VERTEBRATA. 
S3 
spongy mass of minute cells 
communicating with the ex- 
ternal air. Over the surface 
of these cells the blood-vessels 
ramify, and their coats become 
so thin as to admit the union of 
the oxygen with the contain- 
ed blood, the result of which 
is an instant change of its color 
from dark red to vivid scarlet. 
The gills of fishes perform the 
same office as the lungs, water 
being used instead of air. The 
blood thus renewed is return- 
ed by being carried to another 
chamber of the heart, whence 
it is ejected into the other set of 
vessels, called arteries^ which, 
large at first, but branching in- 
to innumerable ramifications, 
convey it to every part, and at 
last, in a manner not thorough- 
ly understood, transfer it into 
the extremities of the veins, 
which, as we have seen, carry 
it back to be renewed again. 
This is what is meant by the 
well-known expression, the ch'- 
culation of the blood, and which 
is carried on not only in man, 
l)ut in all the other vertebrata. 
The young are produced in 
all cases from ova, or eggs, 
which exist in the body of the 
female. In most cases, the 
young animal is excluded from 
the egg while it is within the 
body of the mother; in others 
the egg itself is produced, and 
the young hatched some time 
afterwai'd. The animals of the 
former class are called vivipa- 
rous ; of the latter, oviparous. 
A curious and remarkable 
departure from both of these 
systems is found in the marsu- 
ptialia, as the opossum, kanga- 
roo, and others, which have a 
pouch under the belly, into 
which the young, born in an 
immature state, are received 
and nursed till they are able 
to take care of themselves. 
Vol. L— 5 
Facial Nerve. 
Brachial Plexus. 
Brain. 
' Little Brain. 
Spinal Marrow. 
Internal Cntaneous 
Nerve. 
Median Nerve. 
Cubital Nerve 
Kadial and 
Museulo-Cutaneous 
Nerve. 
Intercostal Nerves 
Femoral Plexus. 
External Peronea' 
Nerve. 
External Saphenous 
Nerve. 
THE NERVOUS SYSTEM IN MAN. 
Temporal Artery. 
Carotid Artery. 
Aorta. 
Renal Artery. 
Iliac Artery. 
BmWBT— -•■ Vertebral Artery. 
ilwlHTf^ihi ' Subclavian Artery, 
Axillary Artei-y- 
Brachial Artery. 
CoBliac Artery. 
m 
iiwm^BK wsaa. Radial Artery. 
Femoral Artery. 
Anterior Tibial 
Artery. 
Art. Pediosa. 
Posterior Tibial 
Artery. 
Peroneal Artery 
THE ABTEKIAl SYSTEM IN MAN. 
