144 



MAMMALIAN ANATOMY 



VARIATIONS IN RIBS. 

 VARIATIONS IN SIZE AND DEVELOPMENT. 



The size and degree of development of the ribs are, of course, 

 largely dependent upon the size and strength of the entire skeleton. 

 Within comparatively narrow limits a rib may, however, exhibit 

 variations which are apparently entirely independent of the size of 

 the animal. The fifth rib, for example, is always longer than the 

 fourth and shorter than the sixth, but the degree of increase or de- 

 crease in its length is by no means constant. 



The following table shows the size of the ribs of nine specimens. 

 The measurements were made by placing the rib on a card ruled in 

 square millimetres, with the caudal aspect uppermost and the curvature 

 away from the observer ; the length (L) represents the extreme dis- 

 tance on a straight line from the vertebral to the sternal end ; the 

 degree of curvature (C) is the shortest distance from this line to 

 the point of maximum convexity wherever that falls on the concave 

 side of the rib. It is probable that the relation between the curvature 

 and the length is to some extent dependent upon the effects of drying 

 after maceration. The width (W) is the greatest diameter of the 

 sternal end of the bony rib. 



