74 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



which form a canal for the transmission of the great blood-ves- 

 sels of the tail. 



The ribs, which are movably attached to the backbone, 

 together with the dorsal vertebrae and breast-bone, compose 

 the thorax, or chest. The articulation with the vertebrae is 

 by means of a rounded head ; in most cases the head has two 

 distinct facets, the pit being formed half on 

 the hinder border of one dorsal vertebra and 

 half on the front border of the next suc- 

 ceeding one, but posteriorly the pit is often 

 shifted, so as to be on a single vertebra. A 

 second articulation is by means of the tu- 

 bercle, a smooth projecting facet on the con- 

 vexity of the rib's curvature and near the 

 head; the tubercle articulates with the 

 transverse process of its vertebra. The ribs, 

 in general, are curved bars of bone, which 

 in small mammals generally and in the 

 clawed orders are slender and rod-like, while 

 in the hoofed mammals they are broader, 

 thinner and more plate-like, especially the 

 anterior ones. The number of pairs of ribs 

 is most commonly 13, but ranges among 

 existing mammals from 9 in certain whales to 24 in the Two-toed 

 Sloth (Choloepus didactylus) . The complex curvature of the 

 ribs, outward and backward, is such that, when they are drawn 

 forward (in Man upward) by muscular action, the cavity of 

 the thorax is enlarged and air is drawn into the lungs, and 

 when they are allowed to fall back, the cavity is diminished 

 and the air expelled. 



Below, a varying number of the ribs are connected by the 

 cartilages in which they terminate with the breast-bone 

 (sternum) ; sometimes these cartilages are ossified and then 

 form the sternal ribs, but there is always a flexible joint between 

 the latter and the true ribs. In certain edentates, notably 



Fig. 18. — Ribs of Wolf 

 from anterior and 

 middle parts of the 

 thorax, cp. , head. t. , 

 tubercle. 



