If we attempt to formulate a definition of protoplasm, we find that our ideas are far from clear, owing to want of knowledge. The questions, What is protoplasm? What is life? are equally unanswerable. Their definition is reciprocal. Protoplasm is the substance, the material, which exhibits life. Life is the complex of phenomena exhibited by protoplasm. All parts of the body are alive, in their degree. The nucleus of a cell lives, as well as its cell-body. Its capsule may be less alive—that is to say, less vibrant—than the soft cell-substance which it encloses; but it lives. So-called intercellular substance, or matrix, is alive. In growing cartilage the matrix does not behave as a dead substance. It does not crack and gape under the pressure of the dividing and multiplying cell-bodies which it contains. If the windows of a house were endowed with the power of spontaneously enlarging, the walls would be crushed. They would bulge, break, tumble. The matrix of cartilage offers as little resistance to the enlargement of the cells which it encloses as the plasma of blood to the multiplication of blood-corpuscles. It grows with the cell-bodies, and must be considered as divisible into areas, each of which is the periphery of a cell. Muscle is alive. So, too, are bone, teeth, hair, nails. But as we proceed outwards we find the quality of aliveness growing less and less apparent, until at last we acknowledge that it is unrecognizable. Vibrations diminish in amplitude and in rapidity, until the material of which the body is made appears to be at rest. Biologists apply the term “protoplasm” to the _most living_ substance of which plants and animals are composed. It may be that there is an entity, protoplasm. It may be that in certain situations this exists in an unmixed state. It may be that the degree of aliveness of a tissue or constituent part of a tissue varies as the quantity of protoplasm which it contains. The tendency of protoplasm to dispose itself in a reticulum in the meshes of which other substances accumulate favours such a view. The cells of the deeper layers of the skin are rich in it. The superficial layers are composed chiefly of keratin. It is possible that the network opens out, and its strands grow thinner and thinner, as keratin accumulates. But it cannot be demonstrated that this is the case. There is no completely satisfactory reason for concluding that the life of a cell of the skin resides in its protoplasmic network, while its keratin is inert. Many attempts have been made to prove that living cells contain something which dead cells do not contain; but no evidence which will bear sifting has, as yet, been adduced in support of this thesis. FOOTNOTE: [1] Proteïn, _subs._, proteïd, _adj._, general terms for complex nitrogenous substances, such as albumin (white of egg), the less soluble globulins, fibrin of blood, casein of milk, etc. CHAPTER III THE UNIT OF STRUCTURE