56 THE HUMAN STERNUM 



or deficient, and the fore limb is adapted to quadrupedal locomotion. This 

 type of sternum, therefore, is associated with the need of pliancy and elasticity 

 in lateral movement of the trunk. 



The mesosternum is comprised of a number of ' segmeats,' usually 

 median and intercostal, and separated from one another by plates of cartilage 

 or by fibrous laminae. The extreme example of this form of mesosternum 

 occurs in the polymorphous Edentata, in which, as a rule, the mesosternal 

 segments regularly alternate with costal attachments (PI. IX, Fig. 6t,). 

 Even here, however, exceptions may occur (^Manis longicaudd) in which there 

 are five mesosternal segments giving attachment to seven and a half sternal 

 ribs (Parker). 



This regular alternation of mesosternal segmentation and costal 

 articulation I have found only exceptionally in other orders, e.g.^ in Cerco- 

 kptes caudivolvulus, Nasua narica, and Mustela 'vulgaris, among Carnivora ; 

 in Paca, Dasyprocta, Atherura africana, among Rodents ; in an exceptional 

 example of Bos indicus among Ungulata ; in an example of Ateles Goejjroyi ; 

 and in a Chimpanzee with only six sternal ribs. 



In most orders the number of mesosternal segments is less than the 

 number of associated ribs, although at the same time there is undoubtedly 

 a general correlation between the two, and an increase in the number of 

 sternal ribs is co-incident with an increase in the number of mesosternal 

 segments. In the rat, for example, there are four mesosternal segments 

 and seven sternal ribs : in the rabbit, five mesosternal segments and eight 

 sternal ribs. I have notes of three examples in the Orang-outan, pos- 

 sessing three mesosternal segments and six pairs of sternal ribs ; and of 

 five examples in which there are four mesosternal segments and seven pairs 

 of sternal ribs. 



While Mammals exhibit considerable variability in the relative 

 number of mesosternal segments and ribs, there is one important point of 

 general agreement. There is usually one mesosternal rib in excess of the 

 number of mesosternal segments (PI. IX, Fig. 64). The penultimate 

 sternal rib is attached to the side of the last mesosternal segment, or along 

 with the last sternal rib to the junction of mesosternum and metasternum. 

 The last sternal rib articulates either with the junction of mesosternum and 



