Heart and brain are the two lords of life. In the metaphors of ordinary speech and in the stricter language of science, we use these terms to indicate two central powers, from which all motives radiate, to which all influences converge. They rule the moral and the physical life: the moral owes to them its continuous supply of feelings and ideas; the physical its continuous supply of food and stimulus. All the composite material which serves to build up the bodily fabric, and repair its daily waste, is only so much "carted material" awaiting the architect, until it has twice passed through the heart—until having been sent by the heart to the lungs it has there received its plastic virtues, and returns to the heart to be thence distributed throughout the organism. So much is familiar to every one; but less familiar is the fact that this transmission of the blood from heart to lungs, and its distribution throughout the organism, are rendered possible and made effective only under the influence of the brain. Life is sustained by food and stimulus. The operation of nutrition itself is indissolubly connected with sensibility. Life is a plexus of nutrition and sensation, the threads of which may ideally be separated, but which in reality are so interwoven as to be indissoluble. This is a paradox which even many physiologists will reject; but it is only a paradox because biological questions have constantly been regarded from a chemical point of view.
To render my proposition free from ambiguity, it is needful to premise that the term heart, by a familiar device of rhetoric, here expresses the whole of that great circulatory apparatus of which it is only a part; and in like manner the term brain here expresses the whole of the sensory apparatus. The reader knows perfectly well that in strict anatomical language the heart is only one organ having a definite function; and that the brain—although the term is used with considerable laxity—is only one portion of the complex nervous mechanism, having also its definite functions. But I am not here addressing anatomists, and for purposes of simplification I shall generally speak of the heart as if it were the whole of the vascular system, and of the brain as if it were the whole of the nervous system. And there is a philosophic truth suggested by this departure from the limitations of anatomical definition, namely, that if the brain as a nervous centre requires to be distinguished from all other nervous centres, it also requires to be affiliated on them: it has its special functions as an organ, but it has also a community of property—i.e., sensibility—with all other nervous centres.
In the study of animal organisms, the scientific artifice called analysis, which separates ideally what nature has indissolubly united, isolating each portion of a complex whole to study it undisturbed by the influences of other portions, has established a division of life into animal and vegetable. The division is as old as Aristotle, but has become the common property of science only since the days of Bichat. It is not exact, but it is convenient. As an artifice it has proved its utility, but like all such distinctions it has a tendency to divert the mind from contemplation of the real synthesis of nature. Even as an artifice the classification is not free from ambiguities; and perhaps it would be less exceptionable if [{624}] instead of vegetal and animal we were to substitute nutritive and sensitive. All the phenomena of growth, development, and decay—phenomena common to plants as to animals—may range under the laws of nutrition. All the phenomena of feeling and motion which specially distinguish animals, will range under the laws of sensibility. Plants, it is true, manifest motion, some few of them even locomotion; but in them it is believed that these phenomena are never due to the stimulus of sensibility.
Viewing the animal organism as thus differentiated, we see on the one hand a complex system of organs—glands, membranes, vessels—all harmoniously working to one end, which is to build up the body, and silently repair its continual waste. They evolve the successive phases of development. They prepare successive generations. On the other hand, we see a complex system of organs—muscles, tendons, bones, nerves, and nerve-centres—also harmoniously co-operative. They stimulate the organs of nutrition. They work first for the preservation of the individual in the struggle of existence; next, for the perfection of the individual in the development of his highest qualities.
But it is important to remember that this division is purely ideal—a scientific artifice, not a reality. Nature knows of none such. In the organism the two lives are one. The two systems interlace, interpenetrate each other, so that the slightest modification of the one is followed by a corresponding change in the other. The brain is nourished by the heart, and were it not for the blood which is momently pumped into it by the heart, its sensibility would vanish. And the heart in turn depends upon the brain, not for food, but for stimulus, for motive power, without which food is inert. That we may feel, it is necessary we should feed; that we may feed, it is necessary we should feel. Nutrition cannot be dissociated from sensation. The blood which nourishes the brain, giving it impulse and sustaining power, could never have become arterial blood, could never have reached the brain, had not the heart which sent it there been subjected to influences from the brain. The blood itself has no locomotive impulse. The heart has no spontaneous power: it is a muscle, and like all other muscles must be stimulated into activity. Unless the sensitive mechanism were in action, the lungs could not expand, the blood would not become oxygenated, the heart would not pump. Look on the corpse from which the life has just vanished. Why is it inert? There is food within it. It has blood in abundance. There is air in the lungs. The muscles are contractile, and the tendons elastic. So little is the wondrous mechanism impaired, that if by any means we could supply a stimulus to awaken the dormant sensibility, the chest would expand, the heart would beat, the blood would circulate, the corpse would revive.
It is unnecessary to point out in detail how dependent the brain is upon the heart; but mention may be made of the fact that more blood is sent to the brain than to any other organ in the body: according to some estimates a fifth of the whole, according to others a third. Not only is a large quantity of blood demanded for the continuous activity of the brain, but such is the peculiar nature of this great nervous centre, that of all organs it is the most delicately susceptible to every variation in the quality of the blood sent to it. If the heart pumps feebly, the brain acts feebly. If the blood be vitiated, the brain is lethargic; and when the brain is lethargic, the heart is weak. Thus do the two great centres interact. They are both lords of life, and both mutually indispensable.
There are two objections which it may be well to anticipate: Nutrition, it may be objected, cannot be so indissolubly blended with sensation as I have affirmed, because, in the first place, most of the nutritive processes go on without the intervention of [{625}] sensibility; and in the second place, the nutritive life of plants is confessedly independent of sensation, since in plants there is no sensitive mechanism whatever. Nutrition is simply a chemical process.
The answers to these objections may be very brief. Nutrition is a biological not a chemical process: it involves the operation of chemical laws, but these laws are themselves subordinated to physiological laws; and one of these laws is the necessary dependence of organic activity on a nervous mechanism wherever such a mechanism exists. Although popular language, and the mistaken views (as I conceive) of physiologists, allow us to say, without any apparent absurdity, that the processes of respiration, digestion, circulation, and secretion go on without feeling or sensation—because these processes do not habitually become distinct in consciousness, but are merged in the general feeling of existence—we have only to replace the word feeling, or sensation, by the phrase "nervous influence," and it then becomes a serious biological error to speak of nutrition as dissociated from the stimulus of nervous centres, as capable of continuance without the intervention of sensibility. The chemical combinations and decompositions do not of course depend on this intervention, but the transport of materials does. All the disputes which have been waged on this subject would have been silenced had the disputants borne in mind this distinction between the chemical and organic elements in every nutritive process. It is not the stoker who makes the steam; but if the stoker were not to supply the fire with coals, and the safety-valve were not to regulate the amount of pressure, steam might indeed be generated, but no steam-engine would perform its useful work. In like manner, it is not the vascular system which makes a secretion; but if the blood did not supply the gland with materials, the secreting process would quickly end, and the blood can only be brought to the gland through the agency of muscular contractions stimulated by nervous influence.
Granting that plants have no sensibility, and that in them the process of nutrition must go on without such an intervention, we are able to demonstrate that in animals in whose organism the sensitive apparatus is an integral portion, the processes of nutrition are more or less under the influence of this apparatus. In saying "more or less," I indicate the greater or less perfection of the organism; for, as every one knows, the perfection of each type is due to the predominance of its sensitive mechanism. In some of the lowest types, no trace of a nervous mechanism can be discovered. A little higher in the scale, the mechanism is very slight and simple. Still higher, it becomes complex and important. It culminates in man. Corresponding with this scale of complexity in the sensitive life is the scale of complexity in the nutritive life. As the two rise in importance they rise in the scale of dependence. Thus a frog or a triton will live long after its brain is removed. I have kept frogs for several weeks without their brains, and tritons without their heads. Redi, the illustrious Italian naturalist, kept a turtle alive five months after the removal of its brain. Now it is needless to say that in higher animals death would rapidly follow the loss of the brain. A somewhat similar parallelism is seen on the removal of the heart. None of the higher animals can survive a serious injury to the heart; but that organ may be removed from a reptile, and the animal will crawl away seemingly as lively as ever. A frog will live several hours without a heart, and will hop, swim, and struggle as if uninjured. Stilling once removed all the viscera from a frog, which, however, continued for one hour to hop, defend itself, and in various ways manifest its vivacity. [Footnote 124]
[Footnote 124: Stilling, Untersuchungen über die Functionen des Rückenmarks, p. 38. ]