Page:The American Cyclopædia (1879) Volume V.djvu/179

COMPARATIVE ANATOMY 175 lower classes, birds differ from each other only slightly in their skeletons ; the bones of the neck, back, chest, and extremities are remark- ably similar in all ; they vary chiefly in the shape and size of their bills, the form of the feet, and the proportions of their bodies. Their bones are white, compact, fragile, and hollow for the introduction of air. The cervical ver- tebrse are the most numerous ; their bodies lock into each other so as to allow a forward motion in the upper and lower ones, and a backward motion in the middle ones ; the transverse processes are long, and have rudi- mentary ribs, especially developed in birds of prey. The bodies of the dorsal vertebrae are short and compressed laterally, and have large transverse processes; they are generally im- movable in birds of powerful flight. The sacral vertebrae are firmly united together, and with the pelvis, in order to give support to the superior parts of the body in flight, and a suffi- cient base for the lower extremities. The bones of the tail are broad and short, penetrated by the spinal cord except the last, which is com- pressed, supporting the tail feathers and an oil gland. The skull is united into a single box at an early period ; the brain cavity is very much larger than in the reptiles and fishes, and is occupied fully by the nervous centre ; the brain of a sparrow is 100 times greater than that of a large marine turtle, in proportion to the size of the animals. The most striking characteris- tic of the class of birds is that the anterior and posterior extremities are entirely dissimilar in appearance and function, though the ana- tomical structure of the wings and legs is the same. The perfect condition of the chest in- dicates the energy of the respiratory system, and the consequent muscular activity. The. ribs are strong, attached to the sternum in front by bony continuations instead of elastic cartilages, movable only at each end, articu- lated to the sides of the vertebrae in the fly- ers, but to the intervertebral cartilages in the runners ; in those which require great solidity of the chest for powerful flight, each rib is strengthened by bony splints running obliquely upward and backward to the succeeding bone, to which it is attached by strong ligaments. The sternum is provided with a keel or crest in front for the attachment of the muscles of the wings, large in proportion to the flying power ; in the ostrich, which does not use its wings to rise in the air, this bone is flat as in man ; in some aquatic birds the breast bone contains several convolutions of the windpipe within its cavity. The shoulder blade is paral- lel to the spine, long and narrow, at the glenoid cavity articulating with the clavicles and cora- coid bones. The coracoid bones, or posterior clavicles, are strong, extending from the glenoid cavity to a transverse groove in the anterior portion of the sternum ; these are called the collar bones by some of the older authors. The true clavicles, the "merrythought" or " wishbone," vary considerably in size, being 217 VOL. v. 12 sometimes quite rudimentary, and in other spe- cies strong arches reaching the sternum. The wing consists of a humerus, radius, and ulna, carpus and metacarpus of two bones each, a small single-jointed ulnar finger, a two-jointed radial finger, and a rudimentary thumb. Prof. J. Wyman has shown in the anas acuta, or pin- tailed duck, an arrangement of the bones and ligaments by which all the segments of the ex- tended wing are retained in a fixed position in- dependently of muscular action, and the flexion or extension of the hand on the forearm wben the latter is flexed or extended on the arm ; during flexion and extension the radius advan- ces and recedes upon the ulna, carrying with it the upper bone of the carpus, and with this last the hand ; when the upper carpal bone, to which the lower is attached firmly, is drawn over the end of the ulna as the radius recedes, the lower is drawn up between the hand and the end of the ulna, maintaining the hand ex- tended, like a wedge, until it is displaced by the reversed action of the radius. ( u Proceed- ings of the Boston Society of Natural History," vol. v., p. 169.) In the lower extremity the iliac bones correspond to the shoulder, the thigh bone to the arm, the leg to the forearm, and the foot to the hand. Though one be covered with feathers and the other bare, the wings and legs present the same analogies that may be traced between the fore and hind legs in all the vertebrata. Form. In the mammalia, or animals bringing forth living young and nour- ishing them with milk, there is a great variety of form. Man, the horse, the whale, the seal, and the bat, notwithstanding their dissimilar external appearance, exhibit in their skeletons and internal structure homologies which show that they belong to the same class ; the whale is nearer to man than to the fish, and the bat is more human than bird-like. The form of the skull varies much in mammals, according to the development of the brain cavity, and the proportions of the face. A direct relation is found to exist between the size of the cranium and face and the intelligence of an animal. In man the cranium, or brain cavity, is directly over the face ; and in proportion as the former retreats behind the latter, and as the face is pro- longed forward, the animal propensities pre- ponderate over the intelligence. The differ- ences between human crania in this respect early attracted the attention of anatomists, and Camper measured them by what is called the facial angle; this is formed between a line drawn horizontally from the opening of the ear to the upper teeth, and a line drawn vertically from the forehead to the same incisors. In the white races this angle is from 85 to 80, thence decreasing in the other races as low as 65. In some of the antique statues this angle is made 90 and in one case 100, which last never ex- isted unless in disease. In children, the fore- head is more prominent than in adults, and their facial angle is usually 90 ; this explains their generally pleasing countenances as well