312 VERTEr.nATE ANIMALS. 



cylindrical bones, articulating proxinially with the carpus, and 

 dist.-illy with the phalanges of the fingers. The most remarkable 

 modification of thi-, normal .state of things occurs in the Euminants 

 a.:id in the Hor.se. In the Ruminants (fig. Hi, A and B), in which 

 the foot is cleft, and consists of two j)erfect toes only, there are two 

 metacarpal bones in the embryo; but these are anchylosed together 

 in the adult, and form a single mass which is known as the "canon- 

 bone" (fig. iii, <•). In the Horse (fig. 22.'!), in which the foot con- 

 sists of no nifire than a single digit, there is only a single meta- 

 carpal bone, on each sitle of which are two little bony spines — 

 the so-called "splint-bones" — which are attached superiorly to the 

 carpus, and are to be reg.arded as rudimentary metacarpals. 



The normal nTUid>er of digits is five, but they vary from one to 

 five. The ]niddle finger is the longest, and most persistent of the 

 digits of the fore-limb ; and in the Horse it is the only one which 

 is left (fig. 22.3). The thumb is very frequently absent. In the 

 Ruminants there are only two fingers which arc functionally useful, 

 these cariyiuL; the hoofs. Normally, each digit is composed of three 

 short bones or jili:iliinii<'x, the innermost digit (the thumb) having 

 two. Tile extremity of the digit is usually protected by a nail, 

 claw, or liO(jf. 



While the fore-limbs are never wanting, the hind-limbs are 

 absent in the so-called "Mutilated Mammals" (namely, the Ceta- 

 ceans and Sirenians). The bones of the hind-limb corresjoond 

 with those of the fore-lindj ; and the foot (pes) coiTesponds with 

 the hand (mamix), consisting normally of five digits, but subject 

 to the same abortion of parts as occurs in the case of the anterior 

 lindi. 



Teeth are ]>resent in the great raajoiity of jNIammals ; but they 

 are only pi-esent in the endiryo of the whalebone Whales, and are 

 entirely absent in the genera J-Jr/iidmi, JIaiu'x, and Mi/rmecophaga. 

 In the Duck-mole ((h-idlliiirJiijnrhint) the so-called teeth are horny, 

 and the .same was the case in the e.xtinct Jihi/tina amongst the 

 iSirenia. In all other Mammals the teeth liave their oi'dinary 

 structure of dentine, enamel, and crusta jietrosa, these elements 

 being variously disposed in ilifi'erent ca,se.s, and the enamel being oc- 

 casionally wanting. In no Mammals are the teeth ever anchylosed 

 with the jaw ; and in all, the teeth are imiilanted into distinct 

 sockets or alveoli, which, however, are \-ery imperfect in some of 

 the Cetacea. 



M.any Manunals have only a single set of teeth throughout life, 

 and these are termed by Owen " monophyodojit." In mo.st cases, 

 howevei-, the first set of teeth— calle<l the "milk" or "deciduous" 

 teeth -is icphiced in the course of growth by a second set of 



