20 



The Anatomy of Birds 



its greatest complexity in the perchers, in which the hypotarsus, as it is termed, is 

 pierced for the passage of four or five tendons. 



In the Crow, for example, there are four large and one small perforation, in the 

 Clamatores but foun, and one of these is closed by cartilage and not by bone; 

 in the Picariae there are but one or two tendinal perforations, and in the divers but 

 one, so that the specialization of the tendons and that of the tarsus go together. 

 The three divisions of the lower end of the tarsus indicate that it is composed 

 of three bones, but these bones are clearly shown only in embryos, in young 

 Ostriches and in Penguins, where the bones, though united, are plainly indicated 

 throughout life, this retaining of a primitive condition being one of the characters 

 which has led many good authorities to place the Penguins in a group contrasting 

 with all other fan-tailed birds. 1 



The leg attaches to the hip-bone, or pelvis, each half of which is composed 

 of the usual three bones, ilium, ischium, and pubis, although these fuse together 

 at an early date and show as separate bones only in very young birds. 



In all birds the pubis is directed backwards, and the greater part of the ilium 

 lies in front of the hip-joint, this being a point where existing birds differ from 

 existing reptiles. In the lower birds, as the Ostriches and Tinamous, the two 

 principal bones of the pelvis, the ilium and ischium, are free from one another 

 behind, this being a primitive character in which these 

 birds resemble reptiles. The Cassowaries are an excep- 

 tion in having these bones united. 



In the vast majority of living birds (the Neognathse) 

 the ilium and ischium are firmly united. The pubic 

 bones, the long, slender, lowermost bones of the pelvis, 

 unite posteriorly in the Ostrich, but are free in other 

 birds and are frequently widely separated ; they may even 

 be nearly lacking, as in Eagles, in which the hinder por- 

 tions only remain. The outward flare and generally open 

 character of the pelvis below has to do with the question 

 of room for the passage of the large and brittle egg. Birds 

 that fly much, and especially sailing and soaring birds like 

 the Petrels and Frigate-birds, have a broad, short pelvis, 

 while in water birds it is long and narrow, much the same condition being 

 found in flightless birds which run much. 



The neck vertebrae of birds are peculiar from the character of their articula- 

 tions, which are saddle-shaped, concave one way and convex the other, a form 

 termed heterocoelous, and one that allows great freedom of movement in both 

 planes. Theoretically the ball-and-socket joint permits the greatest amount 

 of motion, but in practice this form of joint is usually combined with some 

 arrangement which checks its movements. Thus in the snake, while there is 

 the utmost freedom of movement from side to side, there is but little play verti- 

 cally. The neck of the bird is always long and the vertebrae numerous; this is 



1 Recently described fossils indicate that in the earlier Penguins, the metatarsals were not 

 separated to the extent they are in existing species. 



FlG. 17. Right tar- 

 sus of Penguin, Apteno- 

 dytes pennanti. 



