144 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



the vertebrae forward, it is seen that the transverse process of 

 the sacral vertebra, considerably enlarged, supports the pelvic 

 arch, while in the presacral vertebrae these same transverse 

 processes bear short articulated elements, the ribs. It follows 

 from this (i) that the amphibian ribs are not equivalent to the 

 haemal processes in these animals, and (2) that they are struc- 

 tures different from the ribs of fishes. This view is farther 

 substantiated by the conditions which ob- 

 tain in the ganoid Polypterus, where both 

 types of ribs, those of fishes and those 

 of the higher vertebrates, occur in the 

 same segment, the latter lying in the 

 connective tissue between the epi- and 

 hypaxial muscular systems; 



The ribs of the amniotes are clearly 

 homologous with those of the amphibia. 

 They are intersegmental in position, and 

 arise by a condrification and more or less 

 complete ossification of part of the myo- 

 commatous tissue, a mode of development 

 which readily explains their frequent ex- 

 tension to the ventral surface. 



In the fishes the ribs (sometimes 

 lacking, as in some plectognaths and lo- 

 phobranchs) are usually slender, and are 

 frequently firmly united to the vertebral 

 centra ; or, again, they may be movably 

 articulated to short 'basal stumps.' In 

 many physostomous fishes some of the anterior ribs are modi- 

 fied to give rise to a chain of bones connecting the air-bladder 

 with the ear. Besides these ribs, there frequently occur in 

 fishes slender bones in the fleshy portions, the homologies of 

 which remain to be ascertained. Possibly some of them may 

 represent the ribs of the higher forms. These epimerals, epi- 

 centrals, and epipleurals, as they are called, are stated to be 

 without a cartilage stage. 



The ribs of the elasmobranchs are small and cartilaginous, 

 and are more or less intimately united with the vertebral centra. 



FIG. 152. Section 

 through tail of Amblys- 

 toma, showing the two 

 types of rib. ^, epaxial 

 muscles; h, haemal 

 arches ; hy, hypaxial mus- 

 cles; n, notochord; r, 

 true ribs. 



