58 THE HUMAN STERNUM 



in the form of the sternum, by affecting its mode of ossification and 

 influencing the conversion of the embryonic structure into a homogenous 

 or a segmented bone, these differences do not affect its essential structure, 

 as a longitudinal, ventral, thoracic axis. 



The mammalian metasternum. 



The metasternum is not constantly present in mammalia. It is absent 

 in Ornithorhynchus, present in Echidna. It is generally present in Edentata 

 but is absent in Unau (Parker), and in the three-toed and two-toed Sloths. 

 In Sirenia it is present in the Dugong (Parker), but it is not differentiated 

 — if present — in the Manatee. It is generally present in Ungulata, but is 

 absent in the Tapir. Among Cetacea, it is not present in the Dolphin, and 

 is absent or undifferentiated in Balaena and Balaenoptera. 



The mammalian metasternum is always median in position, elongated 

 as a rule, and either broadened out and capped with cartilage at its free end 

 (Rodentia, Chiroptera, Carnivora, Marsupials) or pointed and angular 

 (Echidna, Ungulata). The occasional bifurcation of the metasternum met 

 with in man does not appear to occur in other mammals. 



The position, form, ossification, and occasional absence or want of 

 differentiation of the metasternum, make it difficult to accept the view of its 

 metameric origin from the ventral ends of the ribs. It is essentially median, 

 and a backward prolongation of the ventral thoracic axis. 



Summary 



A survey of the differences in the character of the sternum in the 

 different classes of vertebrates enables one to form a clearer opinion of its 

 morphological position. Its peculiarities appear to be associated, to a certain 

 extent, with differences in the mechanism of respiration, but to a much 

 greater degree with differences in the functions of the fore limb. 



In simple types of Fishes (such as Elasmobranchs) the place of the 

 sternum is occupied by the ventral elements of the shoulder-girdle, which 

 may, as in the dog-fish or skate, form a continuous bar extending across 

 the middle line beneath the heart. With one exception {Notidanus), how- 

 ever, no fish possesses a sternum proper, and the heart is protected and 

 supported by the shoulder-girdle. 



