OSSEOUS SYSTEM. 11 



lingers. A universal characteristic of the vertebrae of a mammifer- 

 ous animal is this : the anterior and posterior surfaces of their 

 bodies are either flat or slightly concave, and collected together by 

 ligament. It is rare for the cervical vertebrae to have, as in the 

 Horse, an articulating cavity posteriorly, and in front a very convex 

 head. 



The Ribs correspond in number with that of the dorsal vertebrae. 

 They are for the most part long, flat, and sometimes very broad 

 from before backward, as in some Edentata, e. g. Myrmecophaga 

 didactyla, where they are in contact in that direction, and even 

 overlap each other like tiles, so as to form a kind of coat of mail ; 

 occasionally, however, they are very small and rounded, as in Mana- 

 tus. The ribs are mostly connected, as in Man, with two vertebrae, 

 and their transverse processes ; in the Monotremata, however, they 

 articulate only with the vertebral bodies. In the Cetacea the pos- 

 terior ribs hang down from the transverse processes alone. In front, 

 the ribs are furnished with their costal cartilages, which in some 

 orders, as the Edentata (also in the Cheiroptera and Cetacea), have 

 a great tendency to become soon converted into bone, and thus into 

 a series of sternal ribs, as is constantly the case in birds. The num- 

 ber of true ribs (those which are attached to the sternum) is usually 

 greater than that of the false, though the Cetacea have far more of 

 the latter ; the Whales have in fact, only one or two true ribs. The 

 Seals, on the contrary, have the greatest number of true ribs. In the 

 Monotremata, the anterior rib-bones are attached by distinct capsular 

 joints to the sternum, and the last costal cartilages are expanded into 

 broad thin plates. 



The Sternum is very generally divided into three portions, the 

 middle one of which, or the body, in place of being represented by 

 a single piece, as in the adult human subject, usually consists of 

 as many pieces as there are true ribs present. In most cases even 

 in the Cetacea, the sternum is broad and compressed from before 

 backward, but more rarely in the lateral direction ; it is very short 

 in the Cetacea, very long in the Carnivora and Edentata. The 

 manubrium sterni presents considerable differences, but generally 

 receives the clavicles, when present, and the first two ribs. It is 

 very broad and conspicuous in the Edentata, and in the Cheiroptera 

 and Monotremata is prolonged into a transverse process, so that it 

 has the form of a T. In the Cheiroptera, the Armadilloes, and the 

 Mole, there is a crest upon the antero-inferior surface of the manu- 

 brium for the attachment of the large and powerfully developed pec- 



