ON THE VERTEBRATE SKELETON. 



291 



Fig. 21. 



sected by the lines N iv and the arrow H iv, with those numbered 26, 27, 

 and 73, and we have to inquire whether through all the modifications which 

 their extreme position subjects them to, we can still trace any evidence of their 

 arrangement according to the vertebrate type. 



A long and slender symmetrical grooved bone, like the ossified inferior 

 half of the capsule of a notochord, is continued forwards from the centrum 

 of the foregoing vertebra, and stands in the relation of a centrum (13) to the 

 vertical plates of the bones 14, which expand as they rise into the broad and 

 thick triangular plates with an ex- 

 posed horizontal superior surface. 

 The arch of which these form the 

 piers, and to the anterior rhinen- 

 cephalic prolongations traversing 

 which arch they stand in the re- 

 lation of neurapophyses, is com- 

 pleted by the two bones (13) : which 

 I, therefore, regard as a divided 

 neural spine. In fishes we have 

 seen that the corresponding ele- 

 ment of the parietal vertebra was 

 similarly divided, whilst the neural 

 spine of the nasal vertebra was 

 single : in the crocodile the re- 

 verse conditions prevail. In a spe- 

 cies of alligator I have observed 

 the bone 13 continued further for- 

 wai'd, expanded, and divided at the 

 middle line, the two divisions form- 

 ing a small disc on the bony palate. 

 The centrum of the nasal vertebra 

 is divided longitudinally at the me- 

 dian line in batrachians, ophidians, 

 and most lacertians ; it is single in 

 chelonians, but retains its carti- 

 laginous state in some species (Emys expansa, e.g.). The neurapophyses 

 (14, 14) transmit the olfactory nerves in all reptiles ; but the ganglions are 

 usually withdrawn backwards into the prosencephalic neural arch, leaving 



ramus in the recent and extinct saurians by pointing out the similarity of the structure to 

 that adopted in binding together several parallel plates of elastic wood, or steel, to make a 

 cross-bow; and also in setting together thin plates of steel in the springs of carriages. Dr. 

 Buckland adds, " Those who have witnessed the shock given to the head of a crocodile by 

 the act of snapping together its thin long jaws, must have seen how liable to fracture the 

 lower jaw would be, were it composed of one bone only on each side." — lb. p. 177. The 

 same reasoning applies to the composite condition of the long tympanic pedicle in fishes. 

 In each case the splicing and bracing together of thin flat bones of unequal length and of 

 varying thickness affords compensation for the weakness and risk of fracture that would other- 

 wise have attended the elongation of the snout. In the abdomen of the crocodile and plesi- 

 osaur the analogous composition of the haemapophyses (abdominal ribs) allows of a slight 

 change of length in the expansion and contraction of the walls of that cavity : and since 

 amphibious reptiles, when on land, rest the whole weight of the abdomen directly upon the 

 ground, the necessity of the modification for diminishing liability to fracture further appears. 

 But what we are here chiefly concerned in is the evidence that the general homology of 

 elementary parts of a natural segment is not affected by the modification of teleological 

 composition of such parts. What happens to the haemapophysial or inferior elements of 

 the inverted arch in the abdominal segments of the crocodile also affects the same elements 

 of a cranial haemal arch ; and the subdivision of the pleurapophyses of the trunk in the 

 sturgeon is repeated in the same elements of the cranial vertebras in osseous fishes. 



Disarticulated rhinencephalic arch, with the anchylosed 

 pterygoids (24) viewed from behind : Crocodile. 



