THE CIRCULATION OF THE BLOOD. Dye 
As an example of the influence of valves and of blood-press- 
ure on the distribution of the blood we may take the case of the 
turtle, in which the subject has been most carefully studied. 
Fia. 234. . Fie, 235, 
Fra, 234.—The heart, dissected from the front, the ventral wall and one of the auriculo-ven- 
tricular valves having been removed. (1 x 6.) The rod, passing from the ventricle into 
the pylangium, shows the course taken by the blood flowing into the carotid and aortic 
trunks. sy, synangium ; p. v’, aperture of entry of pulmonary vein ; va’, sinu-auricular 
valve ; a. 8, inter-auricular septum ; va’. auriculo-ventricular valve; va. s, semi-lunar 
valves ; py, pylangium ; va.l, longitudinal valve (septum) of pylangium ; p. cw’, point of 
origin of pulmo-cutaneous trunk (Howes,) 
Fie, 235.—Heart and arteries of a reptile (boa). r, right. and 1, left auricle ; c, carotid anlery s 
ra, right aortic arch ; la, left aortic arch ; p, pulmonary artery ; rv, right vena cava ; lv, 
left vena cava superior; vi, vena cava inferior. The arrows indicate the course of the 
circulation (after Gegenbaur). 
The structure of the heart and the relations of its main ves- 
sels, etc., will probably be sufficiently clear upon an examina- 
tion of the accompanying figures and the descriptions beneath 
them. 
The right and left auricles pour their blood, kept somewhat 
_ apart by valves, into the cavum venosum. 
Two arterial arches arise from the right-hand part of this 
region, while the pulmonary artery is a branch carrying off 
blood to the lungs from the cavum pulmone. No vessels arise 
from the cavum arterioswm. 
Since the blood flows in the direction of least resistance 
when the ventricle contracts, the venous blood of the cavum 
venosum passes on into the pulmonary artery in which the 
pressure is, of course, lower than in the aortic arches, but, as 
the systole continues, the arterial blood of the cavum arterio- 
17 
