has been successfully performed by Rubner; his experiments were made with the greatest accuracy and with careful attention to all the possible sources of error, and it was found that the quantities of heat as determined by the two methods agreed to within less than 0.5 per cent. These experiments are noteworthy because they furnish us with the first successful experimental demonstration of the accuracy of the general principles, stated above, upon which the available energy of foods is calculated.

Dietetics.-The subject of the proper nourishment of individuals or collections of individuals—armies, inmates of hospitals, asylums, prisons, etc.— is treated usually in books upon hygiene, to which the reader is referred for practical details. The general principles of dieting have been obtained, however, from experimental work upon the nutrition of animals. These principles have been stated more or less completely in the foregoing pages, but some additional facts of importance may be referred to conveniently at this point. In a healthy adult who has attained his maximum weight and size the main object of a diet is to furnish sufficient nitrogenous and non-nitrogenous foodstuffs, together with salts and water, to maintain the body in equilibriumthat is, to prevent loss of proteid tissue, fat, etc. In speaking of the nutritive value of the food-stuffs it was shown that in carnivora (dogs) this condition of equilibrium may be maintained upon proteid food alone, putting aside all consideration of salts and water, or upon proteids and fats, or upon proteids and carbohydrates, or upon proteids, fats, and carbohydrates. When proteids alone are used, the quantity must be increased far above that necessary in the case of a mixed diet, and it is doubtful whether, in the case of man or the herbivora, a healthy nutritive condition could be maintained long upon such a diet, owing to the largely increased demand upon the power of the alimentary canal to digest and absorb proteids, to the greater labor thrown on the kidneys, etc. The experience of mankind, as well as the results of experimental investigation, shows that the healthy diet is one composed of proteids, fats, and carbohydrates. The proportion in which the fats and the carbohydrates should be taken-and, to a certain extent, this is true also of the proteids-may be varied within comparatively wide limits, in accordance with the law of "isodynamic equivalents," provided that the total amount of potential energy represented in the food does not fall below a certain amount, on the average about 40,000 calories per kilo. of body weight. This is illustrated by the following "average diets" calculated by different physiologists to indicate the average amount of food-stuffs required by an adult man under normal conditions of life:

In Voit's diet, which is the one usually taken to represent the daily needs of the body, it will be noticed that the ratio of the nitrogenous to the nonnitrogenous food-stuffs is about as 1 : 5, and basing the estimate upon a man weighing 70-75 kilos., 118 grams of proteid per day would represent a consumption of proteid equal to 1.3 to 1.7 grams per kilo. of weight. Sivén1 has recently attempted to show that this proportion of proteid in food is unnecessarily high. In some experiments upon himself he was able to reduce his daily proteid food to about 0.2 gram per kilo. of body weight and still maintain his body in N-equilibrium, provided the non-proteid portions of his diet were so increased that the total energy of his daily diet remained unchanged. Whether or not so high an amount of proteid per day as 118 grams is most beneficial to the body, under normal conditions of moderate labor, is perhaps an open question. It seems certain that for short periods at least the average individual can keep his body in equilibrium on much smaller amounts. It must be remembered, in regard to these diets, that the amounts of food-stuffs given refer to the dry material: 118 grams of proteid do not mean 118 grams of lean meat, for example, since lean meat (flesh) contains a large proportion of water. Tables of analyses of food (one of which is given on page 278) enable us to determine for each particular article of food the proportion of dry food-stuffs contained in it, and in how great quantities it must be taken to furnish the requisite amount of proteid, fats, or carbohydrates. There is, however, still another practical consideration that must be taken into account in estimating the nutritive value of articles of food from the analyses of their composition, and that is the extent to which each food-stuff in each article of food is capable of being digested and absorbed. Practical experience has shown that proteids in certain articles of food can be digested and absorbed nearly completely when not fed in excess, while in other foods only a certain percentage of the proteid is absorbed under the most favorable conditions. This difference in usableness of the food-stuffs in various foods is most marked in the case of proteids, but it occurs also with the fats and the carbohydrates. Facts of this kind cannot be determined by mere analysis of the foods; they must be obtained from actual feeding experiments upon man or the lower animals. It has usually been stated by those who have worked in this field that the proteids of meats are more completely utilized than those of vegetables. But it is possible that as a generalization this statement is too sweeping, and rests upon the erroneous assumption that the nitrogen in feces represents chiefly undigested proteid. Prausnitz and others have given reasons for believing that the nitrogen in the feces is derived mainly from the intestinal secretions, and that vegetable foods that do not contain much indigestible material, such as rice and bread, are practically completely digested and absorbed in the intestines, their proteids, therefore, being utilized as completely as in the case of meats. Munk gives an inter

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esting table showing how much of certain familiar articles of food would be necessary, if taken alone, to supply the requisite daily amount of proteid or non-proteid food; his estimates are based upon the percentage composition of the foods and upon experimental data showing the extent of absorption of the food-stuffs in each food. In this table he supposes that the daily diet should contain 110 grams of proteid 17.5 grams of N, and non-proteids sufficient to contain 270 grams of C:

Milk.
Meat (lean)

Hen's eggs
Wheat flour
Wheat bread

= =

FFF

Rye bread.

Rice

Corn

Peas

Potatoes

1900

1870

990

520

4500

1000
1100

750

660

750

2550

As Munk points out, this table shows that any single food, if taken in quantities sufficient to supply the nitrogen, would give too much or too little C, and the reverse; those animal foods which, in certain amounts, supply the nitrogen needed furnish only from one-quarter to two-thirds of the necessary amount of C. To live for a stated period upon a single article of food-a diet sometimes recommended to reduce obesity-means, then, an insufficient quantity of either N or C and a consequent loss of body-weight. Such a method of dieting amounts practically to a partial starvation. In practical dieting we are accustomed to get our supply of proteids, fats, and carbohydrates from both vegetable and animal foods. To illustrate this fact by an actual case, in which the food was carefully analyzed, an experimenter (Krummacher) weighing 67 kilograms records that he kept himself in N equilibrium upon a diet in which the proteid was distributed as follows:

For a person in health and leading an active normal life, appetite and experience seem to be safe and sufficient guides by which to control the diet; but in conditions of disease, in regulating the diet of children and of collections of individuals, scientific dieting, if one may use the phrase, has accomplished much, and will be of greater service as our knowledge of the physiology of nutrition increases.

VI. MOVEMENTS OF THE ALIMENTARY CANAL,

BLADDER, AND URETER.

PLAIN MUSCLE-TISSUE.

THE movements of the alimentary canal and the organs concerned in micturition are effected for the most part through the agency of plain muscletissue. The general properties of this tissue will be referred to in the section upon the Physiology of Muscle and Nerve, but it seems appropriate in this connection to call attention to some few points in its general physiology and histology, inasmuch as the character of the movements to be described depends so much upon the fundamental properties exhibited by this variety of muscle-tissue. Plain muscle as it is found in the walls of the abdominal and pelvic viscera is composed of masses of minute spindle-shaped cells whose size is said to vary from 22 to 560 μ in length and from 4 to 22 μ in width, the average size, according to Kölliker, being 100 to 200 in length and 4 to 6 μ in width. Each cell has an elongated nucleus, and its cytoplasm shows a longitudinal fibrillation. Cross striation, such as occurs in cardiac and striped muscle, is absent. These cells are united into more or less distinct bundles or fibres, which run in a definite direction corresponding to the long axes of the cells. The bundles of cells are united to form flat sheets of muscle of varying thicknesses, which constitute part of the walls of the viscera and are distinguished usually as longitudinal and circular muscle-coats according as the cells and bundles of cells have a direction with or at right angles to the long axis of the viscus. The constituent cells are united to one another by cementsubstance, and according to several observers' there is a direct protoplasmic continuity between neighboring cells-an anatomical fact of interest, since it makes possible the conduction of a wave of contraction directly from one cell to another. Plain muscle-tissue, in some organs at least, e. g. the stomach, intestines, bladder, and arteries, is under the control of motor nerves. There must be, therefore, some connection between the nerve-fibres and the muscletissue. The nature of this connection is not definitely established; according to Miller, the nerve-fibres terminate eventually in fine nerve-fibrils that run in the cement-substance between the cells and send off small branches that end in a swelling applied directly to the muscle-cell. Berkley finds a similar 1 See Boheman: Anatomischer Anzeiger, 1894, Bd. 10, No. 10.

2 Archiv für mikroskopische Anatomie, 1892, Bd. 40.
Anatomischer Anzeiger, 1893, Bd. 8.

VOL. I.-24

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ending of the nerves, and in addition describes in the muscularis mucosa of the intestine a large globular end-organ which he considers as a motor plate.

Perhaps the most striking physiological peculiarity of plain muscle, as compared with the more familiar striated muscle, is the sluggishness of its contractions. Plain muscle, like striated muscle, is independently irritable. Various forms of artificial stimuli, such as electrical currents, mechanical, chemical, and thermal stimuli, may cause the tissue to contract when directly applied to it, but the contraction in all cases is characterized by the slowness with which it develops. There is a long latent period, a gradual shortening which may persist for some time after the stimulus ceases to act, and a slow relaxation. These features are represented in the curve shown in Figure 68, which it is instructive to compare with the typical curve of a striated muscle (Vol. II.). The slowness of the contraction of plain muscle seems to depend upon the absence of cross striation. Striped muscle as found in various animals or in different muscles of the same animal-e. g. the pale and red muscles of the rabbit -differs greatly in the rapidity of its contraction, and it has been shown that the more perfect the cross striation the more rapid is the contraction. cross striation, in other words, is the expression of a mechanism or structure adapted to quick contractions and relaxations, and the relatively great slowness of movement in the plain muscle seems to result from the absence of this particular structure. It should be added, however, that plain muscle in different parts of the body exhibits considerable variation in the rapidity with which it contracts under stimulation, the ciliary muscle of the eyeball, for example, being able to react more rapidly than the muscles of the intestines. The gentle prolonged contraction of the plain muscle is admirably adapted to its function in the intestine of moving the food-contents along the canal with sufficient slowness to permit normal digestion and absorption. Like the striated muscle, and unlike the cardiac muscle, plain muscle is capable of

The

FIG. 68.-Contraction of a strip of plain muscle from the stomach of a terrapin. The bottom line gives the time-record in seconds; the middle line shows the time of application of the stimulus, a tetanizing current from an induction coil; the upper line is the curve recorded by the contracting muscle.