IN THE PERENNIBRANCHIATE BATRACHIA. 219 
the higher Reptilia the retarding channels are reduced to two, while in the more ener- 
getic warm-blooded classes the blood is distributed over all the frame by branches of 
a single continuous vessel. A corresponding gradation may be traced in the place of 
origin of the pulmonary arteries, and the consequent impulse received by them from 
the contractions of the heart. In the Siren the pulmonary arteries come off at the end 
of the branchial circulation. In Menopoma they come off from a corresponding situation, 
but receive more of the heart’s impulse, from its not being previously expended on sub- 
divisions of the branchial vessels. In Amphiuma the pulmonary and branchial arteries 
rise together from the end of the aortic bulb. In the higher orders of Reptiles the pul- 
monary arteries proceed by a common trunk from the ventricle itself: and lastly, in the 
warm-blooded classes they have a ventricle expressly appropriated to accelerate the 
circulation through them. 
In the Vertebrate, as in the Molluscous division of the Animal Kingdom, the Muscular 
and Perceptive energies rise in proportion to the perfection of the Respiratory and San- 
guiferous systems. 
PLATE XXXI. 
Fig. 1. The heart in situ of Siren lacertina. 
a. The termination of the inferior vena cava. 
b, 6. The two superior vene cave. 
c,c. The venous sinus. 
d,d. The right or systemic auricle. 
e,e. The left or pulmonic auricle. 
. The ventricle. 
. The elongated bulbus arteriosus. 
. The branchial arteries. These are divided on the right side, and the 
branchial arches turned outwards, to show 
i. The descending aorta, formed by the union of the trunks of the bran- 
chial veins. 
k, k. The pulmonary arteries. 
1, 1. The commencement of the lungs, laid open to show their cellular struc- 
ture, and 
m,m. The pulmonary veins. 
n. The esophagus. 
0,0. The external branchia. 
Fig. 2. The heart and pericardium of the Siren, showing the structure of the auricles. 
The same letters denote the same parts as in the preceding figure. The 
whole extent of the pericardium is here shown. The bristles marked e” e" 
are passed through the pulmonary veins along the common trunk m! into 
VOL. I. 2G 
Tay 
