having on the one hand the cells of the gut and the liver ready to prevent anything, that is not completely deprived of its own characteristics, from passing into the circulation, and, on the other hand, by having the cells of the body passing on to the blood only such substances as have been so far disintegrated as to have lost those features which harmonize them with the cells. Blood which is in circulation thus always shows the same metabolic products and the same substances; and from this point of view we may consider the contents of the blood as being always constant. No doubt the duty of the lymph, which is placed between the cells of the body and the blood, is to guard the blood against an excess of individual products of metabolism. Probably, also, some of the products, which have been insufficiently disintegrated, are finally decomposed by the lymphatic glands, or by the lymph itself.
We are bound in this sense to look upon the lymph system, as indeed we have already pointed out, as an important control station. By means of its own cells, and especially by means of the glands, the lymph watches that no material shall reach the blood which is out of harmony with it.
From the above point of view we gather an insight into the significance of the invasion of organisms of all kinds into an animal organism. The isolation of the whole organism is immediatelv disturbed when
