The third kind of structures which are present in the blood stream are much smaller than the red corpuscles, but are nowhere near so numerous. They are called the _platelets_ and are disk-shaped bodies composed chiefly of protein material, and probably, although not certainly, living cells. Their presence in the blood remained unsuspected up to about the end of the last century, not so much because of their very small size, as because they go to pieces very quickly after the blood is shed. By the time a drop of blood could be gotten under the microscope they would be all gone. They were discovered only as the result of the adoption of a method of treating the blood which preserved them long enough so that they would still be present when the blood was looked at under the microscope. They seem to have something to do with the changes that take place in the blood by which it is caused to clot. Whether this is their only function, we do not know. We have now described the substances which are present in the blood and in the other tissue fluids as a fairly permanent part of their make-up. In addition to these there are present all the materials that are in transit to the cells or from them. These include all kinds of foodstuffs on their way from the digestive organs to the cells. As we shall see in detail later, the digestive organs work over the food that we eat before passing it on to the blood, so that the actual food materials that are being transported by the blood are the digested products of the food rather than the food itself. For example, there are to be found in the blood, in addition to the regular blood proteins which were described a moment ago, the digestion products of the food proteins on their way to serve the needs of the various cells of the body. There will be found also the digestion products of the other classes of foodstuffs. The body fluids contain also the waste products of cell metabolism on the way to be discharged from the body and the special products, such as the hormones, which are manufactured by certain cells and carried through the blood stream to act upon other cells. Of course we realize that not all of the materials that are manufactured by cells are poured out into the blood stream. Such materials as saliva, gastric juice and the like are passed directly from the cells in which they are manufactured into tubes by which they are conveyed to the digestive canal, where they carry on their special work of digesting the food.