STERNUM. 51 



Development teaches us that in the cervical, lumbar, and sacral 

 regions, where no ribs are apparent in the adult, they are present in the 

 embryo, even in Man, and this points back to primitive conditions. 

 The rudimentary character and variety in size of the eleventh and 

 twelfth ribs of Man shows that they are gradually disappearing 

 (cp. p. 53) : a gradual shortening of the thoracic portion of the 

 vertebral column and a corresponding lengthening of the cervical 

 and lumbar regions is also taking place in Mammals generally, and 

 thus the following general rule may be stated: The reduction 

 in the number of ribs is correlated with a higher stage 

 in development of the Vertebrate body. 



It has already been mentioned that sacral ribs are developed, 

 and it is only necessary to add that this statement holds good for 

 all Vertebrates. In other words: the pelvis is always sup- 

 ported by sacral ribs, whether these remain differentiated 

 throughout life (Urodeles), or whether they fuse with the corre- 

 sponding transverse processes of the sacral vertebra? (Amniota). 



III. STERNUM. 



Never present in Fishes, the sternum appears for the first time 

 in Amphibians in the form of a small variously-shaped plate of 

 cartilage situated in the middle line of the chest (Figs. 38 and 74, Sty. 

 It originates from two cartilaginous rods lying in the inscriptiones 

 tendinea3 of the thoracic region, with which the coracoids, or rather 

 the epicoracoid plates of the pectoral arch come into more or less 

 close connection (Fig. 38, St, Co 1 ). In many tailless Batrachians (e.g. 

 Rana), the ventral portion of the pectoral arch is continued forwards 

 in the middle line as a slender bone tipped with cartilage the 

 " orno-sternum " (Parker) (Fig. 38, Ep). The phylogenesis of the 

 Amphibian sternum is still entirely unknown, and it is doubtful 

 whether it ought to be placed in the same category with the simi- 

 larly named structure in the Arnniota. In the latter, the sternum 

 has a costal origin, and is due to a number of ribs on either side of 

 the middle line running together to form a continuous cartilaginous 

 tract. An unpaired cartilaginous sternal plate is formed by the 

 tract of either side becoming more or less completely fused with its 

 fellow, and from this plate the ribs become secondarily segmented 

 off by the formation of true articulations. Later it may become 

 calcined (Reptiles), or converted into true bone (Birds, Mammals). 

 In Reptiles, Birds, and Monotremes the coracoids (Fig. 75, Co, Co 1 ) 

 always come into direct connection with the upper or the lateral 

 edges of the sternum (comp. Fig. 36, St, and Ca, and Figs. 38 and 

 39, St, Co). 



The sternum is greatly developed in Birds, and consists of a 

 broad plate, usually (" Carinate Birds") provided with a projecting 

 keel (crista sterni), which forms a point of origin for the wing- 



E 2 



