484 MANUAL OF ZOOLOGY SECT. 



becoming entirely obliterated. There is a single rounded 

 occipital condyle. The premaxillae are very large and form 

 the greater part of the upper jaw. The sternum is broad 

 and is usually provided with a prominent keel or carina as 

 in the Pigeon, but the carina is absent in the Ratitae, and 

 is rudimentary or absent in some flightless Carinatae. The 

 clavicles and interclavicle unite to form a furcula. The 

 distal carpals and the metacarpals unite to form a carpo- 

 metacarpus. The pubes and ischia run downwards and 

 backwards parallel with one another ; and neither the pubes 

 nor the ischia unite in a symphysis, but remain widely 

 separated at their distal ends, except in the Ostrich, in 

 which the pubes unite distally, and the South American 

 Ostriches, in which the ischia unite while the pubes remain 

 free. Universally characteristic of the skeleton of the hind 

 limb is the union of the tibia with the proximal element of 

 the tarsus to form a tibio-tarsus, and of the distal element of 

 the tarsus with the second, third, and fourth metatarsals to 

 form a tarso-metatarsus, the ankle-joint being situated 

 between these bones. The skeleton always contains air 

 cavities to a greater or less extent. 



The presence of crop, proventriculus and gizzard as in 

 the Pigeon is universal among Birds. The gizzard is most 

 powerful in grain-eating birds, thinner- walled in flesh-eaters. 

 There is a pair of caeca in most Birds at the junction of the 

 large and small intestines. The voice of Birds is always 

 produced not in the larynx as in other higher Vertebrates, 

 but in a syrinx situated either, as in the Pigeon, at the 

 junction of the trachea and bronchi, or at the anterior ends 

 of the bronchi, or the posterior end of the trachea. The 

 system of air-sacs connected with the bronchi described in 

 the account of the Pigeon is of universal occurrence. 



The temperature of the blood is always high. The heart 



