THE ENDOSKELETON 137 



effecting here the rigidity necessary in all parts of the body in 

 adaptation to flight while allowing for play of the respiratory 

 motions. 



The distribution of the two types of ribs among vertebrates 

 is a little unusual, since the second or myocommatous type 

 appears, not only in amphibians and amniotes, the higher 

 groups, but also in the selachians, one of the most primitive. 

 This is one of the many indications of kinship between these 

 and the higher forms, and suggests the direct descent of the 

 amphibians from selachian-like ancestors, thus disposing of 

 the remaining fishes as collateral lines, in which the piscine 

 type attains its special line of development, without relation- 

 ship to the higher classes, save through a common ancestor. 

 The haemal arch ribs, or true " fish-ribs," are characteristic of 

 teleosts, dipnoi, and most ganoids ; in one of the latter, Polyp- 

 terus, both sets appear simultaneously, the myocommatous set 

 being functional, while the haemal arch set is rudimentary, not 

 attached to the vertebrae, and hence of little use to the fish, 

 but of great significance to the anatomical historian. 



In a strict sense it cannot be said that the fish-rib or haemal 

 arch ribs are in all cases exactly homologous with one another, 

 or even that they are in all cases formed mainly from the 

 haemal arches, since recent investigation has demonstrated the 

 existence of other elements, derived directly from the bodies 

 of the vertebrae, and normally supporting the haemal arches, 

 which are often concerned in the formation of the ribs; but 

 not only would an exposition of this lead us too far into details, 

 but would take us away from the main inquiry, since the phe- 

 nomena do not occur on the direct road traced in our present 

 history. ' The conception of these ribs as expanded haemal 

 arches is not in any case far from the truth, since the other 

 elements concerned are themselves functionally if not mor- 

 phologically parts of the haemal arches. 



As shown by amphibians and reptiles every vertebra 5e>~ 

 tween the second (axis) and the sacrum is typically fur- 

 nished with a pair of ribs, which in these Classes are usually 

 free. In birds and mammals certain of these become anchy- 



