s the heart able
to rest out of every twenty-four?

13. State the functions of the capillaries. Show how their structure
adapts them to their work.

14. What kind of physical exercise tends to strengthen the heart? What
forms of exercise tend to injure it? State the effects of alcohol and
tobacco on the heart.

15. How may rheumatism injure the heart?

16. Give directions for checking the flow of blood from small and from
large blood vessels.

PRACTICAL WORK

In showing the relations of the different parts of the heart, a large
dissectible model is of great service (Fig. 24). Indeed, where the time of
the class is limited, the practical work may be confined to the study of
the heart model, diagrams of the heart and the circulation, and a few
simple experiments. However, where the course is more extended, the
dissection of the heart of some animal as described below is strongly
advised.

*Observations on the Heart.*--Procure, by the assistance of a butcher, the
heart of a sheep, calf, or hog. To insure the specimen against mutilation,
the lungs and the diaphragm must be left attached to the heart. In
studying the different parts, good results will be obtained by observing
the following order:

1. Observe the connection of the heart to the lungs, diaphragm, and large
blood vessels. Inflate the lungs and observe the position of the heart
with reference to them.

2. Examine the sac surrounding the heart, called the _pericardium_. Pierce
its lower portion and collect the pericardial fluid. Increase the opening
thus made until it is large enough to slip the heart out through it. Then
slide back the pericardium until its connection with the large blood
vessels above the heart is found. Observe that a thin layer of it
continues down from this attachment, forming the outer covering of the
heart.

3. Trace out for a short distance and study the veins and arteries
connected with the heart. The arteries are to be distinguished by their
thick walls. The heart may now be severed from the lungs by cutting the
large blood vessels, care being taken to leave a considerable length of
each one attached to the heart.

[Fig. 24]

Fig. 24--Model for demonstrating the heart.

4. Observe the outside of the heart. The thick, lower portion contains the
cavities called _ventricles_; the thin, upper, ear-shaped portions are the
_auricles_. The thicker and denser side lies t