GENERAL CHARACTERS OF BIRDS. 651 



there are only two separate carp als, 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 (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 r 



, r FIG. 356. Diagram- 



suppressed. ^ secti ^ of 



In regard to the alimentary system, the young bird. After 

 absence of teeth, the frequent occurrence of Gadow. 

 a crop and a gizzard, the usual shortness *, spinal cord ; /., ver- 

 of the large intestine, the presence of a if v b e r r a ; ' <' gutY *i! 



Cloaca, may be noted. (dotted), somatic layer 



rm T . r j j j *i i of mcsoblast ; spl. 



The heart ts four-chambered ; the single (dotted), splanchnic 

 aortic arch curves to the right side; only Jj*T ao r f ta ; me ^ la i 

 the pulmonary artery rises from the right productive organ ; K. t 

 ventricle ; the two valves between the right kldney - 

 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 14 F. higher 

 than that of Mammals. 



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

 the thorax ; the bronchial tubes expand in irregular branches 

 in the lungs ; the ends of some of these branches are continued 

 into surrounding air-sacs ; these may be continued into the 

 bones, and end in minute air-spaces. The trachea has bony 

 rings, a larynx (without vocal chords) at Us upper end, and a 



