of the oxygen of the atmosphere and the blood. The process seems to be as follows:-When a quantity of atmospheric air has been received into the lungs, by an act of inspiration, it there meets with the venous or dark coloured blood, which has arrived from all parts of the body, loaded with impurities. Several changes then take place. The carbonic acid, which is the cause of its dark colour, is set at liberty; and the protoxide of iron contained in the globules of the blood, becomes a peroxide, by uniting with a part of the oxygen contained in the air inhaled. In this state, the blood (now of a bright red colour) is conveyed, by the action of the heart, through the arteries, to all parts of the system; and in those minute vessels termed "capillaries", the peroxide of iron gives off half its oxygen, and is again reduced to a protoxide; which combines with the carbon and hydrogen set free, in the same situation, by the decomposition of the tissues; and the iron (in the state of carbonate) returns to the lungs, where it exchanges its equivalent of carbonic acid for one of oxygen, and is conveyed to the tissues as before. Thus we see that muscles, tendons, nerves, bones, nails, hair, and all the other solids and fluids of the body, (which are ever in a state of fluctuation,) derive their origin from that important fluid—the blood. This liquid flesh, (as it is some times termed,) as it passes through the capillaries, deposits the materials of each organ, be it muscle, nerve, or bone,-wherever the decomposed particles require renewal; and the same vital current, loaded with the products of chemical transformation-carbon, hydrogen, nitrogen, &c.-finds access

to the various excretory organs, where these effete materials are given off; and, after undergoing its last degree of purification in the lungs, it is returned again to the heart.

174. The blood is thus gradually exhausted, by building up and renewing the solid structure of the body; and itself requires constant renewal, by daily supplies of food. This food is partially prepared for digestion, in the mouth, by mastication and insalivation; it is then passed along the œsophagus into the stomach, where it undergoes various chemical changes, and is converted into chyme; it then passes through an aperture at the right extremity of the stomach (called the "pylorus") into the duodenum, where it undergoes still further change, by the action of several secretions from the duodenum, liver, and pancreas; it is finally elaborated into a white fluid denominated “chyle", which is taken up by minute vessels termed "lacteals"; and passes along the thoracic duct, which terminates in an angle formed by the union of the internal jugular and subclavian veins. Thence it flows-mixed with particles of organized matter, lymph, and venous blood-to the heart; by which it is transmitted to the lungs, where all these different fluids are converted into one substance,arterial blood; to be sent out, by the left side of the heart, to the system for its support. The principal purpose of food, therefore, is to supply nutriment to the body; in order to compensate for the waste constantly taking place, from the decomposition of the tissues by the action of oxygen.

175. But there is also another important end to be

answered by the proximate principles, or ultimate elements, contained in food; namely, the production of animal heat; without which all the varied functions of life would immediately cease. Azotized articles of food, termed also the "plastic elements of nutrition", and (by Dr. Prout) the "albuminous class of aliments", are generally believed to be specially and principally designed for the former purpose. They were found, by Mülder, to be modifications of a certain compound, which he has named "protein" (from newτeuw I hold the first place); which is composed of carbon, hydrogen, nitrogen, and oxygen; as in the formula C", H", N, O1. (Leibig.) The same chemist "has discovered two oxides of protein,—a binoxide and tritoxide; both of which are formed in the animal economy; and constitute, when combined with fatty matter, the buffy coat of inflamed blood. He believes that the protein of the food reaches the right side of the heart, circulates through the lungs, and combines with oxygen,-forming oxy-protein (binoxide, tritoxide, or both): this reaches the nutrient capillaries, and all or part is decomposed ;the oxygen being employed for the disorganization of worn-out tissue; and the protein, thus deoxidized, being deposited to supply its place.-If more protein be set free than is wanted for the growth of tissue, it passes (unchanged) into the veins;-to be again oxidized in the lungs. The tritoxide of protein, being soluble in water, is better enabled to traverse the minutest capillaries, than if it existed merely diffused through the fluid containing it."* 176. The azotized principles, fibrin, albumen (or

* URINARY DEPOSITS, BY DR. G. BIRD. Page 8.

gluten), and casein, are capable of being assimilated, and converted into the various animal organs and tissues; and only differ from each other by small, but essential quantities of mineral substances;-such as sulphur, phosphorus, potash, soda, common salt, and phosphate of lime.

177. The non-azotized principles are supposed to minister principally to the support of respiration, and the production of animal heat. They are divided by Dr. Prout into two groups;-the saccharine, comprehending sugar, starch, and gum; and the oleaginous, which includes oils, fat, and spirits. Liebig asserts, that the nonazotized principles are incapable of supporting life, and nourishing the tissues; but his opinion on this point, as we shall see presently, is questionable. It is pretty well ascertained, however, that one important use (to the Herbivora and to man) of substances containing these principles, is to supply carbon and hydrogen; which, by uniting with oxygen, produce muscular force and animal heat if not supplied with these principles, their organs would be destroyed, by the action of oxygen. Professor Liebig calculates, that about fourteen ounces of carbon are daily burned in the body of an adult man; and that sufficient heat is thus given out to maintain the temperature, and to account for all the gaseous matter and water expelled from the lungs. Although this view of the effects produced by the saccharine principles may be in the main correct, yet certain facts are scarcely reconcileable with their exclusive appropriation to the production of caloric. The diet of the inhabitants of the tropics, for instance, abounds with carbon and hydrogen; which

would be likely to generate a much greater quantity of heat, than could readily escape in the high temperatures to which they are exposed: it is probable, therefore, that a portion of these non-azotized compounds is converted into nutriment, by a union with the nitrogen of the atmosphere (200). We must inquire, secondly, whether vegetables possess the elements and qualities necessary for answering the purposes above mentioned.

178. Some years ago, when organic chemistry had been little investigated, it was generally supposed, that vegetables were deficient in that important element termed "azote" or "nitrogen", which enters largely into the composition of the blood and flesh (or muscle) of all animals. It was therefore concluded, that vegetables are insufficient for the due support and renewal of the human body; and that the flesh of other animals is a necessary article of diet for man. Now, without the aid of chemistry, it is demonstrable, that either the assumption or the inference is incorrect. For as the flesh of sheep, oxen, and other herbivorous animals, is acknowledged to contain as much nitrogen* as the flesh of man, I would simply ask

*The recent researches of Messrs. Macaire and Marcet tend to establish the important fact, that both the chyle and the blood of herbivorous and of carnivorous quadrupeds, are identical in their chemical composition; in as far, at least, as concerns their ultimate analysis. They found, in particular, the same proportion of nitrogen in the chyle, whatever kind of food the animal habitually consumed; and it was also the same in the blood, whether of carnivorous or herbivorous animals; although this last fluid contains more nitrogen than the chyle. (Memoires de la Société de Physique et d'Histoire Naturelle, de Genève. v. 389.) ROGET's Bridge. WATER TREATISE. Vol. 2. P. 53.