CROCODILIA. 83 



then, which we have to notice, is marked in figs. 4, 5, and 6, 7;/, signifying ' pleura- 

 pophysis,' the name of tliese elements. In the segments figured they retain their 

 primitive distinctness, and acquire unusual length, in order to aid in encompassing the 

 dilated canal or cavity for the heart and hmgs ; so modified, these elements are 

 commonly called ' ribs,' or ' vertebral ribs.' 



The elements more constantly employed to protect the vascular or ' hEemal' axis, in 

 other words, to form the inferior or haemal canal, are those marked h in figs. 4 and 6 ; 

 they are the ' heemapophyses,' which are usually articulated, like the neurapophyses, 

 with the centrum, but are displaced by the great centres of the vascular system in the 

 thorax, where they have got the special name of ' sternal ribs,' and also that of ' costal 

 cartilages,' or ' cartilages of the ribs,' when they do not become ossified. The haemal 

 arch in the thorax is usually completed by a median element (//«), called a ' haemal 

 spine,' but which itself becomes vastly expanded in the bird (fig. 4) ; it is, nevertheless, 

 the part in the haemal arch which repeats below, or answers to the part {ns) in the 

 upper arch. In the segments of the trunk and tail, the element {iii) retains its normal 

 size and form as a ' neural spine ;' but where the central axis of the nervous system 

 becomes unusually developed, as in the head, e. g., analogous to the development of 

 the vascular centres in the chest, the neural canal is correspondingly expanded, and 

 the cavity acquires a special name, and is called ' cranium,' just as the analogously 

 expanded haemal canal is called ' thorax.' Into the formation of the wall of the 

 cranium other vertebral elements enter besides the neurapophyses, those e. g. which 

 are numbered 8 and 12 in figs. 10 and 1 1 ; the neural spine (7 and 1 1 in the same figures) 

 retains its primitive distinctness, is expanded horizontally, and, like the ' sternum' in the 

 thorax of the bird {lis, fig. 4, p. 5), it receives a special name, e. g. ' parietal' in fig. 10, 

 and ' frontal' in fig. 1 1 . The elements a a (figs. 4 and 6) form a symmetrical 

 pair of bones or cartilages, attached at one end to the haemal arch, and projecting 

 outwards and backwards. These are the ' prosartemata,' or appendages ; they are, 

 of all the elements of the vertebral segment, those that are least constant in regard to 

 their presence, and, when present, are subject to the greatest amount of development 

 and metamorphosis : they become, e. g., the opercular bones in the frontal segment of 

 the fish ; the branchiostegal rays in the parietal segment ; and the pectoral fins in the 

 occipital segment, and they are developed into the fore limbs and hind limbs, the arms, 

 wings, and legs of other Vertebrata.* 



As the nervous and vascular centres become reduced in size, the bony canals or 

 arches protecting them are simplified and contracted, and the vertebra assumes a 

 symmetrical character. In the Crocodile, the haemal arch, in tlie tail, e. g., is formed 

 by the haemapophyses, which ascend and articulate directly with the centrum ; 

 the pleurapophyses are shortened, directed outwards, and become anchyloscd to 



* The facts and argaments m support of this condusion, are detailed iu my works ' On the Nature of 

 Limbs,' and 'On the Archetype of the Vertebrate Skeleton,' 8vo (Van A^oorst). 



