206 ELEMENTARY ANATOMY. [LESS. 



In Birds it is also transversely and but little antero-pos- 

 teriorly extended, while it sends up a process which is applied 

 to the front of the tibia (Fig. 176, a). It may be perforated 

 by one or two canals for extensor tendons of muscles. 



Thus no projection corresponding with the tuberosity of 

 the os calcis exists in this compound bone. 



Inasmuch as the astragalus of man articulates with the 

 tibia, it has a character which is constant. A distinctness 

 like that which it possesses in him is not only, as we have 

 seen, far from universal, but its distinctness is less than that 

 of its serial homologue in the hand ; for not only may the 

 astragalus anchylose with other tarsal elements, but with the 

 long bone of the limb also, as in Birds. 



The astragalus may be represented by two bones, as in 

 the Salamander and other Tailed-Batrachians, or it may 

 anchylose with the naviculare, as in the Crocodile. 



It may bear a larger proportion to the other tarsal bones 

 than it does in man, as is the case in the Seals. It may 

 articulate widely with the cuboid bone, as in Ruminants. 



One or even two extra ossicles maybe attached to thetibial 

 side of the foot, as in Cercolabes. An extra ossicle is 

 annexed to the astragalus in the male Ornithorhynchus and 

 Echidna. 



The astragalus may project much beyond the os calcis, as 

 in the Two-toed Ant-eater. 



In the Sloths the astragalus has a deep cup-shaped cavity 

 on its outer side to receive the process of the fibula, as before 

 mentioned. That of the Ornithorhynchus has a cup for a 

 process of the tibia. 



The largest bone of the human tarsus, the calcaneum, 

 may have its relative size to the other tarsal bones yet further 

 increased, as in Tarsius, where it attains one-third the length 

 of the spine from atlas to sacrum inclusively taken. It may, 

 on the contrary, be much diminished, as in the true Seals, 

 or still more so, as in Birds. Where it is distinctly ossified 

 in Tailed-Batrachians and Reptiles, it develops no tuberosity, 

 except in the Crocodiles, where that process is still small. In 

 Birds this bone seems very early to be absorbed into the 

 astragalus. 



It never seems to anchylose with any other tarsal element, 

 unless it also anchyloses with the astragalus, as in Birds and 

 in many Reptiles, e.g. Lacerta agilis. 



The tuberosity may be very much twisted in man's own 

 order, e.g. Perodicticus and Lens. It may be all but absent, 



