Page:The American Cyclopædia (1879) Volume II.djvu/672

 652 BIRDS and broad plumes springing from the sides of the hreast. Several other birds, of exquisite plumage, intermediate between the above families, are described by Mr. Wallace. No description can give any idea of the graceful forms and brilliant hues of the paradise birds ; our own beautiful humming birds come nearest to them in fairy-like structure of their plumage, and in the gorgeous, metallic, and ever-chang- ing lustre of their colors. BIKDS (aves), a class of vertebrate biped animals, exclusively oviparous, and with very few exceptions covered with a feathered coat, adapted more or less perfectly for flight. They have frames penetrated through all their parts by air cells, which facilitate motion by impart- ing lightness. By means of nests, which serve as substitutes for internal organs of reproduc- tion, they develop their young after the exclu- sion of the ova. The last two peculiarities distinguish birds from all other animals. The families which have not the power of flight are few both in regard to the number and varieties of species, and to the individuals com- posing them. They are all formed either for motion on the land or in the water exclusively. In all these instances the feathery coverings are incompletely developed, possessing a proximate resemblance to the hairy covering of certain land and water animals. The ostrich and the penguin may be named as typical of these two distinct forms of exception, both in regard to their inability to raise themselves into the air and their exceptional hair-like plumage. In the internal organization of the entire class of birds there are other and more noticeable an- atomic peculiarities. Their skulls are without the sutures that are found in mammalia, form- ing consolidated bones. These are joined to the neck or spinal column by a joint, so con- structed as to give freedom of motion in hori- zontal and lateral directions, without danger of dislocation or injury. In the place of teeth they have upper and lower jaws, forming unitedly the bill, and composed of a hard horny substance. In several families of birds, as the parrots, the upper part of the bill is articulated with the skull. More commonly the skull and upper jaw are united by means of an elastic bony plate, by the interposition of which the brain is protected from injuries to which it would otherwise be exposed. The upper extremities of birds, homologous with the arms or fore legs of other animals, differ essentially in never being used as prehensile organs, or for motion in contact with the earth, as in walking or running. Their use is almost exclusively for flight, and they serve as the basis of the wings. The cervical vertebrae of birds are more numerous than those of mammals. In the latter their number is uniformly Y, while in birds there are never fewer than 10, and in some instances there are as many as 23. The dorsal vertebra are more fixed and limited in their motion than the cervical, and are usu- ally 10 in number, rarely 11, and in some in- stances only 7 or 8. The pelvis in birds is a simple elongated plate, open below, terminated by the rump, which supports the tail feathers. PARTS OF A BIRD. 1. Skeleton. 2. Nictitating Membrane. 8. Brain. 4. Sternum or Breast Bone. The breast bone or sternum is perhaps the most noticeable feature in the bony skeleton of birds. It is also one of the most important parts of the osseous framework, as it forms the base for the insertion of the most powerful of the muscles of flight. Its prolongation or crest determines with infallible accuracy the degree of power of flight of its possessor, and is en- tirely wanting in those destitute of the power of raising themselves in the air. The merry- thought (fercula) should be here mentioned as another peculiarity in birds of flight, and want- ing only in those not possessed of that power. The bony framework of the lower extremities comprises a thigh bone, two leg bones, a meta- tarsal or ankle bone, and the bones of the toes. The last vary in number, arid terminate in nails, of greater or less importance in the an- imal economy, according to the habits of the family possessing them. The variations in the mechanism of the lower extremities are often very curious and striking. The birds which roost, and more especially those which are in the habit of standing long at a time upon one leg, are enabled, by the remarkable arrange- ment of the bones and the muscles attached to them, to do either with very little effort or fa- tigue. As might be expected, in birds of vig- orous flight we find the pectoral muscles pre- senting the greatest development. These often exceed all the other muscles in weight and bulk. The great pectoral and the middle pec- toral are antagonistic forces, alternately de- pressing and elevating the wings, while the small pectorals, or third pair, aid in varying the manner and character of the flight. The mus- cles of the lower extremities vary greatly with the habits of the bird, and especially according