598 



BIRDS. 



processes) are directed backwards parallel to the ischia. There 

 is no pubic symphysis except in the African ostrich (Struthio), 

 and no ischiac symphysis except in the American ostrich (Rhea). 

 In the hind-limb, the fibula is incomplete and united to the 

 tibia ; there are no free tar sal bones, half of them being united 

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

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

 united metatarsals (which thus form a tarso-metatarsus). In 

 other words, the ankle joint is intertarsaL The maximum 

 number of toes is four, of which the first is the hallux, and if 

 there be four, the metatarsal of the hallux is free from the 

 other three. 



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

 frequent occurrence of a crop and a gizzard, the usual short- 

 ness 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 ven- 

 tricle ; the two valves between the 

 right auricle and the right ven- 

 tricle are in part muscular ; the 

 red blood corpuscles are oval and 

 nucleated ; the temperature of the 

 body is from 2 -14 F. higher than 

 that of Mammals. 



The lungs are fixed to the 

 dorsal wall of the thorax ; the 

 bronchial tubes expand in irre- 

 gular branches in the lungs ; the 

 ends of some of these branches are 

 continued into surrounding air 

 sacs ; and these are continued into air spaces in the bones. 

 The trachea is supported by bony rings, and has a voiceless 

 larynx at its upper end, and a syrinx or song-box (with vocal 

 cords) at the origin of the bronchi. 



The 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. 



The testes lie beside the kidneys ; the vasa deferentia run 



FlG. 207. Diagrammatic 

 section of young Bird. (After 

 GADOW.) 



n., Spinal cord; ?/., vertebra 

 r., rib; L., liver; G., gut',som 

 (dotted), somatic layer of meso 

 blast ; spl. (dotted), splanchnic 

 layer of mesoblast ; ao. , aorta 

 7?., reproductive organ ; K. 

 kidney. 



