of the Vertebrate Skeleton. 37 



a rib, what has become of the ribs to the other segments ? 

 And it was the difficulty that there is in meeting this question 

 in the higher Vertebrata, which led me (in a former paper*) 

 to regard the occipital and frontal segments of the skull as 

 standing in the same relation to the parietal segment as the 

 epiphyses of a vertebra stand to its centrum. But remember- 

 ing that, no matter what the potential power may be, it can 

 only give great development to a structure when coincident 

 with functional growth, we should no more be justified in 

 anticipating ribs to all the cranial segments than to all the 

 vertebral segments ; and with many animals parts of the ver- 

 tebral column will be devoid of ribs. Yet as the upper arches 

 of the skull retain characters which long ])reviously became 

 lost to the upper arches of the vertebral column, so we might 

 with more reason expect the lower arches to be present in the 

 skull than in cervical or lumbar vertebras. Accordingly, if 

 we examine a skull, and remove all those bones Avliich we 

 have regarded as modified from a functionally developed rib 

 (which we name the jaws), there will be found in front of 

 their point of attachment, and under the frontal segment, two 

 bones, named the vomeres ; sometimes they become anchylosed 

 into one median bone. And anterior to these bones, and bent 

 up over them frequently, are the ethmoid bones, which simi- 

 larly may become anchylosed. Thus we again have the re- 

 presentative of a rib with its epipleuron. By segmentation 

 the ethmoid developes the nasal bones ; and it is probable that 

 by segmentation the vomer forms the premaxillary. Thus the 

 anterior rib conforms in plan to the posterior rib, and, like it, 

 embraces an organ which, in the lower animals, is only that 

 of smell, but which, by potential growth comes, in the higher 

 vertebrates, to be the respiratory region. So that, just as there 

 are distinct tubes for breathing and for swallowing in the 

 land Vertebrata, so distinct tubes are made for those offices in 

 the skull by the prolongation forward of tli6 dorsal respiratory 

 tube till it is embraced by the first pair of cranial ribs, while 

 the digestive tube, not prolonged so far forward, is embraced 

 by the second pair. 



It is not so easy to find the third pair ; and only on turn- 

 ing to the fish is the homology evident. At each side of the 

 back of the skull is a bone attached to the periotic bones, 

 named the hyomandibular ; and to this bone is attaclied in 

 front the circle of hyoid bones ; and attached to it behind are 

 the opercular bones ; so that there is again a forked rib va- 

 riously segmented for the third arch. 



* " Outlim- of a Tlieory of tlio Skull kc," Anuals, 18GG, xviii. p. :34o. 



