY OF BIBDS. — MYOLOGY. 
193 
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 nmscles, which operate the wings. The 
great pectoral, p, major or p. 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- 
ing 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 downward 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 upivard 
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 flexing 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 
upper 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 represeijted in birds in a rudimentary condition. Macgillivray 
has figured that of the rook as consisting of three fleshy slips, v, v, v, passing fi'om as many 
ribs, 4, 5, 6, to the pleural sac of the lungs, t, t, in fig. 101, p. 206. It is best developed in the 
Apteryx. 
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 
much 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 difi'erent characters in difi'erent groups of birds. It has 
long 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. amhiens, 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 pelvis 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 
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