of a skin which is so thin that perhaps you would be inclined to say it has no thickness at all. The larger channels which are bringing the blood down to the capillaries are the ends of vessels like those which in the rabbit you learnt to call arteries, and the other larger channels through which the blood is rushing away from the capillaries are the beginnings of veins. When you have watched this frog’s foot for some little time, turn away and reflect that in almost every part of your own body, in every square inch, in almost every square line, something very similar might be seen could the microscope be brought to bear upon it, only the corpuscles are smaller and round, the capillaries narrower and for the most part more thick-set, and the race a swifter one. In the muscle of which we were speaking in the last lesson, each of the soft long fibres of which the muscle is composed is wrapped round with a close network of these tiny capillaries, through which, as long as life lasts, for ever rushes a swift stream of blood, reddened by countless numbers of tiny corpuscles. In every part of your flesh, in your brain and spinal cord, in your skin, your bones, your lungs, in all organs and in nearly every part of your body, there is the same hurrying rush through narrow tubes of red corpuscles and of the clear fluid in which these swim. If you prick your finger it bleeds. Almost any part of your body would bleed were you to prick it. So thick-set are the little blood-vessels, that wherever you thrust a needle, be it as fine a needle as you please, you will be sure to pierce and tear some little blood channel, either artery or capillary or vein, and out will come the ruddy drop. =22. What is blood?= It is a fluid; it runs about like water: yet it is thicker than water, thicker for two reasons. In the first place, water, that is pure water, is all one substance. If you were to look at it with ever so powerful a microscope, you would see nothing in it. It is exceedingly transparent--you can see very well through ever such a thickness of clean water. But if you were to try and look through even a very thin sheet of blood spread out between two glass plates, you would find that you could see very little; =blood is very opaque=. If again you examine a drop of your blood with a microscope, what do you see? =A number of little= [Illustration: FIG. 4.--_Red and White Corpuscles of the Blood magnified._ _A._ Moderately magnified. The red corpuscles are seen lying in rows like rolls of coins; at _a_ and _a_ are seen two white corpuscles. _B._ Red corpuscles much more highly magnified, seen in face; _C._ ditto, seen in profile; _D._ ditto, in rows, rather more highly magnified; _E._ a red corpuscle swollen into a sphere by imbibition of water.