104 



COMPARATIVE ANATOMY. 



pterygium, is always unpaired, two bones are present in the 

 fore-arm and shank. The former are called radius and ulna, 

 and the latter tibia and fibula. The hand and foot may also be 

 respectively divided into two sections, a proximal metacarpus 

 and metatarsus, and a distal series of phalanges, which form 

 the skeleton of the fingers and toes (digits). 



Both manus and pes are made up of several series of cylin- 

 drical bones. There are never more than five complete series, 

 which, except as regards number, present essentially similar primary 

 relations throughout the higher Vertebrates. The skeleton of the 

 carpus and tarsus, each of which always consists of a series of small 

 cartilages or bones, shows much variation, but the following may 

 be taken as a ground-type. Round a centrale, which may be 

 double, is arranged a series of other elements, in which three 

 proximal, and a varying number (four to six) of distal, may be dis- 

 tinguished. The proximal, in correspondence with their relations 

 to the bones of the fore-arm and shank respectively, are spoken of as 

 radiale or tibiale, ulnare or fibulare, and intermedium; 

 while the distal are called carpalia or tarsalia / to VI (in the 

 narrower sense). They are counted beginning from the radial or 

 tibial (pre-axial, Huxley) side (Figs. 88 and 89). 



FIG. 90. RIGHT TARSUS OF Discoglossitg pictus, (From above.) 



Aa, astragalus ; Ca, calcaneum ; 1 to 4, the four separate (cartilaginous) tarsalia on 

 the tibial side, in relation with the rudimentary extra finger (7) and digits 1 to 3 

 (II to IV) ; 5 + 6, fibrous band representing the tarsals of digits F"and VI ; 

 I, rudimentary extra digit on tibial side ; // to VI, metatarsals of digits 1 to 5 ; 

 t, single phalanx of the extra tibial digit. 



Amphibia. Whilst the anterior and posterior extremities of 

 Urodelesare formed essentially on the ground -plan described above 

 (Figs. 87 to 89), 1 in the case of the Anura the radius and ulna be- 



1 Numerous secondary fusions of the individual elements of both carpus and 

 tarsus may, however, occur ; this applies also to the Anura. As a rule the anterior 

 extremity is only provided with four lingers, though there are reasons for supposing 

 that it at one time, like the posterior, possessed five complete fingers. The number of 

 phalanges varies in different Amphibians. 



