VERTEBRATES. 



25 



most are present throughout the vertebrate series, hut generally a less number, by rea- 

 son either of the fusion of separate elements, or of the loss of such. In the proximal row 

 we find first an ' intermedium ' wedged in between the radius and ulna, then a 'radiale ' 

 and an ' ulnare ' corresponding to the ends of these bones, and separated by one or 

 two ' centralia.' In the next row are five distal carpals, numbered 1 to 5, from the 

 radial to the ulnar side, and corresponding to the separate digits. The skeleton of 

 each of these is formed of a metacarpal followed by a row of phalanges. 



The nomenclature of the corresponding j)arts in the posterior extremity differs but 

 little from the above. Here also we have an intermedium, then a tibiale, fibulare, one 

 or two centralia, and finally five distal tarsals, followed by the metatarsals and pha^ 

 langes of the toes. From the Amphibia upwards to the Mammalia the structure of 

 the limb skeleton forms one of the most interesting chapters in comparative anatomy. 

 It is impossible within the bounds of the present introduction even to outline the 

 remarkable modifications to which it is subject. Sufiice it to say that these are always 

 in direct response to the nature of the use to which the limb is put, and consist either 

 in the great development of certain elements in compai-ison with others, in the fusion 

 of primitively separate elements, in the loss of such, or in all three combined. 



Instead of the terms described above for the carpals and tarsals, others derived 

 from human anatomy are frequently employed, especially in the group of the Mam- 

 malia. The subjoined table exhibits how these coi-respond. 



POSTEBIOR EXTKBMITY. 



Tibiale Sesamoid 



" 4) 

 " 5) 



Cuboid 



Those familiar with human anatomy will observe an apparently additional bone in 

 the carpus (the central); one in the tarsus (the sesamoid), and one deficient in the carpus 

 (the pisiform). The nature of the last will be presently referred to; as to the central, 

 although independent in the adults of many mammals, it is only separate in the human 

 foetus, being afterwards, except in rare instances, absorbed in the scaphoid. The sesa- 

 moid is an independent bone in some mammals, but is generally absorbed in the 

 navicular, which accordingly is usually equivalent to the centrale -|- tibiale. 



A different interest attaches to the j^isifoi'm bone ; it is to be regarded as a rudi- 

 ment of a sixth digit on the ulnar side of the manus, and is represented also in most" 

 turtles as well as on the corresponding border of the pes in some tailed amphibians. 

 Of rarer occurrence is a rudiment of a sixth digit on the tibial side of the pes, but 

 the monotremes, as well as certain rodents, possess one. 



If the dotted lines in Fig. 31 be now studied, it will be appai-ent that it is possible 

 to represent the bones of the manus or pes as a principal axis, like the metapterygium 

 of the selachian fin, giving rise to a series of radially arranged pieces. The axis is 

 here represented as passing through the humerus, ulna, intermedium, carpale,^ and the 

 radial digit (pollex or thumb). There is abundant evidence to show that the radial 

 border of the limb -of the higher vertebrate is comparable to the metapterygial border 

 of the selachian fin, but there is also evidence in the development of the tailed am- 



