516 



Comparative Morphology of Chordates 



rectrices. Its skeleton consists of several freely movable vertebrae 

 followed by a more or less elongated and pointed bone, the pygostyle 

 (Fig. 401), formed by fusion of several embryonic vertebrae. In the 

 pigeon, there are 6 movable caudal vertebrae, and the pygostyle 

 consists of 4 or more vertebrae. But to these 10 or more vertebrae 

 which are literally postsacral for the pigeon may be added the 5 

 reptilian caudals (postsacral for the reptile) which have become in- 

 corporated into the pigeon's synsacrum. 



The skull of the bird (Figs. 401, 405), as regards the number and 

 general arrangement of its constituent bones, is very similar to that 

 of a reptile. The following several factors, however, combine to give it 

 a highly characteristic form. Relative to the size of the body, the bird's 

 brain is somewhat larger than that of the reptile, but of very different 

 form, being shorter and thicker (Fig. 416). The eyes are extraordinarily 

 large, but the nasal organs are not especially well developed. The jaws 

 are narrow and taper forward into the pointed beak. Therefore the 

 skull, viewed dorsally, has a roughly triangular outline (Fig. 4055). 

 The much-enlarged posterior cranial region contains the short, compact 



Fig. 405. (A) Skull of Callus domesticus. (a) articular; (at) recess leading into 

 Eustachian tube; (b) pterygoid; (c) occipital condyle; (d) palatine; (e) rostrum; 

 (f) mandibular foramen; (fo) fenestra ovalis; (fr) fenestra rotunda; (fz) zygo- 

 matic process of frontal; (g) supra-angular; (h) dentary; (is) interorbital septum; 

 (j) jugal; (l) lacrimal; (m) maxillary; (mp) maxillo-palatine process; (n) nasal; 

 (of) optic foramen; (pm) premaxillary; (pt) posterior tympanic recess; (q) quad- 

 rate; (qj) quadratojugal; (sf) olfactory foramen; (sz) zygomatic process of squa- 

 mosal; (tf) foramen for trigeminal nerve. (Courtesy, Marshall and Hurst: "Prac- 

 tical Zoology," London, John Murray & Co.) 



(B) Columba livia. Skull of a young bird (about six weeks old); dorsal view. 

 (fr) Frontal; (ju) jugal; (mx) maxillary; (na, na', na") nasal; (pa) parietal; (p.mx) 

 premaxillary; (qu) quadrate. (From Parker: "A Course of Instruction in Zootomy." 

 By permission of The Macmillan Company, publishers.) 



