140 HISTORY OF THE HUMAN BODY 



point. One of the sternal elements, usually the largest, lies 

 in the fourth myocomma, in close connection with the over- 

 lapping coracoids, and as at the same point in higher salaman- 

 ders there is a definite sternal plate of a rhomboid shape, this 

 latter has evidently developed from the element in question, 

 while the others have been lost. This must also be the same 

 piece found in frogs and other tailless amphibians, again in 

 the same relationship to the coracoids, and entering into a 

 more or less complete connection with the two halves of the 

 shoulder-girdle in forming the skeletal armature that covers 

 the pectoral region. As the ribs of all amphibia are very short 

 and rudimentary, and do not reach even half way around the 

 body, there is never the slightest attempt at a connection be- 

 tween them and the sternal piece, a characteristic that defi- 

 nitely distinguishes this archisternum from the neosternum 

 of the Amniota. This last organ, the second form of 

 sternum, is characteristic of reptiles, birds, and mammals, 

 and is not only always connected with several pairs of 

 thoracic ribs, but undoubtedly owes its origin to them, being 

 probably due to the fusion of the ribs in the mid-ventral 

 line. This fusion forms in reptiles and birds a flat plate, 

 especially extensive in the latter, where it serves as a place of 

 origin for the enormously developed muscles of flight, but in 

 the mammals the sternum, continuous with the ribs while in 

 the cartilaginous state, ossifies in the form of a series of sepa- 

 rate elements, the sternebra, one for each pair of ribs involved. 

 The original number of these elements may be retained 

 throughout life, as in most mammals, or may become reduced 

 by a secondary fusion to a smaller number. 



The confinement of the sternum to the thoracic region 

 leaves the ventral abdominal surface unprotected, an affair 

 of no great moment so long as an animal remains small, or 

 not very much elongated, but when, as in the Crocodilia, the 

 elongation of the body greatly increases the extent of the 

 unprotected surface, while at the same time the increase in 

 size renders the body ponderous, the pressure exerted on the 

 abdominal viscera by the weight of the body as the animal 



