138 MAMMALIAN ANATOMY 



the head than toward the median line. In this way it is that while 

 the ribs increase in length and, to some extent, in the degree of dorso- 

 ventral curvature, the thorax increases in dorso- ventral depth, but 

 only slightly in transverse width. It must be remembered that the 

 positions of the ribs vary in respiration and with the motions of the 

 trunk and limbs. 



SPECIAL CHARACTERS OF THE DIFFERENT RIBS. 



The First Rib (Fig. 97) is the shortest, and, relatively to its length, 

 the stoutest. It is less curved, and presents no distinct angle. It is 

 also less flattened, its surfaces and borders being more rounded. It is 

 hardly twisted on its long axis. 



The head is large, and, since it fits into a single articular facet 

 on the body of the first thoracic vertebra, its articular surface is not 

 divided by a crest and pit into two facets. The neck is long and stout, 

 and forms an angle of more than forty-five degrees with the vertebral 

 part of the body ; the tubercle is very high. Its articular facet faces 

 dorsally and medially, and, while convex from the cephalic to the 

 caudal edge, is flat from side to side. The terminal surface for the 

 costal cartilage is circular. 



The Second Rib (Fig. 97) may be distinguished from the first by 

 its greater size and its flatter and more twisted form. 



The head is smaller, and its articular surface is divided by the crest 

 and pit into two facets. The neck is more slender. The tubercle is 

 smaller and less prominent, and its articular surface is not so decidedly 

 convex from the cephalic to the caudal edge, and is faintly concave 

 from side to side. There is an angle, two or three millimetres below 

 the tubercle. The surface for the costal cartilage is oval. 



The Third Rib (Fig. 97) is about a fourth longer and considerably 

 wider than the second rib. It is more strongly bowed, and is twisted 

 to a greater degree on its long axis. 



The head is not so spherical. The neck is stouter and less ex- 

 cavated on the dorsal surface. The tubercle is lower, and its articular 

 surface is small and oval. The angle of the rib is pronounced. The 

 sternal end is compressed, so that its terminal surface is a long oval. 



The Fourth Rib (Fig. 98) is a fourth longer than the third, and 

 a trifle more slender. Its dorso-ventral curvature is greater, and its 

 sternal end bends slightly toward the tail. 



