Page:Popular Science Monthly Volume 11.djvu/442

426 as a compound of so many tissues, each tissue corresponding to one of the fundamental qualities of protoplasm, to the development of which it is specially devoted by the division of labor. It must, however, be remembered, that there is a distinct limit to the division of labor. In each and every tissue, in addition to its leading quality, there are more or less pronounced remnants of all the other protoplasmic qualities. Thus, though we may call one tissue par excellence metabolic, all the tissues are, to a greater or less extent, metabolic. The energy of each, whatever be its particular mode, has its source in the breaking up of the protoplasm. Chemical changes, including the assumption of oxygen and the production, complete or partial, of carbonic acid, and therefore also entailing a certain amount of secretion and excretion, must take place in each and every tissue. And so with all the other fundamental properties of protoplasm; even contractility, which, for obvious mechanical reasons, is soonest reduced where not wanted, is present in many other tissues besides muscle. And it need hardly be said that each tissue retains the power of assimilation. However thoroughly the material of food be prepared by digestion and subsequent metabolic action, the last stages of its conversion into living protoplasm are effected directly and alone by the tissue of which it is about to form a part.

Bearing this qualification in mind, we may draw up a physiological classification of the body into the following fundamental tissues:

 1. The eminently contractile: the muscles. 2. The eminently irritable and automatic: the nervous system. 3. The eminently secretory or excretory: digestive, urinary, and pulmonary, etc., epithelium. 4. The eminently metabolic: fat-cells, hepatic cells, lymphatic and ductless glands. 5. The reproductive: ovary, testis. 6. The indifferent or mechanical: cartilage, bone, etc.

All these separate tissues, with their individual characters, are, however, but parts of one body; and in order that they may be true members working harmoniously for the good of the whole, and not isolated masses, each serving its own ends only, they need to be bound together by coordinating bonds. Some means of communication must necessarily exist between them. In the mobile, homogeneous body of the amœba, no special means of communication are required. Simple diffusion is sufficient to make the material gained by one part common to the whole mass, and the native protoplasm is physiologically continuous, so that an explosion set up at any one point is immediately propagated throughout the whole irritable substance. In the higher animals the several tissues are separated by distances far too great for the slow process of diffusion to serve as a sufficient means of communication, and their primary physiological continuity is broken by their being imbedded in masses of formed material, the product of