696 MAMMALIA. 
one aortic trunk, which curves over the left bronchus. The 
red blood corpuscles are, when fully formed, non-nucleated, 
and appear as slightly biconcave discs, circular in outline, 
except in the Camelide, 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 potkilothermal, 
Fic. 382.—Diagram of skull bones (partly after Flower and 
Weber), the membrane bones shaded. 
BO., Basioccipital; £O., exoccipital; C., condyle; SO., 
supraoccipital; Par., parietal; 7, frontal; Wa., nasal; Pyzx., 
premaxilla; A7Z., mesethmoid; Z., lachrymal; 7z., turbinal; 
PS.,_presphenoid; OS., orbitosphenoid; AS., alisphenoid; 
BS., vasisphenoid ; SQ., squamosal; P., periotic; 7., tympanic}; 
Pi, palatine; Pt, pterygoid; JAZx., maxilla; Jz., jugal; 7.4, 
tympano-hyal; S.H., stylo-hyal; £.H/., epi-hyal; C.H., cerato- 
hyal; &.7/., basi-hyal; 72.H., thyro-hyal. 
The lungs are invested by pleural sacs, and lie freely 
in the chest cavity. Within the lungs the bronchial tubes 
Jork 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 form a urogenital canal; in the female, 
