The thoracic cavity or chest in Mammals is always enclosed by a series of ribs, the number of which varies with that of the dorsal vertebrae. In most cases each rib articulates by its head with the bodies of two vertebrae, and by its tubercle with the transverse process of one of these vertebrae (the lower one). In the Monot?'emata (e.g., the Duck-mole), the ribs articulate with the body of the vertebra only; and in the Whales, the hindermost of the ribs, or all of them, articulate with the transverse processes only, and not with the centra at all.
There are usually no bony pieces uniting the ribs with the sternum or breast-bone in front, as in Birds; but the so-called pieces placed one behind the other, but usually anchylosed together to form a single bone. It is placed upon the ventral surface of the body, and is united with the vertebral column by the ribs and their cartilages. It is generally a long and narrow bone, but in the Cetacea it is broad. It is only in some burrowing animals (such as the Moles) and in the true flying Mammals (the Bats), that the sternum is provided with any ridge or keel for the attachment of the pectoral muscles, as it is in Birds. The sternum is primitively composed of three pieces, an anterior piece or praesternum, a middle piece or mesosternum, and a posterior piece or xiphisternum. The praesternum is the "manubrium sterni" of human anatomy, and is the portion of the sternum which lies in front of the attachment of the second pair of ribs. All the other ribs are connected with the mesosternum. The xiphisternum is the "xiphoid cartilage" of human anatomy, and it commonly remains throughout life more or less unossified. In the Mono-tremes there is a T-shaped bone above or in front of the praesternum, but this is to be regarded as belonging to the shoulder-girdle, and as representing the "episternum" or "interclavicle" of the Reptiles.
"sternal ribs" of Aves are represented by the "costal cartilages" of the Mammals. In some cases, however, the cartilages of the ribs do become ossified and constitute sternal ribs. Sometimes, as in the Armadillos, there is a joint between the vertebral ribs and sternal ribs. More rarely, as in the Mono-tremes (fig. 357, D), an intermediate piece is found between the vertebral and costal portions of the rib. Only the anterior ribs reach the sternum, and these are called the "true" ribs; the posterior ribs, which fall short of the breast-bone, being known as the "false" ribs.
The sternum or breast-bone (fig. 353) is formed of several
Fig. 353. - A, Sternum of Man, with the costal cartilages. B, Sternum and costal cartilages of the Dog: p Presternum; m Mesosternum; x Xiphisternum.
The normal number of limbs in the Mammalia is four, two anterior and two posterior; and hence they are often spoken of as "quadrupeds," though all the limbs are not universally present, and other animals have four limbs as well. The anterior limbs are not known to be wanting in any Mammal, but the posterior limbs are absent in the Cetacea and Sirenia.
As regards the structure of the anterior limb, the chief points to be noticed concern the means by which it is connected with the trunk. The scapula or shoulder-blade is never absent, and it is in the form of a broad flat bone (rarely long and narrow), applied to the outer aspect of the ribs, and much more developed than in the Birds. The coracoid bone, which forms such a marked feature in the scapular arch of Aves, is fused with the scapula, and only articulates with the sternum in the Duck-mole and Echidna (Monotremata). In all other Mammals the coracoid forms merely a process of the scapula, and does not reach the top of the breast-bone. The collar-bones or clavicles never unite in any Mammal to form a "furculum," as in Birds; but in the Monotremes they unite with an " interclavicle," placed in front of the sternum. The clavicles, in point of fact, are not present in a well-developed form in any Mammals except in those which use the anterior limbs in flight, in digging, or in prehension. The Cetacea, the Hoofed Quadrupeds (Ungulata) and some of the Edentata, have no clavicles. Most of the Carnivora and some Rodents possess a clavicle, but this is imperfect, and does not articulate with the top of the sternum. The Insectivorous Mammals, many of the Rodents, the Bats, and all the Quadrumana, have (with Man) a perfect clavicle articulating with the anterior end of the sternum.
The humerus, or long bone of the upper arm (brachium), is never wanting, but is extremely short in the Whales, in which the anterior limbs are converted into swimming-paddles. In many Mammals, as in the Monkeys, and Felidae (constituting the most typical group of the Carnivora), the median nerve and brachial or ulnar artery are protected on their way down the arm by a canal placed a little above the elbow, and formed by a process - the "supra-condyloid" process - which is sometimes present in man as an abnormality.
In the fore-arm of all Mammals the ulna and radius are recognisable, but they are not necessarily distinct; and the radius, as being the bone which mainly supports the hand, is the only one which is always well developed, the ulna being often rudimentary. In the Cetacea the ulna and radius are anchylosed together; and in most of the Hoofed Quadrupeds they are anchylosed towards their distal extremities. In the flying Mammals or Bats the ulna is hardly recognisable, being reduced to its upper third and fused with the radius. The forearm attains its greatest perfection in man, in whom the radius can rotate upon the ulna, so as to allow the back of the hand to be placed upwards or downwards, these movements being known respectively as "pronation" and "supination." In the Monkeys only is there any approach to this power of rotation.