202 ELEMENTARY ANATOMY, [LESS. 



Distinct malleoli may be wanting, and are so in Vertebrates 

 below Mammals. The lower end of the tibia of all adult 

 Birds is very different from that of the other classes. This, 

 however, is due to its being made up of the proximal part 

 of the tarsal element, on which account it cannot here be 

 compared with the lower end of the tibia of man and 

 Mammals. Nevertheless, exceptional forms (like the Ostrich) 

 show us that the true tibia of Birds is destitute of malleoli. 



The tibia never assumes that crossed position in relation 

 to the fibula which the radius assumes in relation to the ulna, 

 and which is termed pronation. Nevertheless, a modified 

 action may take place in certain Marsupials, viz. the Phalan- 

 gers and Wombat. 



Crucial ligaments and inter-articular cartilages may be 

 wanting, as in Tailed-Batrachians, though present in Birds 

 and Saurians. 



14. From what has been already said it is clear that the 

 FIBULA of man can only be illustrated by bones of Mammals, 

 Sauropsidans, and Batrachians. 



The fibula of man in its subordination to the tibia, occu- 

 pies a medium position, as has been noticed in describing 

 that bone. 



It may be relatively more reduced in size than in man, as 

 is the case in Birds and in Ruminants ; or it may be com- 

 pletely absent when the tibia is rudimentarily represented, 

 as is the case in Bal&na mysticetus and Boa constrictor. 



The conditions as to its anchylosis with the tibia have 

 already been mentioned. 



In its reduced state the fibula may be quite styliform, as in 

 Birds ; or it may be developed inferiorly, but atrophied at its 

 upper end, as in Bats. 



It may be represented only by a small ossification in the 

 place of the external malleolus, as in the Ox, and with this 

 there may exist (though widely separated from it) a little 

 styliform rudiment of the upper end of the fibula, as in the 

 Elk. 



The fibula may be much bowed outwards, as in the Cha- 

 meleon and the Bat Molossus, but it is generally nearly 

 straight, or quite so, as in Birds. 



The upper end of the bone may join the femur, as in 

 Marsupials and Birds, and this end may be produced 

 into a large process like the olecranon of the ulna (as in 

 Cook's Phalanger), to which even a sesamoid bone may be 

 annexed. 



