1468 



BIRDS 



Skull 



the ankle, of a Mammal; while the canon bone represents the metatarsus, plus the 

 lower 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 ridge 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 which pass beneath. Such pulleys enable the fleshy portions of 

 trie muscles to be placed high up in the limb, and thus cause the centre of gravity 

 of the body to be near the wings, an arrangement essential for flight. In addition 

 to the three toes articulating with the lower end of the canon bone, most birds have 

 another toe, corresponding to the first or great toe of the human foot, of which the 



inetacarpal is loosely attached to a facet on the 

 inner edge of the hinder surface of the canon 

 bone as shown in the figure of the canon bone 

 of a buzzard in our fourth volume. No bird 

 has any trace of the fifth toe. The number of 

 joints in each toe, in place of not exceeding 

 three as in ordinary Mammals, increases regu- 

 larly from the first to the fourth toe. 



As the structure of the base of 

 the skull is of some importance in 

 classification, a few words are necessary on this 

 point. In the first place, the skull of a bird is 

 characterized by the great size of the sockets 

 for the eyes, which are separated from one 

 another merely by a thin bony partition. The 

 aperture for the nostrils (immediately below Na in the figure on p. 1464) may be 

 either short and rounded, when the skull is said to be holorhinal (as in that figure); 

 or they may form elongated slits, as in a pigeon, when the condition is termed 

 schizorhinal. In all Birds most of the component bones of the skull are completely 

 united, without any trace of the original lines of division, in the adult state, and in 

 ornithology it is usual to apply the terms upper and lower mandible to the two parts 

 of the beak. 



With regard to the bones of the palate, the introduction of a number of tech- 

 nical terms is unavoidable. In the middle of the hinder part of the lower surface of 

 a bird's skull can be seen a pointed rod of bone, known as the sphenoidal rostrum, 

 which may carry, as in (A) of the figure, a pair of basipterygoid facets (f). In ad- 

 vance of this is a single or double bone, termed the vomer ( Vo). On the two sides 

 of this central axis are two pairs of slender bones, of which the hinder are termed 

 pterygoids (Pt), and articulate with the basipterygoid processes when present; while 

 the front pair are named palatines (/*/). From the sides of the upper jaw or maxil- 

 l<z (Mx}, are given off two maxillo-palatine processes (Mxfi~), projecting in the middle 

 line toward the vomer. Now when the vomer, as in the fowl and capercaillie (A) 

 is pointed in front, while the maxillo-palatines remain separate both from it and 

 from one another, the skull is said to be schizognathous (cleft palate). When, on 

 the other hand, as in the duck (B), the maxillo-palatines unite in the middle line, 



LOWER END OF THE LEFT TIBIA 

 OF A CRANE (A), AND A YOUNG 

 OSTRICH (B). 



