structure is much simpler
than that of the rabbit's lung, in which the branching bronchi, the
imperfect cartilaginous rings supporting them, alveoli, arteries and
veins, form together a quasi-solid mass.

Section 6. The mechanism of respiration is fundamentally different
from that of the mammal. The method is as follows:-- The frog opens
its anterior nares, and depresses the floor of the mouth, which
therefore fills with air. The anterior nares are then closed, and the floor
mouth rises and forces the air into the lungs-- the frog, therefore,
swallows its air rather than inhales it. The respiratory instrument of
the rabbit is a suction pump, while that of the frog is a "buccal force
pump."

Section 7. The heart is not quadrilocular (i.e., of four chambers), but
trilocular (of three), and two structures, not seen in Lepus, the
truncus arteriosus and the sinus venosus, into the latter of which
the venous blood runs before entering the right auricle, are to be
noted. The single ventricle is blocked with bars of tissue that render
its interior, not an open cavity, but a spongy mass. Figure 2, Sheet
11, shows the heart opened; l.au. and r.au. are the left and right
auricles respectively; the truncus arteriosus is seen to be imperfectly
divided by a great longitudino-spiral valve (l.s.v.); p.c. is the
pulmo-cutaneous artery -going to the lungs- [supplying skin and
lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and
limbs]; and c.a. the carotid artery going to the head; all of which
vessels (compare Figure 1) are paired.

Section 8. It might be inferred from this that pure and impure blood
mix in the ventricle, and that a blood of uniform quality flows to lungs,
head, and extremities; but this is not so. The spongy nature of the
ventricle sufficiently retards this mixing. It will be noted that the
opening of pulmonary arteries lies nearest to the heart, next come the
aortic and carotid arches, which have a common opening at A.
Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a
close meshwork of arterioles, the carotid gland, forming a sponge-like
plug in this vessel.] is a spongy mass of matter, the carotid gland
inserted upon the carotid. Hence the pulmonary arteries yawn nearest
for the blood, and, being short, wide vessels, present the least
resistance to the first rush of blood-- mainly venous blood for the right
auricle. As they fill up, the back resistance in them become