MAMMALIAN CHARACTERISTICS 



781 



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

 710 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, but dijfer from other 

 Vertebrates, which are cold-blooded or poikilothermal. 



The lungs are invested by pleural sacs, and lie freely 

 in the chest cavity. Within the lungs the bronchial tubes 



Fig. 466. — Diagram of skull bones (partly after Flower and 

 Weber), the membrane bones shaded. ' 



BO., Basioccipital ; EG., exoccipital ; C, condyle ; SO., supra- 

 occipital ; Par., parietal ; Fr., frontal ; Na., nasal ; Pmx 

 premaxilla ; ME., mesethmoid ; L., lachrymal ; Tu., turbinal 

 PS., presphenoid; OS., orbitosphenoid ; AS., alisphenoid 

 BS., basisphenoid ; SQ., squamosal ; P., periotic ; T., tvmpanic , 

 PL, palatine ; PL, pterygoid ; Mx., maxiUa ; Ju., jugal ; T.h!, 

 tympano-hyal ; S.H., stylo-hyal ; E.H., epi-hyal ; C.H., cerato- 

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



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 urinogenital sinus. Except in Monotremes, 

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

 the genital duct, to form a urinogenital canal ; in the female, 

 except in Monotremes and a few other cases, the urethra and 

 the genital duct open into a common vestibule. 



In the more primitive Mammals the testes lie in the abdo- 

 men ; in the majority they descend permanently {or in a 



