A number of lowly animals claim attention from the medical man for they are parasites of the human body; many fungi, too, cause disease. A certain amount of chemical work with the microscope also falls to the lot of the medical man. We have stated many times in our pages that, both plants and animals, are made up of one or more cells and that the cells of plants are each surrounded by a more or less rigid cell wall, whilst those of animals have no definite cell wall. As a start in our medical investigations we may examine some cells from our own person. There is no need for alarm, the operation is quite painless. After having prepared our microscope and got ready a clean slide, on which we place a drop of clean water, we must take some clean, blunt instrument such as a tooth-brush handle and gently scrape the inside of the cheek. Having done so, dip the part of the tooth-brush with which the scraping has been done, in the drop of water, then cover the water with a cover slip. A careful examination of our object with a fairly high magnification will show that, though the scraping was very gentle and quite painless, something has been removed from the mouth. We shall see a number of cells, somewhat overlapping like the tiles of a roof. They are almost colourless, and have ill-defined margins, certainly no cell wall, and each cell contains a dark spot near its centre. These cells from our own person will give us a good idea of the appearance of animal cells, for we are animals, much as we may dislike the idea. The dark spot in the middle of each cell is called a nucleus, it is the most active part of the cell and when the latter is about to divide, as it does when growth takes place, the nucleus always divides first. [Illustration: 1. CRYSTALS FROM HUMAN BLOOD It is often possible to identify the source of blood by means of crystals which can be obtained from it, for each race of animal has its own special form of blood crystal. The large crystal at the margin exhibits the typical shape. 2. CRYSTALS FROM THE BLOOD OF THE BABOON These show some resemblance to human blood. ]