The medical man may be called upon to decide two questions concerning human blood; he may wish to know whether it is healthy and, in the case of certain crimes, he must be able to state positively whether certain stains are caused by human blood or not. Dealing with these questions in order, let us see how we would proceed. There are many people, far too many in towns who are described as bloodless, the expression is of course an exaggeration for no bloodless person could continue to exist. What really happens is that such people are deficient in red blood corpuscles. If we examine two samples of blood, one from a perfectly healthy subject and one from a so-called bloodless subject we shall probably not be able to detect any difference between the two, but the experienced medical man will soon see that one sample has too few red blood corpuscles. An unhealthy state of the blood is often indicated by the shape of the blood crystals. Let us see how we may obtain some of these crystals. If we add a drop of ether to our drop of blood upon the slide, and wait a few moments till the ether has evaporated, we shall notice when we examine our object again, under a high magnification, that a number of prismatic crystals have formed, especially towards the edges of the slide. Now in certain diseases of the blood, these crystals are no longer formed in their usual shape, thus pointing out to the medical man that something is amiss. Again there are many blood parasites just as there are external parasites of man. In the disease known as malaria, very small parasites are introduced into the blood by mosquitoes. Each of these little parasites enters a red blood corpuscle, divides up into many smaller individuals, causes the corpuscles to burst, then each little parasite attacks more corpuscles. Some of these little bodies are sucked up, along with the blood of malaria patients, by other mosquitoes and, if the insects are of the particular kind which spread malaria, the parasites complete their development within the mosquito. Patients suffering from that terrible African malady known as sleeping sickness, have blood parasites resembling minute eels with long threadlike tails. These parasites are the cause of the malady and are introduced into the blood by flies, closely related to house flies. There are a very large number of blood parasites of one kind and another, so it is clear that a knowledge of blood is very important to the medical man.