mity of the back bone.
36. The rectum.
37. The testicles.]

Between these various surfaces is the real interior of the body, in
which there are many sorts of living tissues,[2] each, of which, in
addition to maintaining itself, has some function necessary for the
maintenance of the body as a whole. Many of these tissues have for
their main purpose the adjustment and cooerdination of the activities
of the different organs to the needs of the organism as a whole. The
activity of certain of the organs is essential for the maintenance of
life; without others life can exist for a time only; and others, such
as the genital glands, while essential for the preservation of the
life of the species, are not essential for the individual. There is a
large amount of reciprocity among the tissues; in the case of paired
organs the loss of one can be made good by increased activity of the
remaining, and certain of the organs are so nearly alike in function
that a loss can be compensated for by an increase or modification of
the function of a nearly related organ. The various internal parts are
connected by means of a close meshwork of interlacing fibrils, the
connective tissue, support and strength being given by the various
bones. Everywhere enclosing all living cells and penetrating into the
densest of the tissues there is fluid. We may even consider the body
between the surfaces as a bag filled with fluid into which the various
cells and structures are packed.

[Illustration: FIG. 9.--A LONGITUDINAL SECTION THROUGH THE FEMALE
PELVIS.

1. The Fallopian tube which forms the connection between
the ovary and the uterus.
2. The ovary.
3. The body of the uterus.
4. The uterine canal.
5. The urinary bladder represented as empty.
6. The entrance of the ureter.
7. The pubic bone.
8. The urethra.
9. The vagina.
10. The common external opening or vulva.
11. The rectum and anus.]

[Illustration: FIG. 10.--THE LUNGS AND WINDPIPE. Parts of the lungs
have been removed to show the branching of the air tubes or bronchi
which pass into them. All the tubes and the surfaces of the lungs
communicate with the inner surface of the body through the larynx.]

The nervous system (Fig. 8) represents one of the most important of
the enclosed organs. It serves an important function, not only in
regulating and cooerdinating all functions, but by means of the special
senses which are a part of it, the relations of the organism as a
whole with the env