the _vena cava
superior_, or _upper vena cava_, brings back the blood from the arms and
head, the other, the _vena cava inferior_, or _lower vena cava_, brings
back the blood from the body and lower limbs.

In the preceding figure, H, is the heart, which is divided into four
compartments; two, called _auricles_, used for receiving the blood, and
two, called _ventricles_, used for sending out the blood. A, is the
_aorta_, or great artery, which sends its branches to every part of the
body. In the upper part, at _a_, _a_, _a_, are the main branches of the
_aorta_, which go to the head and arms. Below, at _a_, _a_, are the
branches which go to the lower limbs. The branches which set off at X,
X, are those by which the intestines are supplied by vessels from the
_aorta_. Every muscle in the whole body, all the organs of the body, and
the skin, are supplied by branches sent off from this great _artery_.
When the blood is thus dispersed through any organ, in minute vessels,
it is received, at their terminations, by numerous minute veins, which
gradually unite, forming larger branches, till they all meet in either
the upper or lower _vena cava_, which returns the blood to the heart. V
I, is the _vena cava inferior_, which receives the blood from the veins
of the lower parts of the body, as seen at v, v. The blood, sent into
the lower limbs from the _aorta_, is received by minute veins, which
finally unite at v, v, and thus it is emptied through the lower _vena
cava_ into the heart: _o_, _o_, represent the points of entrance of
those tributaries of the _vena cava_, which receive that blood from the
intestines, which is sent out by the _aorta_ at X, X. In the upper part,
V S, is the _vena cava superior_, which receives the blood from the head
and arms; v, v, v, are the tributaries of the upper _vena cava_, which
bring the blood back from the head and arms; _d_, _d_, represents the
course of the _thoracic duct_, a delicate tube by which the chyle is
carried into the blood, as mentioned on page 89; _t_, shows the place
where this duct empties into a branch of the _vena cava_.

It thus appears, that wherever a branch of the _aorta_ goes to carry
blood, there will be found a tributary of the upper or lower _vena
cava_, to bring it back.

The succeeding engravings, will enable the reader to form a more
definite idea of this important function of the system,--the circulation
of the blood. The heart, in man, and in all warm-blooded a