696 



MAMMALIA. 



one aortic trunk, which curves over the left bronchus. Tht 

 red blood corpuscles are, when fully formed, non-nucleated, 

 and appear as slightly biconcave discs, circular in outline, 

 except in the Camelidcz, where they are elliptical. There is 

 no renal-portal system. 



Mammals are warm-blooded or stenothermal, i.e. their 

 body-temperature does not change with that of the surrounding 

 medium. In this they agree with Birds, and differ from other 

 Vertebrates, which are cold-blooded or poikilothermaL 



FlG. 382. Diagram of skull bones (partly after Flower and 

 Weber), the membrane bones shaded. 



O. t Basioccipital ; EO., exoccipital ; C., condyle; SO., 

 supraoccipital ; Par., parietal; Fr. t frontal; Net., nasal; Pmx. 



tympano-nyai , o./j., siyio-nyai ; .zi./?., < 

 hyal ; B.H., basi-hyal ; Th.H., thyro-hyal. 



The lungs are invested by pleura I sacs, and lie freely 

 in the chest cavity. Within the lungs the bronchial tubes 

 fork repeatedly into finer and finer branches. At the top 

 of the trachea there is a complex larynx with the vocal 

 chords. 



The kidneys are generally compact and rounded bodies ; 

 the ureters open into the bladder, except in Monotremes, in 

 which they enter a urogenital sinus. Except in Monotremes, 

 the outlet or urethra of the bladder unites in the male with 

 the genital duct, to fonn a urogenital canal ; in the female, 



