70 COMPARATIVE ANATOMY 



the cartilaginous facet on the trans.verse process. In the " false " 

 ribs, these characters become to a greater or less extent lost in 

 passing from before backwards, so that the posterior ribs have a 

 more rudimentary character. As already mentioned (p. 62), 

 vestiges of ribs are present in the lumbar and sacral regions, and 

 unite with the corresponding transverse processes. There are 

 usually thirteen pairs of ribs, but their number may vary between 

 nine (Hyperoodon) and twenty-four (Choice pus). These facts 

 indicate that there has been a gradual phylogenetic reduction in 

 the number of ribs, and the occasional presence of supernumerary 

 ribs is to be explained as a reversion. 1 



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 (Fig. 55). It 

 arises as a paired cartilaginous plate 2 derived in the first in- 

 stance from chondrifications in an intermuscular septum on the 

 median border (linea alba) of the rectus abdominis muscle, and 

 therefore may be looked upon as comparable to a pair of 

 " abdominal ribs." Such cartilaginous structures must have been 

 present in greater numbers in the ancestors of existing Urodeles 

 (cf. p. 67). In many tailless Batrachians (.#., Ranidas) the ventral 

 portion of the pectoral arch is continued forwards in the middle 

 line, from where the two clavicles meet, as a slender rod, the 

 omosternum (Fig. 55, D) : this has a similar origin, and the 

 proximal portion both of it and of the sternum becomes ossified. 

 Thus the sternum and omosternum of Amphibians are not to be 

 considered as corresponding to differentiations of the pectoral arch 

 (coraco-sternum), a view which is often held, but as consisting of 

 skeletal parts which primarily belong to the body-wall, and only 

 secondarily come into connection with the limb-skeleton. 



In most Urodeles and certain Anurans (e.g. Pipa, Discoglossus, 

 Bombinator, Alytes), this cartilaginous sternal plate is inserted into 

 the grooved median margins of the two overlapping coracoids (Fig. 

 55, B, c). In Rana, on the other hand (D), in which the two 



1 A primitive and a secondary type of thorax may be distinguished. The 

 former is the more usual, and occurs in most Mammals even up to the lower 

 Apes : it is characterised by an elongated form, and by the dorse-ventral 

 diameter being much greater than the transverse diameter. The latter occurs in 

 anthropoid Apes and Man, in which the dorso-ventral diameter has, both 

 ontogenetically and phylogenetically, become considerably reduced relatively : 

 the broad thorax is thus more cask-like in form, and may often even be flattened 

 dorso-ventrally. A somewhat similar modification is seen amongst insectivorous 

 Bats. 



- It is unpaired from the first in Triton and Rana, but this is probably due 

 to an abbreviation of development. 



