96 ANATOMY OF VERTEBRATES. 



are cold-blooded animals the gymnothorax and slow- worm, 

 e. g.- -in which this arch supports no appendage ; there are others 

 -Lepidosiren and Protopterus, fig. 41, 52 in which it supports 

 an appendage in the form of a single many-jointed ray, ib. 57. 

 In other fishes, the number of rays progressively increase, until, 

 in those called ( rays ' par excellence, fig. 64, they exceed a hundred 

 in number, and are of great length, forming the chief and most 

 conspicuous parts of the fish. The more common condition of 

 the appendage in question is that exhibited in the Cod, fig. 34, 

 So developed, it is called in Ichthyology the f pectoral fin,' ib. P : 

 otherwise and variously modified in higher animals, the same part 

 becomes a fore-leg, a wing, an arm and hand. 



Proceeding to the next segment, in advance, in the Cod-fish's 



o ~ * * 



skull, we find that the bone which articulated with the centrum 

 of the occipital segment is continued forward beneath a great pro- 

 portion of the skull. In quadrupeds, however, the corresponding 

 part of the base of the skull is occupied by two bones ; and if the 

 single long bone in the fish be sawn across at the part where the 

 natural suture exists in the beast, we have then little difficulty in 

 disarticulating and bringing away with it a series of bones similar 

 in number and arrangement to those of the occipital segment. 



In the skeletons of most animals the centrums of two or more 

 segments become, in certain parts of the body, confluent, or they 

 may be connate ; they form, in fact, one bone, like that, e. g., 

 which human anatomists call ( sacrum.' By the term ( confluent ' 

 is meant the cohesion or blending together of two bones which 

 were originally separate ; by ( connate,' that the ossification of 

 the common fibrous or cartilaginous bases of two bones proceeds 

 from one point or centre, and so converts such bases into one 

 bone : this is the case, e. g., in the radius and ulna of the frog, 

 and in its tibia and fibula. In both instances they are to the eye 

 a single bone ; but the mind, transcending the senses, recognises 

 such single bone as being essentially two. In like manner it 

 recognises the ( occipital bone ' of man as essentially four bones ; 

 but these have become ' confluent,' and were not ' connate.' The 

 centrums of the two middle segments of the fish's skull are con- 

 nate, and the little violence above recommended is requisite to 

 detach the penultimate segment of the skull. When detached, 

 the bones of it are seen to be so arranged as to form a neural and 

 a hremal arch. In the neural arch, fig. 78, the centrum, neura- 

 pophyses, diapophyses, and neural spine are distinct: moreover, 

 the neural spine in the Cod, and many other fishes, is bifid, or 

 split at the median line. The centrum is called ' basiphenoid,' 5 ; 



