184 HISTORY OF THE HUMAN BODY 



sidered by some to be a sixth digit, a " post-minimus." It is 

 probably a row of sesamoids employed here to widen the 

 paddle and thus increase its effectiveness. In the cetaceans the 

 external digits suffer some reduction while the two middle 

 ones, II and III, are lengthened. 



In adaptation to flight the chiridial appendage forms a 

 framework for a thin surface, without appreciable weight, and 

 formed either of integument or of integumental structures of 

 some sort. There have been at least three independent and 

 perfectly successful attempts to solve this most difficult me- 

 chanical problem, each one involving profound changes in the 

 limb skeleton. In the extinct pterodactyls the principal modi- 

 fication consisted of an extreme lengthening of the little finger 

 to form a framework for the wing membrane; in the bats a 

 similar result has been attained by lengthening all the digits 

 except the pollex ; and in the birds, where the development of 

 feathers necessitates the formation of a firm framework with- 

 out motion between the parts, there is formed a bone complex 

 composed of carpal and metacarpal elements and several 

 phalanges. [Fig. 50; (d) to (f).] 



At the conclusion of this subject it may not be without in- 

 terest to review briefly the subject of the transition between 

 the two types of free limb, the ichthyopterygium and the 

 cheiropterygium, in which, although as yet no theory has 

 gained general credence, or has even passed beyond the stage 

 of an ingenious speculation, many interesting suggestions 

 have been advanced, some of which may be near the truth, as 

 may at any time be shown by the discovery of the fossil re- 

 mains of transition animals, unknown at present. 



In general the similarity between fin-rays and digits is seen 

 by everyone, and this probable homology is rendered more 

 likely by the fact that the bones of the digits in animals with 

 the hand-form of limb are preformed in cartilage, thus sug- 

 gesting the fin-rays of the selachians and ganoids. Both are 

 divided by joints into movable segments; both are supplied 

 with flexor and extensor muscles ; and in some fishes the rays 

 are prolonged by the addition of horn threads, in structure 



