480 



COMPARATIVE ANATO]\IY. 



appendage as a large bone, wliicli forms a piece of about tlie same size 

 as tbe rest of the appendage ; tliis may be called tlie humerus. In 

 Plesiosaurus two succeeding pieces, which in Ichthyosaurus are still 

 indifferent, are also increased in size ; these correspond to the fore- 

 arm : radius and ulna; these are succeeded by two transverse rows of 

 smaller pieces, which form a carpus, and these again by longer rows 

 of bones, which represent the metacarpus and the phalanges of the 

 fingers. The segmentation which affects the appendage after the stem 



and rays are broken up into several pieces, 

 may here be seen in its different stages. 



The arrangements which are presented 

 by the Amphibia are similar in character; 

 for although one finger is atrophied, we can 

 fill up the void by the aid of the arrange- 

 ments seen in the hinder limb, where they 

 are complete. The stem of the archiptery- 

 gium must, therefore, be sought for in a 

 lateral series of skeletal pieces, which ex- 

 tends from the humerus, through the ulna 

 to the fifth finger, and in the carpus con- 

 sists of two pieces. The other skeletal 

 pieces are arranged on these rays. One 

 ray begins with the radius, and extends 

 into the first finger. A second, third, and 

 fourth begin in the carpus, and end in the 

 second, third, and fourth fingers. The 

 primitive carpus is therefore composed of 

 ten pieces ; five carpals carry the fingers, 

 three are attached to the bones of the fore- 

 arm ; these are the radial, intermedium, and 

 ulnare ; two centralia (cc) are enclosed by 

 these two sets. 



The change in the function of this appendage is connected with 

 a rotation of the humerus on its own axis, and this rotation may be 

 observed in the individual development of higher forms. It brings 

 about a difference in the position of the limb as compared with that 

 of lower forms. 



Fig. 265. Diagram of the 

 fore-limb of an Amphi- 

 bian. The dotted lines 

 indicate the rays, which 

 remain attached to the stem 

 of the Archipterygium. 



§ 364. 



A more or less complete copy of the typical form of limb derived 

 from the archipterygium is retained in all divisions of the Verte- 

 brata. In all there are often unmistakable traces of the characteristic 

 relations, in opposition to which numerous deviations, due chiefly to 

 reduction and concrescence, make their appearance. These modifi- 

 cations are clearly due to the varied uses to which the limbs ai-e put, 

 just as the complete atrophy of some parts, or even of the whole limb, 

 are due to their being no longer required. 



