956 ANIMAL PHYSIOLOGY. 
Passing on to the vertebrates, in the lowest group, the fishes, 
the heart consists of two chambers, an auricle and a ventricle, 
the latter being supplemented by an extension (bulbus arterio- 
sus) pulsatile in certain species; and an examination of the 
course of the circulation will show that the heart is through- 
out venous, the blood being oxidized in the gills after leaving 
the former. : 
Among the amphibians, represented by the frog, there are 
two auricles separated by an almost complete septum, and one 
Fig. 232. Fie 233. 
Fie. 282.—The frog’s heart, seen from the front, the aortic arches of the left side having been 
removed. (1x 4.) ca, carotid ; ¢. gl, carotid gland ; ao, aorta ; aw’, right auricle ; aw’, 
left auricle ; pr. c, vena cava superior ; pt. c. vena cava inferior ; p. cu, pulmo-cutaneous 
trunk ; tr, truncus arteriosus ;_v, ventricle (Howes). 
Fia. 233.—The same, seen from behind, the sinus venosus having been opened up to show the 
sinu-auricular valves. (1x 4.) p.v. pulmonary vein ; s.v, sinus venosus ; va’’, sinu-au- 
ricular valve. Other lettering as in Fig. 232 (Howes). 
ventricle characterized by a spongy arrangement of the mus- 
cle-fibers of its walls. 
In the reptiles the division between the auricles is complete, 
and there is one ventricle which shows imperfect subdivisions. 
In the crocodile, however, the heart consists of four per- 
fectly divided chambers. Of the two aortic arches, one arises 
together with the pulmonary artery from the right ventricle, 
and, as it crosses over, the left communicates with it by a small 
opening, so that, although the arterial and the venous blood 
are completely separated in the heart, they intermingle outside 
of this organ. 
In birds the circulatory system is substantially the same as 
in mammals; but in all vertebrate forms below birds the blood 
distributed to the tissues is imperfectly oxidized or is partially 
venous. 
