THE ANATOMY OF BIRDS. — OSTEOLOGY. 
133 
also variously carinate or ridged, sulcate or grooved. In a few cases they are rounded under- 
neath, so as to be nearly circular in cross-section, as is the case with those of the fish-hawk 
(Pandion). They are always horny (comeows). They take name from and are reckoned by 
their respective digits : thus, 1 d. = claw of 1 ^ ; 2 cl. = claw of 2 1, etc. 
Fig. 53 ter. — Foot of Parra gyvmostoma, nat. size, showing the long, straight claws. (From Pr. U. S. Nat. 
Mus. The spurred wing of the same bird is also shown. See p. 114.) 
Spurs (Lat. calcar, a spur) are developed on the metatarsal bones of a few birds. They 
are of the nature of claws, being hard, horny modifications of the epiderm : but they have 
nothing to do with the digits. They possess a bony core upon which they are supported, 
like the horns of cattle. Such grow^ths chiefly occur in gallinaceous birds : the spurs of the 
domestic fowl are a familiar case. Sometimes there are a pair of such weapons on each foot, 
as in the Pavo Mcalcaratus. The only instance of their occurrence among indigenous birds of 
North America is offered by the wild turkey (Meleagris gallipavo). Metatarsal spurs are 
characteristic of the male sex ; they are offensive weapons, and belong to the class of ^'second- 
ary sexual characters" (p. 90). (For wing-spurs, as shown in fig. 53 ter, see p. 114.) 
§ 4.— AN INTRODUCTION TO THE ANATOMY OF BIRDS. 
Anatomical Structure now affords ornithologists many and the most important of the 
characters used in classification. In fact, few if any of the groups above genera can be 
securely established without consideration of internal parts and organs, as well of exterior 
modifications of structure. Therefore, the student who really ''means business" must be on 
speaking terms at least with avian anatomy. For example, none could in the least intelli- 
gently understand a wing or a leg without knowing the bony framework of those members. 
Yet, for me to adequately set this matter forth would be to occupy this whole volume with 
anatomy ; whereas, I can only devote a few pages to the entire subject. In such embarrass- 
ment, which attends any attempt to treat a great theme in a short way that shall not also be a 
small way, attention must be mainly confined to those points which bear most directly upon 
systematic ornithology as distinguished from pure anatomy, in order to bring forward the 
structures which are more particularly concerned in the classification of birds. I wish to 
give a fair account of the skeleton, as osteological characters are of the utmost importance for 
the determination of natural affinities ; and to continue with some notice of prominent features 
of the muscular, vascular, respiratory, digestive, urogenital, and nervous systems, and 
organs of the special senses, as the eye and ear. The tegumentary system has already been 
treated at some length (pp. 82-91) ; so has the osseous system, so far as the bones of the limbs 
are concerned (pp. 106-109, 118-122, 127 ). What further I shall haA-e to say is designed 
merely as an introduction to the rudiments of avian anatomy, and is supposed to be addressed 
to beginners only. 
