d
bowels; _q_, the _vena cava superior_; _r_, the _left auricle_; _s_, the
left _coronary artery_, which distributes the blood exclusively to the
substance of the heart.

[Illustration: Fig. 15.]

ORGANS OF DIGESTION AND RESPIRATION.

Digestion and respiration are the processes, by which the food is
converted into blood for the nourishment of the body. The engraving on
the next page (Fig. 16) shows the organs by which these operations are
performed.

In the lower part of the engraving, is the stomach, marked S, which
receives the food through the _gullet_, marked G. The latter, though in
the engraving it is cut off at G, in reality continues upwards to the
throat. The stomach is a bag composed of muscles, nerves, and
blood-vessels, united by a material similar to that which forms the
skin. As soon as food enters the stomach, its nerves are excited to
perform their proper function of stimulating the muscles. A muscular
(called the _peristaltic_) motion immediately commences, by which the
stomach propels its contents around the whole of its circumference, once
in every three minutes.

[Illustration: Fig. 16.]

This movement of the muscles attracts the blood from other parts of the
system; for the blood always hastens to administer its supplies to any
organ which is called to work. The blood-vessels of the stomach are soon
distended with blood, from which the _gastric juice_ is secreted by
minute vessels in the coat of the stomach. This mixes with the food,
and reduces it to a soft pulpy mass, called chyme. It then passes
through the lower end of the stomach, into the intestines, which are
folded up in the abdomen, and the upper portion, only, of which, is
shown in the engraving, at A, A. The organ marked L, L, is the liver,
which, as the blood passes through its many vessels, secretes a
substance called _bile_, which accumulates in the gall-bladder, marked
B. After the food passes out of the stomach, it receives from the liver
a portion of bile, and from the _pancreas_ the _pancreatic juice_. The
pancreas does not appear in this drawing, being concealed behind the
stomach. These two liquids separate the substance which has passed from
the stomach, into two different portions. One is a light liquid, very
much like cream in appearance, and called _chyle_, of which the blood is
formed; the other is a more solid substance, which contains the refuse
and useless matter, with a smaller portion of nourishment; and this,