THE HUMAN STERNUM 55 



the presternum. It appears to be much commoner for it to project 

 forwards in front of the first rib than not : and this forward projection 

 may be a single median spine, or may be bi-lobed. There is no forward 

 projection in Ungulata as a rule (except in the Peccary, Gazelle, Chamois 

 (PL IX, Fig. 62), and Ox, in which there is a slight median projection) ; 

 nor in sterna of the human type (Primates, Tamandua^ Myrmecophaga jubata 

 (PL IX, Fig. 63), Centetes, a.nd flying Squirrel). 



The process in most mammals is single and median, and gives attach- 

 ment to the clavicles (when present). It occurred in a characteristic form 

 in all the Carnivora examined (PL IX, Fig. 64), and, usually, also in 

 Rodents. It was found in most Marsupials (JDasyurus mangoei, Wombat, 

 Kangaroo, Hypsiprymnus minor, tree Kangaroo, vulpine Phalanger, and Koala) ; 

 among Edentata in the climbing Ant-eater, three-toed Sloth, and giant 

 Armadillo ; among Ungulates in the Gazelle, Peccary, Chamois, and Ox ; in 

 the Mole and Elephant ; and in Hyrax, in which case it is tri-lobed. 



A bilateral projection in front of the presternum is less common. 

 It was present in Petaurus, Bandicoot, Porpoise, Manatee (PL X, Fig. 67), 

 Tatusia, Armadillo, two-toed Sloth, Water Rat, and Java Loris. 



The mammalian mesosternum. 



The mesosternum may be absent or indistinguishable from the pre- 

 sternum {e.g.. Manatee (PL X. Fig. 67), Balaend). In Mammalia it is the portion 

 of the sternum in which the most striking differences occur. In the embryo it is 

 at first a simple bar of cartilage in which ossification (endo-chondral or 

 periosteal) occurs, so as to produce in the adult of different orders two 

 extreme types — the segmented and the unsegmented. The simple 

 unsegmented type of sternum is associated with a powerful development 

 of pectoral muscles, and with a specialization of structure of the shoulder- 

 girdle. It occurs in Man, Orang-outan, and Gibbon (I have not met with 

 an example of an unsegmented mesosternum in the gorilla or chimpanzee) ; 

 in Cetacea (in the Porpoise the mesosternum is fused with the presternum and 

 metasternum), Cheroptera, and in the Mole. 



The segmented type of mesosternum is characteristic of animals in 

 which the shoulder-girdle is reduced to its simplest form, the clavicle is absent 



