Now we may perform another operation upon ourselves, rather more painful than the last one but not very serious. We wish to examine some human blood, so we tie a handkerchief tightly round one of our fingers and with a clean needle—be sure that it is clean—make a puncture in the finger tip. The handkerchief bandage will prevent our feeling any pain. We must put the drop of blood we have obtained in the centre of a clean slide and examine it under the microscope. While the blood is still liquid, we shall see a number of circular discs floating about, they are very small being only 1/3200 inch in diameter. The centre of each disc appears darker than the rim, but this darker shade is only apparent. Using a term we introduced in our chapter on the lens, we may call the discs double concave. Now let us watch our objects for a moment and we shall notice that they begin to arrange themselves in chains, they appear like a number of draughtsmen placed one upon the other, our drop of blood is now beginning to clot. A simple experiment may be attempted at this stage; we must run a drop of water on to our little blood discs and watch carefully what happens. We shall see that they change their shape gradually, from being double concave they become flat sided, and this is not all, for they continue to change till both sides bulge outwards and they may be described as double convex.
These little discs are known as red blood corpuscles. Their shape is some indication of the animal to which they belong; those of man, as we have seen, are circular and so are those of most of the higher animals, except the camel tribe, which has oval, red blood corpuscles. Birds, reptiles and fishes have corpuscles agreeing in shape with those of the camel. In size there is a great deal of difference between the corpuscles of various animals; a member of the deer family has the smallest and a creature related to our Newts, called Proteus, has the largest. The size of the blood corpuscles bears no relation to the size of the animal to which they belong, those of the frog measure 1/1108 inch, or nearly three times the size of the red blood corpuscles of man. Without much difficulty it should be possible to obtain other samples of blood and the red corpuscles should always be examined, needless to say the blood should always be in a fresh condition. In addition to the red corpuscles we may notice a few smaller circular bodies, they are the white corpuscles. If we have any difficulty in finding them in our own blood, we can examine another specimen of frog’s blood, in which they are more easily seen. It has been calculated that in a cubic inch of blood from a healthy human being there are eighty millions of red and a quarter of a million of white blood corpuscles. Although there is such an enormous difference in the sizes of red corpuscles from various animals, the white corpuscles are remarkably constant, measuring about 1/3000 inch in warm-blooded animals and 1/2500 inch in reptiles. Sometimes, as we are examining the white blood corpuscles under the microscope, we shall notice that they behave in a similar manner to the Proteus Animalcule, which we described in our [chapter on Pond Life]. A kind of creeping motion takes place and the corpuscle loses its circular shape. In our own blood this movement lasts but a very few moments, in frogs’ blood, by keeping our slide moderately warm, we may witness the movement for some time.
It is perfectly easy to watch the circulation of blood in the foot of a frog. For this purpose we require a piece of apparatus known as a frog plate, this is merely a flat brass plate perforated with a number of holes, through which tape is passed to bind the frog down and having also a hole the size of the opening in the microscope stage, into which a circle of glass is usually fitted. Such a plate may be made of wood and will serve our purpose quite as well as the more expensive brass plate. We must bind our frog firmly, but not too tightly with wet rag, leaving one leg exposed. This free leg must now be fastened in such a manner that one of the webs between its toes comes over the opening in the plate, finally the toes must be carefully tied with string so that they remain apart and the web is fully expanded. Having fixed our frog in a satisfactory position let us examine his blood-vessels under a fairly high power. We shall find that there are large and small vessels in his webbed foot; we will devote our observation to the small vessels, called capillaries, from the Latin Capillus, a hair, because of their small size. Probably the blood will not flow when we make our first examination and this points to one of two things, either the frog is bound too tightly or he has not recovered from the alarm he experienced at his treatment. In the latter event he will not be long recovering and his blood circulation will soon be in full working order; in the former case, we must loosen his bandages. In some of the very small blood-vessels we shall notice that the blood always flows in one direction, in others it does not appear to have any definite direction. In either case, however, we can see the red corpuscles flowing rapidly along the central stream of the blood-vessel whilst the white corpuscles travel much more slowly along the sides.
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.
From our remarks concerning the sizes of the red corpuscles in the blood of man and various animals, one might be excused from thinking that it would be quite easy for anyone with a little experience to recognise human blood. As a matter of fact it is very difficult, it is always a doubtful matter to rely upon size alone. We have mentioned blood crystals and these give us a slightly better clue to the origin of the blood, for the blood crystals of different animals vary in shape, far more than their corpuscles. Those of the guinea-pig, for example, are little four sided pyramids; those of the mouse, eight sided and so on. Some apes, however, have blood crystals very similar to those of man, so similar that the two may be confused. The microscopist who is compelled to give an opinion concerning the origin of a sample of blood, especially blood which is some days old, is faced with no light task.
Without the assistance of the microscope, medical men would never have discovered the cause of many of these insect-borne diseases, they could never have traced the development of the blood parasites in the bodies of insects. For many years it was thought that the disease malaria was caused by the damp air of low-lying land, in fact the word malaria is derived from two Italian words meaning bad air. It was not till the advent of the microscope that the true cause of the disease was learned and the discovery was made that the only connection between the malady and dampness is that the mosquitoes which carry the disease can only thrive in damp situations.
Medical work with the microscope also entails a study of external and internal parasites of man other than those which infest the blood. Some of the larger human parasites we know only too well, they force their unpleasant attentions upon us from time to time, but fortunately they are not as a rule serious and are soon got rid of. The cleanest of us, in these days of universal travel, cannot avoid the visitations of the lively flea, he is at worst an annoying companion, but he has a cousin who is responsible for the passing of the germs of the dreaded plague from rats to man. Plague long remained a mystery, which, without the help of the microscope, would probably have remained unsolved to this day. Many of these apparently harmless blood-sucking insects may prove to be disease carriers, as medical knowledge and microscopic investigation is brought to bear upon them.
The majority of the smaller external parasites are beyond the reach of the amateur microscopist. A number of course are available to medical students, but we write for the ordinary enthusiast and not for the specialist. Many interesting little mites, closely allied to the cheese-mites, cause skin diseases and the creatures themselves as well as their tunnels in the skin are interesting.
There is one little parasite which we nearly all of us carry without knowing it, and it makes quite an interesting object for the microscope. Its name is Demodex Folliculorum and it dwells in the sweat glands, especially those round about the nose. It is very minute and requires a high magnification for its examination. The adult is worm-like and tapering in the hinder two-thirds of its body, whilst the front third, to which is attached four pairs of short, fleshy legs, is stouter. From the eggs which may be heart or spindle shaped, little six-legged grubs arise; later these grubs change into the eight-legged adults. All the changes take place in the sweat glands, and the creatures live with their heads turned away from the gore of the gland.