6 2 8 



are usually well developed, and connected by an interclavicle, 

 which may be connected with the apex of the sternum. The 

 fore-limb has not more than three digits (I., II. and III.), the 

 three metacarpals are fused (except in Archaeopteryx), and 

 there are only two separate carpals, the others fusing with the 

 metacarpals, and thus forming a carpo-metacarpus. The 

 thumb is often clawed, the second digit rarely. 



The ilia of the pelvis may be firmly fused to the complex 

 sacrum; the acetabulum is incompletely ossified; the pubes 



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 more 

 or less united to the tibia ; the proximal 

 tarsal bones are 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 

 matic section "of (which thus form a tarso-metatarsus). 

 young bird. After As in reptiles, the ankle-joint is therefore 



intertarsal. The maximum number of 

 ., Spinal cord; *., ver- foes is four, ofwhich the fir st is the 



som 

 ^ 



spl 



FIG. 309. Diagram- 

 matic : 

 young b 

 Gadow. 



tebra 

 liver 



r., rib; L., 

 G., gut; som. 



hallux : if there be four, the metatarsal 

 of the hallux is free from the other three 

 (dotted), splanchnic fused metatarsals ; if there are onlv three 



layer of mesoblast : 'JTTT T ^ 7 



ao., aorta; R., re- the hallux has been suppressed. 



productive organ ; A., j n re cr ar i f o f] ie alimentary system. 



kidney. , - - 7 - / 



the absence of teeth, the frequent occur- 

 rence 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 tivo valves between the right auricle and 

 the right ventricle are in part muscular ; there is -no renal- 

 portal svstem : the red blood corpuscles are oval and nucleated : 

 the blood temperature is from 2- 14 F. higher than tJiat of 

 Mammals. 



The non-expansible lungs are fixed to the dorsal wall of 

 the thorax ; the bronchial tubes expand in irregular branches 



