GENERAL CHARACTERISTICS. 



2 95 



FRONT VIEW OF THE RIGHT HUMERUS 

 OF A GULL. 



one another. In the wrist the numerous bones found in Mammals are, how- 

 ever, reduced to two ; and the metacarpus and hand are flattened and specially 

 modified. Thus there are never more than three digits, which are usually without 

 claws, although among recent birds two may be 

 thus armed. The thumb, or first digit, is repre- 

 sented merely by one or two joints (as shown 

 in the skeleton on p. 292), and carries the so- 

 called bastard-wing ; while the other two digits 

 represent the index and middle fingers of the 

 human hand. Their respective metacarpals, as seen in the figure cited, are united 

 at their two ends so as to form a single bone ; while the index finger has two 

 flattened joints, and the third finger (not present in the figure) but one. 



In the hind-limb there is a still wider departure from the Mammalian type. 

 The uppermost bone in a bird's leg (A of the accompanying figure) is the thigh- 

 bone, or femur ; below this comes the tibia, or larger bone of the lower leg, on the 

 outer side of which is a small splint (not shown in the figure) representing the 



fibula. Below the tibia comes another long bone, 

 terminating (except in the ostrich, where they 

 are reduced to two) in three pulley -like sur- 

 faces, known as trochleas, to which are articu- 

 lated the toe-bones. Obviously, then, this third 

 long bone corresponds to the metatarsus of a 

 mammal, consisting in fact of the three middle 

 metatarsals of the typical five-toed limb welded 

 together, in the same manner as two such 

 metatarsals are united in the hind-limb of a 

 ruminant mammal. It may, therefore, be called 

 either the metatarsus or the cannon-bone. The 

 reader will, however, now ask what has become 

 of the ankle or tarsus in the bird's leg. To 

 this it may be replied that its upper bones have 

 united to the lower end of the tibia ; while the 

 lower row has joined the upper end of the 

 cannon-bone. The figure on p. 290 exhibits the 

 lower end of the tibia of an adult crane and 

 of a young ostrich ; and it will be seen that in 

 the latter the upper ankle-bone is still distinct, 

 while in the former it has become completely 

 united with the tibia. A precisely similar state of things takes place in the for- 

 mation of the cannon-bone. It will, therefore, be apparent that the tibia of a bin! 

 corresponds to the tibia, plus the upper half of the ankle, of a mammal : while the 

 cannon-bone represents the metatarsus, plus the low, ■ half of the ankle. Hence. 

 while the ankle-joint in a mammal occurs between the tibia and the upper row of 

 ankle-bones, in a bird it is placed between the upper and lower rows of the ankle. 

 The bony bridge seen at a in the tibia of the crane is very commonly present in 

 birds; it acts as a pulley for the tendons of the muscles of the front of the leg 



BONES OF THE RIGHT LEG OF A MOA 

 B, CANNON - BONE OF SAME ON A 

 LARGER SCALE. 



