296 
BIRDS. 
which pass beneath. Such pulleys enable the fleshy portions of the 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 cannon-bone, most birds have another 
toe, corresponding to the first or great toe of 
the human foot, of which the metacarpal is 
loosely attached to a facet on the inner edge 
of the hinder surface of the cannon-bone—as 
shown in the figure of the cannon-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 
Skull 
of the skull is of some import¬ 
ance in classification, a few words are neces¬ 
sary on this point. In the first place, the 
skull of a bird is characterised by the great size 
of the sockets for the eyes, which are separated from one another merely by a 
thin bony partition. The apertures for the nostrils (immediately below Na in the 
figure on p. 292) 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 schizorltinal. In all Birds most of the component 
bones of the skull are completely united together, 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 
technical 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 advance 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 (PI). From the sides of 
the upper jaw or maxilloe (Mi c), are given off two maxillo-palatine processes (Mxp), 
projecting in the middle line towards 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, so as to form a bridge in front of the vomer, the construc¬ 
tion is termed desmognathous (bridged palate). In a third modification, as 
exemplified in the raven (C) and all other living passerine birds, the maxillo- 
palatines, although extending beneath the vomer, do not unite either with that 
bone or with one another, while the vomer itself is expanded and abruptly 
truncated in front; this arrangement being termed cegithognathous (passerine- 
v, 
LOWER END OF THE LEFT TIBIA OF A 
CRANE (a), AND A YOUNG OSTRICH (B). 
