These molecules are, for the most part, cellular, each cell living a little life of its own, generated with a definite individuality, doing its own life-work, then shrivelling in decay, dying in the midst of vital surroundings, suffering cremation, and thereby contributing to the animal heat necessary for the life of its successors, and even giving up a portion of its substance to supply them with absorption-food. The cell walls are mainly composed of gelatin, or the substance which produces gelatin, as already explained, while the contents of the cell are albuminous matter or fat, or the special constituents of the particular organ it composes. A description of all these constituents would carry me too far into details. I must, therefore, only refer to those which constitute the bulk of animal food, and which are altered in the process of cooking. In the lean of meat, _i.e._ the muscles of the animal, we have the albuminous juices already described, the gelatinous membranes, sheaths, and walls of the muscle fibre, and the fibre itself. This is composed of _muscle-fibrin_, or _syntonin_, as Lehmann has named it. Living blood consists of a complex liquid, in which are suspended a multitude of minute cells, some red, others colourless. When the blood is removed and dies, it clots or partially solidifies, and is found to contain a network of extremely fine fibre, to which the name of _fibrin_ is applied. A similar change takes place in the substance of the muscle after death. It stiffens, and this stiffening, or _rigor mortis_, is effected by the formation of a clot analogous to the coagulation of the blood. The chief difference between blood-fibrin and muscle-fibrin or syntonin is, that the latter is readily soluble in water, to which only 1/1000 of hydrochloric acid has been added, while in such a solution blood-fibrin only becomes swollen. If the gastric juice contains a little free hydrochloric acid, this difference is important in reference to food. I should, however, add that the existence of such free acid in the human gastric juice is disputed, especially by Gruenewaldt and Schroeder. The conflict of able chemists on this point and others concerning the composition of this fluid leads me to suppose that the secretions of the human stomach vary with the food habitually taken; that flesh-eaters acquire a gastric juice similar to that of carnivorous animals, while vegetable feeders are supplied with digestive solvents more suitable to their food. This idea is supported by the testimony of rigid vegetarians. They tell me that at first the pure vegetarian diet did not appear to satisfy them, but after a while it became as sustaining as their former food. This is explained if, in consequence of the modification of the gastric and other digestive juices, the vegetarian food became more completely digested after vegetarian habits became established.