730 



PHYLUM CHORDATA : CLASS AVES — BIRDS 



the distal end of the tibia {which is therefore called a tibio- 

 tarsiis), the others being united to the proximal end of three 

 united metatarsals {which thus form a tarso-metatarsus). As 

 in Reptiles, the ankle-joint is therefore intertarsal. The 

 maximum number of toes is four, of which the first is the 

 hallux : if there be four, the metatarsal of the hallux is free 

 from the other three fused metatarsals ; if there are only 

 three, the hallux has been suppressed. 



In regard to the alimentary system, the absence of teeth, 



the frequent occurrence of a crop and 

 a gizzard, the usual shortness of the 

 large intestine, the presence of a 

 cloaca, may be noted. 



The heart is four -chambered ; the 

 single aortic arch curves to the right 

 side ; only the pulmonary artery rises 

 from the right ventricle ; the two 

 valves between the right auricle and the 

 right ventricle are in part muscular ; 

 there is no renal portal system ; the 

 red blood corpuscles are oval and 

 nucleated ; the blood temperature is 

 from 2° to 14° F. higher than that of 

 Mammals. 



The non-expansible lungs are fixed 

 to the dorsal wall of the thorax ; the 

 bronchial tubes expand in irregular 



the ends of some 

 are continued into 

 air-sacs ; these may be 

 continued into the bones, and etid in 

 The trachea has bony rings, a larynx 

 {without vocal chords) at its upper end, and a syrinx or 

 song-box {with vocal chords) at the origin of the bronchi. 

 Expiration is the more active part of the respiratory process. 



The {metanephric) kidneys are three-lobed, and lie embedded 

 in the pelvis ; the ureters open into the cloaca ; there is no 

 bladder ; the urine is semi-solid, and consists chiefly of urates. 

 Water must be mainly got rid of by evaporation from the 

 walls of the air -sacs and air-passages. 



The testes lie beside the kidneys ; the vasa defer en tia run 



som 



Fig. 431. — Diagram- 

 matic section of 

 youngbird. — After 

 Gadow. 



n., Spinal cord ; v., ver- 

 tebra ; r., rib ; L., 

 liver ; G., gut ; som. 

 (dotted), somatic layer 

 of mesoblast ; spl. 

 (dotted), splanchnic 

 layer of mesoblast ; 

 ao., aorta ; R., re- 

 productive organ ; K., 

 . kidney. 



minute air-spaces. 



branches in the lungs 

 of these branches 

 surrounding 



