proper career of circulation. It is needless here to dwell on the manner in which the most refined principles of hydraulics are brought into play, or to speak of the manner in which forces of compression and elasticity are introduced; how that there are valves which open only in one way to let the current pass, or how some of these, as in the like human contrivances, are tied down in their action by cords. Moreover, since it is required that the animal shall go in search of its food, muscles of locomotion, which act upon purely mechanical principles on the bony skeleton, must be resorted to, and so the animal structure becomes a most elaborate and complicated machine.

In this regard the human body may be spoken of as a mere instrument Physical as or engine, which acts in accordance with the principles of mepect of man. chanical and chemical philosophy, the bones being levers, the blood-vessels hydraulic tubes, the soft parts generally the seats of oxidation. But if we limit our view to such a description, it presents to us man in a most incomplete and unworthy aspect. There animates this machine a self-conscious and immortal principle-the soul.

The soul: its

nature and responsibilities.

Though in the most enlarged acceptation it would fall under the province of physiology to treat of this immortal principle, and to consider its powers and responsibilities, these constitute a subject at once so boundless and so important, that the physiologist is constrained to surrender it to the psychologist and theologian, and the more so since the proper and profitable treatment of it becomes inseparably involved with things that lie outside of his domain.

Yet under these circumstances, considering the ever-increasing control which scientific truth exerts over the masses of men, considering too how much the welfare of the human family depends on the precision and soundness of its religious views, it is the duty of the physiologist, if for the reasons that have been specified he yields this great subject to others, to leave no ambiguity in the expression of the conclusion to which his own science brings him. Especially is it for him, whenever the opportunity offers, to assert and to uphold the doctrine of the oneness, the immortality, the accountability of the soul, and to enforce those paramount truths with whatever evidence the structure of the body can furnish.

For this reason, he can not recall but with regret the existing use of many terms, such as mind, intellect, vital principle, spirit, which, though they were at first doubtless employed as expressions of the functions or qualities of the soul, have in the course of time gathered other meanings and confused the popular ideas. They have brought about a condition of things in science not unlike that which prevailed in theology during the reign of polytheism. Constrained, perhaps, himself by the necessities of language to use such phraseology, it is for him at the outset to leave no doubt of the views he entertains, and, as far as he can, prevent such

THE VITAL PRINCIPLE.

25

expressions from frittering away the great truth that, as there is but one God in the universe, so there is but one spirit in man.

On one of these terms, the vital principle, I may make a few remarks, since, from being a mere expression of convenience, it has by de- The vital grees risen among physicians and physiologists to the rank of principle. designating an existing agent, by some regarded as of the same kind as light, heat, electricity, or gravitation-nay, even superior to them, since it is its peculiar attribute to hold them all in check. Animated by this extraordinary power, organic substances are supposed to withstand every external influence, and to submit to physical agents only after this principle has left them. Such a preposterous doctrine will not bear the touch of exact science for a moment. It is only a relic of the old metaphysical system of philosophizing, which accepted a name in lieu of an explanation, which preferred the dogma of the horror of a vacuum to the more simple but material view of the pressure of the air. By the aid of this imaginary principle, complete physiological systems have been woven, in which every act and every condition of the animal economy is spontaneously explained, and nothing remains for solution. But by the student of nature, whose mind has been trained in positive science, the imposture is detected. He sees at a glance that this is not the style of the Great Artist. The problems of organization are not to be solved by empirical schemes; they require the patient application of all Importance of the aids that can be furnished by all other branches of hu- physical sciman knowledge, and even then the solution comes tardily. ology. Yet there is no cause for us to adopt those quick but visionary speculations, or to despair of giving the true explanation of all physiological facts. Since it is given us to know our own existence, and be conscious of our own individuality, we may rest assured that we have what is in reality a far less wonderful power, the capacity of comprehending all the conditions of our life. God has framed our understanding to grasp all these things. For my own part, I have no sympathy with those who say of this or that physiological problem, it is above our reason. My faith in the power of the intellect of man is profound. Far from supposing that there are many things in the structure and functions of the body which we can never comprehend, I believe there is nothing in it that we shall not at last explain. Then, and not till then, will man be a perfect monument of the wisdom and power of his Maker, a created being knowing his own existence, and capable of explaining it. In the application of exact science to physiology, I look for the rise of that great and noble practice of medicine which, in a future age, will rival in precision the mechanical engineering of my times. In it, too, are my hopes of the final extinction of empiricism. Even now this method is attended with results which must commend it to every thoughtful mind, since it is connecting

ence in physi

26

SUBDIVISIONS OF PHYSIOLOGY.

itself with those great truths which concern the human family most closely, and is bringing into the region of physical demonstration the existence and immortality of the soul of man, and furnishing conspicuous illustrations of the attributes of God.

CHAPTER II.

OF FOOD.

The natural Subdivisions of Physiology.—Of Food: its Sources and Classification—its Value not altogether dependent on its Composition.-Of Milk: its Composition, and Use of its Water, Casein, Sugar, Butter, and Salts.- Variations in the Composition of Milk.-Of Bread.—Of mixed Diets. Of the embryonic Food of Birds.-Nutrition of carnivorous and herbivorous Animals.-Food formed by Plants and destroyed by Animals.-Uses of mixed Food and Cooking.-Absolute Amount of Food.

PHYSIOLOGY possesses a very great advantage over many other sciences Subdivisions of in offering its leading problems and doctrines in a certain physiology. well-marked order or sequence, a connected whole, with only here and there points of digression, but those points often of very striking interest. Thus pursuing the train of reflections entered on in the preceding chapter, we should have to consider the nature of the food, the manner of its preparation by the process of digestion, the mechanism by which it is taken up from the cavities in which it has been so prepared, and that by which it is distributed to every part. We should have to show the way in which it becomes incorporated as a portion of the living mass, its duration in that condition, and the manner of its decay. We should have to show by what physical means and through what mechanism the air is introduced to effect the destruction of the dying parts, and how, as the consequence of this, a fixed temperature is maintained. The causes which lead to variations of this temperature, and the manner in which the wasted products are removed by the skin, the lungs, the kidneys, might next obtain our attention. The complicated machinery necessary to accomplish all these purposes requires to be made to act in unison in all its different parts, a condition which introduces to us the nervous system. A consideration of the structure and gradual development of this system leads to the structure of the various organs of sense, and to the operations of the intellectual principle itself. Thus in succession we should have to treat of digestion, absorption, circulation, respiration, secretion, nutrition, and innervation, and to close the whole with the consideration of reproduction. This is the order which I propose to follow, and shall devote this chapter to the nature and qualities of the food.

HISTOGENETIC AND CALORIFACIENT FOOD.

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Sources of food

The supply of food to animals requires a more complicated provision than it does to plants, in which the elaborating organs, the leaves, presenting themselves superficially, are always in for animals and contact with the air, from which much of their nutrition is plants. derived. And as one portion after another becomes exhausted, it is renewed by simple mechanical agencies, such as the trembling of the leaf, the warmth of the sun, or the winds.

Food, therefore, comes spontaneously to plants, which need no powers of locomotion. And though, as we shall hereafter find, muscular movement requires as its essential condition the waste of tissue, it is not necessary for their nutrition that plants should destroy organized substance. But an animal must seek its food, and for this purpose is endowed with locomotion, involving the destruction of tissue. In a chemical point of view, plants are organizing, and animals destroying machines. Nor is this general assertion controverted by the apparent exceptions which are here and there presented, as, for example, that the herbivora can form sugar and fat from food in which those substances did not pre-exist, and the salts of the biliary acids, which are never found in plants.

To obtain for animals the necessary supply of nutriment, the resources of nature are displayed in the most wonderful contrivances. According as their modes of life may be, one takes its food with its teeth, another with its lips, another with its fore member, another winds around it its whole body. The geometrical spider weaves a net, and lies in wait for his prey; the ant lion digs a pit in the sand. Some rely upon labor, some upon force, some upon fraud. Man depends upon all.

histogenetic

Viewed as regards its physiological distinction, the food is generally considered as of two kinds: Histogenetic or tissue-making, and Classification Calorifacient or heat-making. Histogenetic food furnishes the of food into chemical substances-carbon, hydrogen, oxygen, nitrogen, sul- and calorifaphur, chlorine, phosphorus, iron, potash, soda, lime, &c. Ca- cient. lorifacient food furnishes carbon and hydrogen mainly. In consequence of this chemical constitution, tissue-making food is sometimes called nitrogenized, and heat-making non-nitrogenized food. The former is also sometimes designated nutritive, and the latter respiratory.

It is, however, to be distinctly understood that these divisions are only adopted for the sake of convenience, and that they have no natural foundation. Thus it will be found, when we examine the functions which the fats discharge, that though they are non-nitrogenized bodies, and are, therefore, considered as belonging to the class of respiratory food, there is every reason to believe that they are essentially necessary to tissue development, and that the metamorphoses of nitrogenized bodies can only go on in their presence. They are, therefore, as truly essential to nutrition as are the latter substances.

28

CLASSIFICATION OF FOOD.

So, too, as respects the albumenoid bodies, of which it would be incorrect to speak as though they were limited to nutrition. In their decay or descending metamorphosis in the organism, they give rise to the evolution of heat, and are at last dismissed under the aspect of products of oxidation. They are, therefore, as far as this goes, as much respiratory food as are the fats themselves.

Other classifi

Perhaps the most convenient subdivision of food articles cations of food. is presented in the four following groups:

1st. Carbohydrates, or compounds in which carbon is united with hydrogen and oxygen, their proportion being that for forming water. Starch, sugar, gum, cellulose, are examples.

2d. Hydrocarbons. Compounds containing unoxidized hydrogen. The oils, fats, and alcohol, are examples.

3d. Albumenoid bodies. These contain nitrogen. Albumen, fibrin, casein, are examples.

4th. Salts. Any classification of food articles which does not contain this group is imperfect; for salts are not only absolutely essential to organic processes, but also to the construction of many tissues. As an example of the former case, the chloride of sodium may be mentioned; and of the latter, the phosphate of lime.

does not depend wholly on its composition.

It has been supposed that the tissue-making power of any kind of Value of food food depends on the quantity of nitrogen it contains, and that its value may therefore be determined by chemical analysis. Upon this principle tables have been constructed, showing the agricultural worth of different articles of forage for domestic animals. But, as will be found hereafter, when we consider the physiological effect of the allotropism of bodies, these tables are not of the use supposed. Without entering into details at present, the case of gelatin may be taken as an example; this, though a substance abounding in nitrogen, possesses no tissue-making value, but in reality belongs to the calorifacient class, and therefore its administration in the sick-room, under the various well-known forms of jellies, soups, etc., is altogether deceptive as regards any nutritive power, since it undergoes speedy oxidation in the system, and the products of its change escape by the kidneys and the lungs. The value of food is not only dependent on the occurrence of certain chemical elements; they must also be present in certain allotropic

states.

The same remark applies to the tables which have been constructed, showing the amount of caloric furnished by different varieties of heatmaking food. The quantity of heat set free during the combustion of a substance depends not only on the nature of the elements composing it, but also on the particular states in which they occur. Combustibles may have the same chemical composition, but very different heating power.