620 



AVES 



[CH. 



saddles, being concave from side to side and convex from above 

 downwards in front and exactly the opposite curvatures behind 

 (Fig. 309). This arrangement allows great freedom of movement, 

 and the head and flexible neck of a bird may be said to play the 

 part which hand and arm play in human economy. The fore-limb 

 of Birds, from its function as a wing, is rendered totally useless for 



prehension. As all know, a 

 Bird is able to twist the head 

 completely round and look 

 straight backwards. In doing 

 so, of course it 

 Sys*l^ a] squeezes the skin 

 of one side of the 

 neck and stretches that of 

 the other, and so the great 

 jugular vein, which carries 

 blood from the head, is liable 

 to be blocked on one side 

 (Fig. 311). To obviate this 

 difficulty, the two jugulars 

 are connected by a cross 

 piece just under the head, so 

 that the blood from both sides 

 can always have a free pass- 

 age. The carotid arteries, 

 which take blood to the 

 head, come close together at 

 the base of the neck and run 

 up just under the vertebrae. 

 As they are placed close to 

 the axis of rotation and are 

 further protected by curved 



FIG. 310. Diagram of arterial arches of rods growing out from the 



a Bird viewed from the ventral aspect. ver t e brae and forming arches 



i, it, in, iv, v, vi. First to sixth arterial .1 ,1 



arches. 12. Tracheal (ventral carotid). OV6r tnem > tne y are never 



13. Common carotid (dorsal carotid), compressed, however much 



14. Systemic arch. 17. Dorsal aorta, ik U*-J H- *. M i 

 19. Pulmonary. 20. Innominate. 21. the bird twisfcs lts neck - 

 Subclavian (ventral type). 24. Coeliac. Turning now to the con- 

 sideration of the internal 



organs, we have first to notice the structure of the heart. In 

 Birds the ventricle is completely divided into two, a condition 

 found only in the Crocodiles among Reptiles, and even there the 



