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:

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 μ 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 observers1 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.

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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.

giving submaximal as well as maximal contractions; with increased strength of stimulation the amount of the shortening increases until a maximum is reached. This fact may be observed not only upon isolated strips of muscle from the stomach, but may be seen also in the different degrees of contraction exhibited by the intestinal musculature as a whole when acted upon by various stimuli.

In his researches upon the movements of the ureter Engelmann' showed that a stimulus applied to the organ at any point caused a contraction that, starting from the point stimulated, might spread for some distance in either direction. Engelmann interprets this to mean that the contraction wave in the case of the ureter is propagated directly from cell to cell, and this possibility is supported by the fact, before referred to, that there is direct protoplasmic continuity between adjoining cells. This passage of a contraction wave from cell to cell has, in fact, often been quoted as a peculiarity of plain muscle-tissue. In the case of the ureter the fact seems to be established, but in the intestines, where there is a rich intrinsic supply of nerve-ganglia, it is not possible to demonstrate clearly that the same property is exhibited. The wave of contraction in the intestine following artificial stimulation is, according to most observers, usually localized at the point stimulated or is propagated in only one direction, and these facts are difficult to reconcile with the hypothesis that each cell may transmit its condition of activity directly to neighboring cells. Upon the plain muscle of the ureter Engelmann was able to show also an interesting resemblance to cardiac muscle, in the fact that each contraction is followed by a temporary diminution in irritability and conductivity; but this important property, which in the case of the heart has been so useful in explaining the rhythmic nature of its contractions, has not been demonstrated for all varieties of plain muscle occurring in the body.

A general property of plain muscle that is of great significance in explaining the functional activity of this tissue is exhibited in the phenomenon of "tone." By tone or tonic activity as applied to muscle-tissue is meant a condition of continuous contraction or shortening that persists for long periods and may be slowly increased or decreased by various conditions affecting the muscle. Both striated and cardiac muscle exhibit tone, and in the latter at least the condition may be independent of any inflow of nerve-impulses from the extrinsic nerves. Plain muscle exhibits the property in a marked degree. The muscular coats of the alimentary canal, the blood-vessels, the bladder, etc., are usually found under normal circumstances in a condition of tone that varies from time to time and differs from an ordinary visible contraction in the slowness with which it develops and in its persistence for long periods. Such conditions as the reaction of the blood, for example, are known to alter greatly the tone of the blood-vessels, a less alkaline reaction than normal causing relaxation, while an increase in alkalinity favors the development of tone. Tone may also be increased or diminished by the action of motor or Pflüger's Archiv für die gesammte Physiologie, 1869, Bd. 2, S. 243.

inhibitory nerve-fibres, but the precise relationship between the changes underlying the development of tone and those leading to the formation of an ordinary contraction has not been satisfactorily determined.

The mode of contraction of the plain muscle in the walls of some of the viscera, especially the intestine and ureter, is so characteristic as to be given the special name of peristalsis. By peristalsis, or vermicular contraction as it is sometimes called, is meant a contraction which, beginning at any point in the wall of a tubular viscus, is propagated along the length of the tube in the form of a wave, each part of the tube as the wave reaches it passing slowly into contraction until the maximum is reached, and then gradually relaxing. In viscera like the intestine, in which two muscular coats are present, the longitudinal and the circular, the peristalsis may involve both layers, either simultaneously or successively, but the striking feature observed when watching the movement is the contraction of the circular coat. The contraction of this coat causes a visible constriction of the tube that may be followed by the eye as it passes onward.

MASTICATION.

Mastication is an entirely voluntary act. The articulation of the mandibles with the skull permits a variety of movements; the jaw may be raised and lowered, may be projected and retracted, or may be moved from side to side, or various combinations of these different directions of movement may be effected. The muscles concerned in these movements and their innervation are described as follows: The masseter, temporal and internal pterygoids raise the jaw; these muscles are innervated through the inferior maxillary division of the trigeminal. The jaw is depressed mainly by the action of the digastric muscle, assisted in some cases by the mylo-hyoid and the genio-hyoid. The two former receive motor-fibres from the inferior maxillary division of the fifth cranial, the last from a branch of the hypoglossal. The lateral movements of the jaws are produced by the external pterygoids, when acting separately. Simultaneous contraction of these muscles on both sides causes projection of the lower jaw. In this latter case forcible retraction of the jaw is produced by the contraction of a part of the temporal muscle. The external pterygoids also receive their motor fibres from the fifth cranial nerve, through its inferior maxillary division. The grinding movements commonly used in masticating the food between the molar teeth are produced by a combination of the action of the external pterygoids, the elevators, and perhaps the depressors. At the same time the movements of the tongue and of the muscles of the cheeks and lips serve to keep the food properly placed for the action of the teeth, and to gather it into position for the act of swallowing.

DEGLUTITION.

The act of swallowing is a complicated reflex movement which may be initiated voluntarily, but is for the most part completed quite independently