The whole process of respiration appears most clearly developed in the case of a man exposed to starvation. Currie mentions the case of an individual who was unable to swallow, and whose body lost 100 lb. in one month. The more fat an animal contains the longer will it be able to exist without food, for the fat will be consumed before the oxygen of the air acts upon the other parts of the body. There are various causes by which force or motion may be produced. But in the animal body we recognise as the ultimate cause of all force only one cause, the chemical action which the elements of the food and the oxygen of the air mutually exercise on each other. The only known ultimate cause of vital force, either in animals or in plants, is a chemical process. If this be prevented, the phenomena of life do not manifest themselves, or they cease to be recognisable by our senses. If the chemical action be impeded, the vital phenomena must take new forms. The heat evolved by the combustion of carbon in the body is sufficient to account for all the phenomena of animal heat. The 14 oz. of carbon which in an adult are daily converted into carbonic acid disengage a quantity of heat which would convert 24 lb. of water, at the temperature of the body, into vapour. And if we assume that the quantity of water vaporised through the skin and lungs amounts to 3 lb., then we have still a large quantity of heat to sustain the temperature of the body. _III.--The Chemistry of Blood-Making_ Physiologists conceive that the various organs in the body have originally been formed from blood. If this be admitted, it is obvious that those substances alone can be considered nutritious that are capable of being transformed into blood. When blood is allowed to stand, it coagulates and separates into a watery fluid called serum, and into the clot, which consists principally of fibrine. These two bodies contain, in all, seven elements, among which sulphur, phosphorus, and nitrogen are found; they contain also the earth of bones. The serum holds in solution common salt and other salts of potash and soda, of which the acids are carbonic, phosphoric, and sulphuric acids. Serum, when heated, coagulates into a white mass called albumen. This substance, along with the fibrine and a red colouring matter in which iron is a constituent, constitute the globules of blood. Analysis has shown that fibrine and albumen are perfectly identical in chemical composition. They may be mutually converted into each other. In the process of nutrition both may be converted into muscular fibre, and muscular fibre is capable of being reconverted into blood. All parts of the animal body which form parts of organs contain nitrogen. The principal ingredients of blood contain 17 per cent. of nitrogen, and there is no part of an active organ that contains less than 17 per cent. of this element.