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.