58 



COMPARATIVE ANATOMY 



number of ribs which reach the sternum, and in some cases the 

 sternal, as well as the vertebral ribs may become ossified. In both 

 " true " and "false" ribs (p. 56), a capitulum, a neck, a tubercul-um, and 



a body may be distinguished (Fig. 42). 

 The capitulum of the former usually 

 articulates with its own centrum as well 

 as with that next in front, in the 

 region of the epiphysis ; the tuber- 

 culum articulates with the cartilagin- 

 ous facets on the transverse process. 

 In the " false " ribs, these characters 

 are to a greater or less extent lost. 

 As already mentioned (p. 51), rudi- 

 ments of ribs are present in the 

 lumbar and sacral regions, and unite 

 with the corresponding transverse 

 processes. 



Fi. 42. COSTAL ARCH OF 

 MAN. 



WK, centrum of vertebra ; 

 Pt, transverse process ; 

 Ps, neural spine ; Cp, 

 body of rib ; Ca, capitu- 

 lum ; Co, neck ; Tb, tuber - 

 culum ; Kn, cartilaginous 

 sternal rib ; St, sternum. 



This fact, as well as the rudimentary 

 character and variety in size of the eleventh 

 and twelfth ribs and the occasional presence 

 of a thirteenth rib in Man, shows that a reduc- 

 tion in the number of these structures is here 

 taking place : 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 it may be stated that the 

 reduction in the number of ribs is correlated with a higher stage in 

 development of the Vertebrate body. 



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. 43). It 

 arises as a paired cartilaginous plate 1 in the inscriptiones tendineaB 

 of the rectus abdominis muscle, and therefore may be looked upon 

 as corresponding to a pair of "abdominal ribs." Such cartilaginous 

 abdominal ribs must have been present in greater numbers in 

 the ancestors of existing Urodeles (comp. Necturus, p. 56). In 

 many tailless Batrachians (e.g., Rana) the ventral portion of the 

 pectoral arch is continued forwards in the middle line, from where 

 the two clavicles meet, as a slender omosternum (Fig. 43, D) : 

 this has a similar origin, and the proximal portion both of it 

 and of the sternum become ossified. Thus the sternum and 

 omosternum of Amphibians are not to be considered as correspond - 



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

 an abbreviation of development. 



