Bones of the Lower or Posterior Extremities.—These consist of a thigh-bone (femur); leg-bones (tibia and fibula), for there are two, though the fibula is very small, and becomes anchylosed to the tibia ; one metatarsal bone (at the lower end of which there are as many processes as there are toes, each process furnished with a pulley for moving its corresponding toe) ; and the toes. Of these, three generally are directed forwards and one backwards. This back toe, or great toe, is wanting in some birds In the swallows it is directed forwards ; in the climbing birds the outer toe as well as the back toe are directed backwards. The number of joints is generally progressive ; the back too has 2, the next 3, the middle too 4, and the outer toe 5 joints.
"The stork, and some others of the Grallte (Waders)," says 3facartney, "which sleep standing on one foot, possess a curious mechanism for preserving the leg in a state of extension, without any or at least with little muscular effort. There arisen from the fare part of the head of the metatarsal bone a round eminence, which pareses up between the projections of the pulley, on the anterior part of the end of the tibia. This eminence affords a sufficient degree of resistance to the flexion of the leg to counteract the effect of the oscillations of the body, and would prove an insurmountable obstruc tion to the motion of the joint if there were not a socket within the upper part of the pulley of the tibia to receive it when the leg is in the bent position. The lower edge of the socket is prominent and sharp, and presents a sort of barrier to the admission of the eminence that requires a voluntary muscular exertion of the bird to overcome, which being accomplished, it slips in with some force like the end of a dislocated bone." Muscles of Motion and External Infevnmvnfa.
" The muscles," Writes Blumenbach, " in this class are distinguished by possessing a comparatively weak irritable power, which is soon lost after death ; and by their tendons becoming ossified as the animal grows old, particularly in the extremities, but sometimes also in the trunk." The pectoral ninnies, as we might expect from the form of the sternum, exhibit, generally speaking, the greatest development They are three in number, taking their rise chiefly from the ample breast bone, and all being brought to bear on the head of the arm (humene4). Of these, the first or great pectoral is said, as a general proposition, to weigh more than all the other muscles put together. Rising from the keel or erect of the breast-bone, the merry-thought, and last rib++, it is inserted in that rough linear elevation which may be observed on tho bone of the arm of most birds. This bone it strongly depresses, and so produces the rapid and powerful motions of the wing, which, acting on the surrounding air, carries the bird forward in its flight. An an antagonist to the great pectoral muscle, the middle pectoral, which lies under it, and whose office it is to elevate the wing, puts forth its tendon over the point where the merry-thought is joined to the clavicle and shoulder-blade. This point of junction acts as a pulley for the tendon which is inserted in the upper part of the bone of the arm ; and by this contrivance the elevating power is situated on the lower surface of the body. The third or email pectoral, aide the great pectoral in depressing the wing. Thus some birds are enabled to dart away with the rapidity of an arrow, while others soar to a height invisible to the gaze of man.
',Ve have already seen that the pelvis is prolonged backwards to a considerable extent This formation furnishes room for the attach ment of the mnscles which set, the posterior extremities in motion, and enables them to perform the functions of walking, hopping, swimming, climbing, and perching. To this end there are a set of
muscles which go from the pelvis to the toes. One of the flexor or bending tendons given off from a muscle which comes from the bone of the pubis runs in front of the knee, and all the flexors go behind the heel, so that the mere weight of the bird will bend the toes. Any one may satisfy himself that thin operation in purely mechanical, and not the result of muscular action, by making the experiment on a dead bird ; when he will find that the flexion of the knee and heel will at once bend the toes. This admirable contrivance, useful as it generally is, shows itself in the most striking manner when brought to bear on the limbs of those birds which roost in trees. When all the voluntary powers are suspended, such a bird enjoys the most profound repose, and the most secure position on its perch, without an effort.
The integuments of birds are composed of the same parts as those of the rnammifera, with the addition of feathers, the peculiar covering common to the whole class. The beak is covered with horn, and at its base, an in the birds of prey, there is a fleshy part called the core. The lower extremities are protected above by a scaly skin, and the bottom of the foot and toes by a callous modification of the same integument Some, the turkey for instance, are furnished with hair in certain situations. The feathers vary infinitely. When a bird has just left the egg its covering is a downy kind of hair, several little bundles taking their rise from one common bulb. This is the origin of the future feather. A dark cylinder soon makes its appearance, from the upper extremity of which the sprouting feather emerges, while the lower extremity receives the blood-vessels which supply the vascular nourishing pulp of the barrel. When this pulp has per formed its office, and the stalk and other parte of the feather are fully developed, it shrivels up into the well-known substance which every one finds in a quill when he cuts it for the purpose of making a pen. The details of the development of the feathers are highly interesting, and have been described at length by F. Cuvier in the 316moire4 du Museum,' tom. xiii., and also in the article ' Ares' in the Cyclopredia of Anatomy and Physiology.' The care which nature takes for the development of that particular part of the plumage first which the wants of the particular bird demand is remarkable. A young partridge runs off as soon as it is hatched to pick up the pups of the ant, which tho parent bird scratches up for it Some time elapses before it is necessary that it should fly; we accordingly find that the body from the moment of its birth is protected with a close-act downy covering, while all the strength is thrown into the thighs, legs, bill, and neck. The wings are gradually developed afterwards. A young thrush or a young blackbird is hatched nearly naked, and while its body presents only a few scattered bunches of weak downy hairlike feathers, great progress maybe observed in the formation of the quills and other wing-feathers ; because from the habits of the bird it is necessary that it should be able to fly as noon as it leaves the met As a general rule the plumage of the cock bird far exceeds in bril liancy that of the hen ; and in all such cases the young at first put on the more sombre garb of the mother. When the cock and lieu are without much difference in this respect, the young have a particular distinguishing plumage of their own.