THE HUMAN STERNUM 57 



metasternum, or more rarely with the metasternum alone. The so-called 

 metameric arrangement fails through the mode of attachment of the penul- 

 timate rib and the presence of a rib in excess of the number of mesosternal 

 ' segments.' 



It has been shown that only in exceptional cases {e.g., Edentata) does 

 a true alternation of mesosternal segments and costal articulation occur. 

 There are, on the other hand, orders in which specialization of the sternum 

 occurs in the opposite direction, leading to equally exceptional conditions. 

 Such examples occur among Ornithodelphia, Sirenia, and Cetacea. The 

 arrangement of the ribs and mesosternum in Ornithorhynchus and Echidna 

 has already been discussed. In Sirenia, Parker" describes the sternum of 

 the Manatee as representing the presternum alone, and receiving the attach- 

 ments of only the second pair of ribs. In two examples in the Liverpool 

 IVTuseum, a sternum of a simple type occurs, with a pair of bilateral forward 

 projections and a pointed posterior extremity. The first ribs are cervical, 

 and the sternum receives the second and third pairs (PI. X, Fig. 67). 

 From Parker's description of the Dugouts sternum, which I have not had 

 an opportunity of examining personally, it is evident that there is a com- 

 plete sternum, comprising presternum, mesosternum (of three segments), 

 and metasternum. Yet there appear to be only two or three pairs of 

 sternal ribs. 



Among Cetacea, in the Dolphin there is no metasternum. The 

 presternum receives one-and-a-half pairs of ribs, and the mesosternum, 

 composed of two elements, articulates with two-and-a-half pairs of ribs. 

 There are, in two examples, thirteen and fifteen pairs of ribs respectively 

 altogether, and in both cases four pairs of sternal ribs. In Balaena mysticetus 

 a simple sternum occurs (Parker) with a pointed posterior extremity, and 

 only one pair of ribs articulating with it. 



Examples such as these are as much at variance with the notion of a 

 metameric sternum with a costal origin as the Edentate sternum is in 

 harmony with it. Reviewing the whole series, and bearing in mind the 

 general mammalian arrangement and the exceptional conditions of an 

 opposite character which occur, one is forced to the conclusion that, while 

 the differences in costal attachments are probably responsible for differences 



