80 ANTHROPOLOGY. 
The blood, after passing through the lungs, is brought back to the left auri- 
cle, which, contracting at the moment when the left ventricle is expanding, 
torces the blood into the left ventricle. The contraction of this ventricle 
drives the blood into the aorta, its reflux into the auricle being prevented 
by the mitral valve. The semilunar valves of the pulmonary artery and 
aorta prevent the reflux of blood into their respective ventricles. 
Pl. 180, fig. 1, right half of the heart from before: *, right auricle; *, right 
ventricle. 1g, 2, left half of the heart from before: *, left auricle; ’, left 
ventricle. ig. 4, larynx, trachea, pericardium, and lungs: °, pericardium. 
Fg. 5, heart from before: ', right sinus; °, right auricular appendage (the 
two constituting the auricle); *, superior vena cava; *, inferior vena cava; 
*° left sinus and appendage, or left auricle; ”*, pulmonary veins; °, sulcus 
transversalis; *, longitudinal furrow or fissure; “, right ventricle; ™, pul- 
monary artery; ”, left ventricle; “, aorta. Sg. 6, section of the right half 
of the heart: *, right auricle; *, fossa ovalis; *, Kustachian valve; *, open- 
ing of the great coronary vein with the valvula Thebesii; °, right ventri- 
cle with its columnz carnex; °, a point of the tricuspid valve with the 
-chorde tendinez; ’, pulmonary artery with two of the semilunar valves. 
Fig. 7, section of the left half of the heart: ', left auricle with the open- 
ings of the pulmonary veins; ’, left ventricle; *, mitral valves; *, aorta 
with two semilunar valves. /%g. 8, direction of the muscular fibres of 
the heart: »”°*, fibres of the auricles; *, fibres of the ventricles; °, open- 
ings for the large vessels of the heart; °, place where the fibres twist 
round each other at the apex, to become united with the deep layers; 
’, place where the superficial anterior and posterior fibres interlace and 
become united with the deep fibres; **, openings of the pulmonary artery 
and aorta. 
2. SPECIAL ANATOMY OF THE ARTERIES. 
The aorta, situated on the base of the heart, may be considered as the 
main trunk of a tree, whose ramifications ever increasing constitute the 
arteries, the ultimate branches being the capillaries. The various arterial 
-divisions are named partly from their regional situation, as subclavian, 
axillary, &c., partly from their relative position, as deep or superficial, 
-and partly from their distinction, as cerebral, ophthalmic, &c. The 
‘names ‘‘aorta” and “artery” show the opinion entertained by the 
-ancients with respect to their functions, being supposed to conduct vital 
‘air exclusively, from being found empty of blood after death, yet still 
. distended. 
The arterial tubes are of a dense structure, and when empty preserve 
their form without collapsing. They are composed of three principal coats 
or tissues. The first or externa! is fibro-cellular, strong and resisting, and 
-connects the vessels with the surrounding parts. The second, or middle, or 
proper coat of the arteries is thickest, and consists of yellowish and rather 
dry fibres, elastic but not brittle; although circular they do not form com- 
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