42 PAKTETAL SEGMENT, OK VERTEBRA. 



of adequate extent and firmness is thus insured for the 

 support of the pectoral fins. The carpal bones of these 

 fins are four in number, progressively increasing in length 

 from the ulnar to the radial side of the wrist. The meta- 

 carpo-phalangial rays are thirteen in number ; the upper- 

 most or ulnar one being the strongest, and articulating 

 directly with the ulna. 



Proceeding to the next segment, in advance, in the cod- 

 fish's skull, we find that the bone which articulated with 

 the centrum of the occipital segment is continued forward 

 beneath a great proportion 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 oc- 

 cipital segment. 



In the skeletons of most animals the centrums of two 

 or more segments become, in certain parts of the bod}^, 

 confluent, or they may be connate ; they form, in fact, one 

 bone, like that, e. g., which human anatomists call " sa- 

 crum." 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 com- 

 mon 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, trans- 

 cending the senses, recognizes such single bone as being 

 essentially two. In like manner, it recognizes the " oc- 

 cipital bone" of man as essentially four bones ; but these 



