TBE RESPIRATORY MECHANISM. III IllaHHIRIMilo" ourselves the chief organ of external respiration is the lungs. When the inspiratory muscles are contracted the chest is enlargol and air flows into the lungs. When the expiratory muscles are con tracted the chest is diminished in size and air is forced out of the lungs. In ordinary respira tion the expiratory act is entirely passive; the chest. expanded by the action of the inspiratory muscles, sinks hack into its normal position when these muscles cease to act. In forced breathing, however, ihe expiratory muscles mine into action. By 11leallS of these respiratory movements the air in the bows is continually renetyed, the sup ply of oxygen is maintained, and the carbon di oxide is removed. The oxygen contained in the ainsacs of the lungs diffuses through their thin walls, and, coming into contact with the blood, it unites chemically with the coloring matter of the red corpuscles, the haonoglobin. In thiA combina tion it is carried to the heart, and thence dis tributed over the body through the various arte ries. 1Vhen the blood reaches the capillaries the minimumd of hiemoglobin and oxygen is broken or dissociated by the physical conditions there prevailing, the liberated oxygen passes into solution in the blood plasma and lymph, and is thus conveyed to the tissues. On the other band, the general nutritive change or metabolism of the tissues results in the formation among other things of carbon dioxide. This substance is a waste product, and if it accumulates in the tis sues. brings on a suppression or perversion of the normal nutritive processes. Under normal condi tions, however, it is rapidly removed by the and blood. As the oxygen passes from the blood to the tissues by diffusion through the thin-walled capillaries. so the carbon dioxide as rapidly as it is formed streams in the opposite direction from tissues to blood. Each gas follows the physical law diffusion from a point of greater to one of less tension. Within the blood the carbon dioxide is held mainly in chemical combination, partly with the proteids of the blood plasma, partly with the proteids of the blood corpuscles. When the blood reaches the lungs this loose chemical union breaks down, the carbon dioxide is liberated and diffuses into the air-sacs of the lungs, whence it is given off in the expired air. The process of respiration, therefore, may be divided into two parts, external and internal respiration. Under the former term we include all the processes involved in the movement of air into and out of the lungs, and the exchange of oxygen and carbon dioxide between the blood and the air in the lungs. By internal respiration we mean the exchange of oxygen and carbon dioxide he tween the tissues and the blood as well as the promises of nit rition by means of which the oxy gen is used and the carbon dioxide What v.e call arterial blood differs from venous blond in that it contains more oxygen and less carbon dioxide. and to this difference in the gaseous contents is due also the well-known ya Tiatin in color, arterial blood having a scarlet tint, while the venous blood is purplish or crim son. The machinery for the movements of respi ration, that is, the respiratory muscles; are un der the control of the (pntral nemais system. These muscles may be influenced within certain limits by direct voluntary effort; but a far more important factor is their unconscious or reflex regulation exerted through the respiratory nerve cells or nerve centre fonnd in the oblon gata. By the activity of this centre the respira tory movements are kept continually in play and the extent of the respirations is adjusted to the needs of the body.
DmEsnoN AND NUTRITION. The living matter of the animal body is characterized, as compared with plant protoplasm. by its limited powers of assimilation. While the latter can construct living matter from comparatively simple inor ganic material, such as carbon dioxide, water, and nitrogen containing salts, the former re quires food in the more complex form of organic material. Since in the last analysis this organic food is derived from the plant kingdom, it may be said that the maintenance of animal life is only possible through the synthetic activity of plant protoplasm. All of our varied foods are found upon analysis to be composed of essentially the same materials united in different propor These constituent materials of our foods are known as food-stuffs and are usually classi fied as proteids, fats, carbohydrates, water, and salts. Of these substances the water, inorganic salts, and proteids are absolutely essential. The two former are necessary to the composition and reactions of the living tissues, but they do not directly furnish any energy to the body. The requisite amount of water is controlled by the sensation of thirst, and the proper proportions of the inorganic salts are provided in our foods without the necessity of any conscious selection on our part, except perhaps in the case of sodium chloride. Proteid foods have a different value.
They are complex nitrogen-containing compounds which in the body are destroyed and reduced to much simpler substances, namely, carbon di oxide, water, and urea. This destruction of pro teid is essentially an oxidation, and as much heat is given off in the process as would be lib erated outside the body by burning proteid to the same end-products. Proteids are an absolutely necessary constituent of food, because they con tain nitrogen in a form capable of being used in the construction of living matter. Fats and carbohydrates, since they contain no nitrogen, cannot be used alone in the synthesis of proto plasm. They are nevertheless valuable foods, since they are readily destroyed or burned in the body with the liberation of energy in the form of heat or muscular work. In a normal diet pro teids, fats, and carbohydrates are usually com bined in certain proportions, and experience as well as direct physiological experiments show that within limits the fats and the carbohy drates may be interchanged, and furthermore the greater the amount of these two substances used the less will he the amount of proteid required; or as we say in physiology, fats and carbohy drates are `proteid-sparers.' The nutritive history of these three energy yielding foods may be summarized briefly as fol lows: The proteids in whatever form they may be taken are digested partly in the stomach by the action of the gastric juice and partly in the intes tines by the action of the pancreatic juice. By the act of digestion the food proteids are converted into simpler and more soluble forms known as peptones and protoses, which are then absorbed into the blood and carried to the various tissues. here they are utilized in part to form protoplasm, either to replace that broken down in metabolism or to supply material for growth. But much the larger part of the proteid is simply destroyed in the tissues with the transformation of some of its chemical energy into a corresponding amount of heat. The fats are prepared for di gestion in the stomach, but undergo the impor tant change that fits them for absorption after they are brought into contact with the pancreatic juice in the small intestine. After absorption they are found in the blood and lymph for a time, but soon pass into the tissues. Here they may be deposited as part of our normal store of body-fat, hut with the usual diet of adult life they are supposed to be completely burned. It is known from experiments that one gram of fat burned outside or inside the body yields as much heat as 2.2 grams of proteid. The carbohydrates include the starches and the sugars which from a nutritive standpoint have the same value. The starches form the bulk of our diet, and they are digested partly in the mouth by the action of the saliva, but mainly in the small intestine by the action of the pancreatic secretion. Under the influence of these secretion. the starches are converted into a form of sugar. which is then absorbed into the blood. As the blood from the intestines passes through the liver this absorbed sugar is removed and again con verted into a form of starch known as glycogen, which is deposited or stored in the liver cells. From time to time the glycogen is reconverted to sugar and given off to the blood. The regulating mechanism controlling this production and con version of glycogen is so adjusted that under the normal conditions of life the blood always con tains a nearly constant amount of sugar. Sugar, therefore. is the final form in which all of our carbohydrate food is brought to the tissues—and under the influence of the living matter it is eventually oxidized to carbon dioxide and water. In the long run, therefore, the final fate of our food is to he burned and furnish energy in the form of heat, muscular work, etc. The continual consumption of food is necessary to maintain the body temperature, and in the body we have very complex regulating mechanisms which con trol the loss of heat and to a certain extent its production, with the normal result that the tem perature of the body remains nearly constant un der all the varying conditions of life. It has been shown with scientific accuracy that all the detectible energy given off from the body is de rived directly from the food consumed. The energy value of any food can, therefore. be de termined by ascertaining the amount of heat pro duced by burning it, or more conveniently by ascertaining the heat value of proteids, fats, and carbohydrates, and then analyzing the food to de termine how much of these three foodstuffs is contained in it.