168 COMPARATIVE ANATOMY 



(Ratitse) the second (HI], and even the third (IV") also (e.g. 

 Struthio). Claws may be present in the young only (e.g. Opistho- 

 comus, Sterna, Fig. 133). 



The tarsus is still more reduced in Birds than in Reptiles, and 

 consists in the embryo of three elements, two small proximal and 

 a broader distal, which in many cases (e.g. Penguin) consists 

 primarily of four distinct pieces. The former (tibiale and fibulare) 

 unite later with the distal end of the tibia, thus forming a tibiotarsus, 

 while the latter, which corresponds to tarsalia / to V, becomes 

 included in the base of the metatarsus. Thus the foot of adult 

 Birds no longer possesses any distinct tarsal elements, though, as 

 in Chelonians and Lizards, it really moves by an intertarsal 

 articulation. Of the original five metatarsals, the fifth soon dis- 

 appears, while the second, third, and fourth become united with 

 one another and with the distal element of the tarsus to form a 

 single bone, the tarsometatarsus (Fig. 132), grooves at the ends 

 of which indicate its compound nature, which is especially well 

 seen in Penguins. The first metatarsal remains to a greater 

 or less extent independent. 



The number of toes varies between two (Struthio) and four : 

 that of the phalanges is normally 2, 3, 4, 5, reckoning from the 

 first to the fourth digit. The tibia, even from the first, greatly 

 exceeds the splint-like fibula in size, and the two bones become 

 united distally. 1 



Mammals. In Mammals the anterior extremity either 

 remains in the condition of a simple organ of locomotion, serving 

 for progression on land, or it may give rise to a digging or a 

 prehensile organ ; or, again, may become modified in adaptation to 

 an aerial (Bats) or aquatic (Pinnipedia, Cetacea, Sirenia) mode 

 of life. 



The humerus, which may possess a supracondyloid foramen near 

 its distal end, is variously modified as regards form and relative 

 length and the presence of ridges and elevations for the insertion 

 of muscles ; and the same is true as regards the femur and its 

 ridges or trochanters. 



The tibia is the more important bone of the shank, and 

 the fibula often becomes united with it to a greater or less 

 extent distally and sometimes proximally also, usually taking 

 no part in the knee joint. The two shank-bones lie parallel, 

 and are at most very slightly movable on one another (e.g. 

 climbing Marsupials). The fibula never disappears entirely, 

 but in some cases (Bats, Ruminants) only its distal end is 

 recognisable as the lateral (external) malleolns. 



The radius and ulna are connected with the humerus by a 

 hinge-joint at the elbow, only allowing movement in one plane, 

 and primarily their relations to one another are similar to those 

 of the tibia and fibula. This is the case in Monotremes and in 



1 For the pneumatic character of Birds' bones, of, under Air-sacs, 



