54 THE HUMAN STERNUM 



that, although the mesosternum is segmented, there is no metameric co- 

 ordination between it and the ribs. 



The mammalian presternum. 



The presternum is the one element of the mammalian sternum which 

 is constantly present. It is sometimes the sole representative of the bone, 

 as in some Cetacea {Balaenoptera, Balaend), and in the Manatee. In 

 Balaena (Turner) only one pair of ribs reaches the sternum. In the 

 Manatee (PI. X, Fig. 67), two ribs articulate with it on each side ; and it 

 possesses a bi-lobed projection in front of the articulation and a pointed 

 process behind it. 



Usually the presternum articulates with the first and half the second 

 pair of ribs. Besides the Manatee, exceptions to this rule are Tamandua, 

 Beaver, Coypu, and Hylobates (two and a half ribs), and Echidna and Tamandua 

 bivittata (two ribs). 



The presternum is usually composed of a single bony element. A 

 remarkable exception is found in Hylobates, in which it is normally made up 

 of two elements, one in front of the other, and receiving two and a half 

 pairs of ribs (corresponding to a condition which occurs occasionally in 

 man, as observed already, by inclusion in the presternum of the first piece 

 of the mesosternum). 



The form of the presternum varies considerably. That characteristic 

 of the human sternum occurs among Primates, but only exceptionally in 

 other orders, as among some Rodents [Pteromys volucella, Arvicola amphibia, 

 and in Centetes among Insectivora. 



In those orders where the shoulder-girdle is deficient and the clavicle 

 is absent (as in Ungulata), the presternum becomes considerably reduced in 

 size : in contrast to the mesosternum, which is greatly increased in width, 

 particularly in its posterior portion. On the other hand, in exceptional cases, 

 the presternum is keeled on its ventral surface in a more or less marked 

 degree. This character has been noted in the lesser Rat Kangaroo 

 {Hypsiprymnus minor), Tree Kangaroo {Dendrolagus inustus). Vulpine Phalanger 

 ( Trichosurus vulpinus), Hystrix cristata, Cheroptera, Mole, and Aye aye. 



Special attention has been given to the form of the anterior end of 



