142 ANATOMY OF VERTEBRATES. 



amphibious reptiles, when on land, rest the whole weight of the 

 abdomen directly upon the ground, the necessity of the modifi- 

 cation diminishing liability to fracture further appears. These 

 analogies are important, as demonstrating that the general homo- 

 logy of the elements of a natural segment of the skeleton is not 

 affected or obscured by their subdivision for a special end. 

 The purposive modification of the hsemapophyses of the frontal 

 vertebra is but a repetition of that which affects the same 

 elements in the abdominal vertebrae. 



Passing next to the haemal arch of the parietal vertebra, fig. 

 93, h, ii, we are first struck by its small relative size. Its 

 restricted functions have not required it to grow in proportion 

 with the other arches, and it consequently retains much of its 

 embryonal dimensions. It consists of a ligamentous ' stylohyal,' 

 retaining the same primitive histological condition which obstructs 

 the ordinary recognition of the pleural element of the lumbar haemal 

 arches ; of a cartilaginous ' epihyal,' 39, intervening between this 

 and the ossified haemapophysis, or ceratohyal, 40 ; and of the haemal 

 spine, 41, which retains its cartilaginous state, like its homotypes, 

 in the abdomen : there they get the special name of ' abdominal 

 sternum,' here of ' basihyal.' The basihyal has, however, coalesced 

 with the thyrohyals to form a broad cartilaginous plate, the anterior 

 border rising like a valve to close the fauces, and the posterior 

 angles extending beyond and sustaining the thyroid and other 

 parts of the larynx. The long bony ' ceratohyal ' and the com- 

 monly cartilaginous ' epihyal ' are suspended by the ligamentous 

 e stylohyal ' to the back part of the tympanic at its junction with 

 the paroccipital process ; the whole arch having, like the man- 

 dibular one, retrograded from the connection it presents in Fishes. 



This retro2;radation is still more considerable in the succeedino- 

 haemal arch, fig 92, n l ; fig. 57, 51. In comparing the occipital 

 segment of the Crocodile's skeleton with that of the Fish, fig. 81, 

 the chief modification that distinguishes that segment in the Cro- 

 codile is the apparent absence of its haemal arch. We recognise, 

 however, the special homologues of the constituents of that arch 

 of the Fish's skeleton, fig. 34, in the bones 51 and 52 of the Cro- 

 codile's skeleton, fig. 57 ; but the upper or suprascapular piece, 50, 

 fig. 92, retains, in connection with the loss of its proximal or cranial 

 articulations, its cartilaginous state: the scapula, si, is ossified, as 

 is likewise the coracoid, 52, the lower end of which is separated 

 from its fellow by the interposition of a median, symmetrical, 

 partially ossified piece called ' episternum.' The power of recog- 

 nising the special homologies of so, 51, and 52 in the Crocodile, 



