The sweat glands, as we have already seen, make up part of the service of the removal of waste in that they help to carry away water from the body. To a very slight extent, but much slighter than is commonly believed, they get rid also of waste products of cell activity; for example ammonia, which is one such product, can frequently be detected in the sweat by its odor; practically, however, the importance of sweat glands is not in their discharge of waste organic substances, but only in their discharge of water. In this they are playing a very important part in the regulation of body temperature, so that the further account of the sweat glands as well as of the skin of which they form a part is left for the chapter in which the regulation of temperature is described. CHAPTER XVIII MORE ABOUT THE USE OF FOOD BY THE BODY We have now brought our description of the various things that happen in our bodies up to the point where we may begin to make some kind of summary of them; particularly in respect to what goes on within our individual cells. We have seen how some of our cells are muscle cells by which we make motions; others are the cells of sense organs by which we get the necessary information for guiding our activities; still others are in the nervous system and carry on the adjustments by which we act in accordance with the indications of our surroundings and also of our past experience; finally we have the cells which manufacture materials, as in the various digestive glands or in the glands which make hormones. We have also looked into the kind of materials which these cells require, where they come from, and how they are prepared in the digestive tract, and we have seen how these prepared materials and oxygen are conveyed to the different parts of the body where the cells can get them and finally how the waste products which all these cells give off are gotten rid of. We are now to turn to the process of metabolism itself as it takes place within the cells. In Chapters III and IV we looked into the use of food for power development and for repairing the wastage of protoplasm, as well as for the making of new protoplasm in growth. We have seen also that the body contains much nonliving material, as in the bones and teeth, which must come from the food and which must be put in place as the result of metabolism on the part of living cells.