RIBS are elastic arches of bone. which, with the vertehral column behind. and the sternum or breast-hone in front, constitute the osseous part of the walls of the chest. In man, there are 12 ribs on each side. The first 7 are more direetly connected through iniervenin cartilages with the sternum than the remainder. and hence they are termed rertebro-sternal or true ribs; while the other five are known as false ribs, and the last two of these, from being quite free at their anterior extremities. are termed fottfiragritas. glnuce at a skeleton, or at a plate representing the articulated bones, will show that the ribs vary very considerably both in their direction and size. The upper ribs tire nearly horizontal, but the others lie with the anterior extremity lower than the posterior; this obliquity increasing to the 9th rib, and then slightly decreasing. They increase in length from the first to the eighth, and then again diminish. The spaces between the ribs are termed the intercostal spaces. On examining a rib taken from about the middle of the series. we find that it presents two extremities (a posterior or vertebral, and an anterior or sternal), and an intervening portion, termed the body or shaft. The posterior extrem ity presents a head. a neck, and a tuberosity. The head is marked by two concave articular surfaces divided by a ridge, the lower facette being the larger. These surfaces fit into the cavity formed by the junction of two contiguous dorsal vertebrae, and the ridge serves for the attachment of a ligament. The neck is a flattened portion proceed ing from the head; it is about an melt long, and terminates at an eminence termed the tuberosity or tubercle, from whence the shaft commences. On the lower surface of this tubercle is a small oval surface, which articulates with a corresponding surface on the upper part of the transverse process of the lower of the two contiguous vertebrae. The shaft presents an external convex, and an internal concave surface. A little in front of the tubercle, die rib is bent inward, and at the same time upward, the point where this takes place being called the angle. The upper border of the rib is thick
and minuted, •while the lower border is marked by a deep groove, which lodges the inter costal vessels and nerve.
The ribs of mammals are mostly connect:xi, as in man, with the bodies of two ver tehrre, and with the transverse processes of the posterior one. In the monotremata, however, they articulate with the vertebral bodies only; while in the cetacea, the pos terior ribs hanedown from the transverse processes alone. Their number, on each side, with that of the dorsal vertebrar. The greatest number, 23, occurs in the two-toed sloth, while in the cheiroptera, 11 is the ordinary number. In birds, each rib articulates by means of a small head with the body of a single vertebra near its anterior border, and with the corresponding transverse process by means of the tubercle. Moreover, each rib possesses a diverging appendage," which projects backward over the next rib, so as to increase the consolidation of the thoracic framework, necessary for flying. The dorsal vertebrae here never exceed 11, and are commonly 7 or 8 hr number, and the ribs proceeding from them are connected with the sternum, not by cartilage, as in mammals. but by true osseous sterbal ribs, which are regularly articulated at one end with the sternum, and at the other with the termination of the spinal ribs. In the elle Ionian reptiles, the ribs i(as well as the vertebra and the sternum) deviate remarkably from the normal type, the lateral parts of the carapace consisting mainly of anchylosed ribs united by dermal plates. In the crocodiles, there are only twelve pair of true or dorsal ribs; while in the other saurians, and in the ophidians, the ribs arc usually very DUIller011S. In the frogs. there are no true ribs; the reason probably being, that any bony element in their thoracic walls would interfere with the enormous tho•acico abdominal enlargement which these animals periodically undergo at the breeding period.
In the language of the transcendental anatomists, a rib is to be regarded as a pkura pophysis—one of the elements of a typical vertebra (q.v.).