be exercised to develop correct ideas of the principles and processes
derived from these sciences. Too much latitude has been taken in the past
in the use of comparisons and illustrations drawn from "everyday life." To
teach that the body is a "house," "machine," or "city"; that the nerves
carry "messages"; that the purpose of oxygen is to "burn up waste"; that
breathing is to "purify the blood," etc., may give the pupil phrases which
he can readily repeat, but teaching of this kind does not give him correct
ideas of his body.

The method of teaching, however, that uses the pupil’s experience as a
basis upon which to build has a value not to be overlooked. The fact that
such expressions as those quoted above are so easily remembered proves the
value of connecting new knowledge with the pupil’s experience. But _the
inadequacy of this experience must be recognized_ and taken into account.
The concepts of the average pupil are entirely too indefinite and limited
to supply the necessary _foundation for a science_ such as physiology.
Herein lies the great value of experiments and observations. They
supplement the pupil’s experience, and increase both the number and
definiteness of his concepts. No degree of success can be attained if this
phase of the study is omitted.

The best results in physiology teaching are of course attained where
laboratory work is carried on by the pupils, but where this cannot be
arranged, class experiments and observations must suffice. The Practical
Work described at the close of most of the chapters is mainly for class
purposes. While these serve a necessary part in the development of the
subject, it is not essential that all of the experiments and observations
be made, the intention being to provide for some choice on the part of the
teacher. A note-book should be kept by the pupil.