BIRD-STRUCTURE AND DEVELOPMENT. 219 



coids, but more properly termed metacoracoids. Superiorly these metacora- 

 coids join the scimitar-like scapulae, or shoulder-bones, and at their junction 

 form the cavity for the reception of the head of the upper wing-bone, or 

 humerus. Joining the upper ends of the metacoracoids, and extending 

 downwards towards the sternum, is the well-known merry-thought, or 

 furcula ; although in this modification peculiar to birds, the furcula really 

 represents the united collar-bones, or clavicles, of man. In ordinary 

 mammals there are no separate representatives of the metacoracoids ; but 

 such occur in the egg-laying mammals, although differing widely from those 

 of birds. Before leaving this region of a bird's skeleton, it is important 

 to mention that in the backbone at least in the neck the individual 

 joints, or vertebrae, are articulated together by saddle-shaped surfaces, 

 such a mode of articulation being unknown, in any other class of vertebrate 

 animals. 



As regards the structure of the bones of the wing, very few words will 

 suffice. The two upper segments of this part of the skeleton are essentially 

 similar to the same part in mammals. The bones of the wrist, or carpus, are, 

 however, reduced to two in number; while those of the metacarpus and 

 fingers are flattened and otherwise modified, the number of digits never ex- 

 ceeding three. The three digits respectively represent the thumb, index, and 

 middle fingers of the human hand. Of these, the thumb has either one or 

 two joints, the index two, and the third digit one. The metacarpals of the 

 index and third fingers are united together by a thin plate of bone, which 

 may be perforated. 



In a bird's leg the general plan of structure is a specialised modification of 

 that obtaining in reptiles. The grand distinction between the leg of a reptile 

 and that of a mammal, lies in the fact that, whereas in the latter the ankle-joint 

 occurs at the top of the two rows forming the ankle, or tarsus, in the former 

 the joint is situated between the two rows. A bird's leg is formed on the 

 latter plan, but has undergone the further modification that the two rows of 

 tarsal bones are respectively welded to the bones above and below them. We 

 have first of all the thigh-bone, or femur ; next comes the tibia (with the 

 aborted fibula at its upper end), to the lower extremity of which are attached 

 the upper row of tai-sal^ones, thus forming a tibio-tarsus. Below this comes 

 a compound bone, formed by the lower row of tarsal bones and the three 

 middle metatarsal bones of the typical five-toed foot, this compound bone 

 being termed the tarso-metatarsus. Usually this bone terminates below in 

 three pulley-like surfaces, to which are attached the bones of the toes ; but 

 in the ostrich there are only two of these pulleys, and likewise only two toes. 

 When the first toe, or halhex, is present, it is attached to the hinder surface 

 of the lower end of the tarso-metatarsus. No trace of the fifth toe ever re- 

 mains. Whereas in mammals each of the toes, save the thumb, has three 

 joints, in a bird (as also in a reptile) the number of these joints increase from 

 the second to the fourth toe. 



The chief character which distinguishes the class Aves from the other 

 classes of vertebrate animals is the possession of feathers. The production of 

 the young from an egg is not confined to the birds, but is shared by some 

 mammals and reptiles ; nor is the power of flight a sole prerogative of the 

 birds, but their feathery covering is a really distinctive characteristic. 



That birds existed on the earth in very remote times is undoubted, and 

 that they were very different in form and structure from those which we 

 have around us now, is equally certain. As the special character of the class 



