PHYSIOLOGY 17 kidney. In the old language the function of the kidney is to secrete urine. When we come to inquire into the matter, we find, in the first place, that the secretion of urine that is, the quantity and quality of the urine escaping from the duct of the kidney in a given period is partly determined by the quantity of blood passing through the kidney and the circumstances of its passage. Now the quantity of blood reaching the kidney at any one time is dependent partly on the width of the renal arteries, partly on the general pressure of the blood in the arterial sys tem. The width of the renal arteries is in turn dependent on the condition of their muscular walls, whether con tracted or relaxed ; and this condition is determined by the advent of nervous impulses, the so-called vaso-motor im pulses, arising in the central nervous system and passing down to the renal arteries along certain nerves. The emis sion of these vaso-motor impulses from the central nervous system is further determined, on the one hand by the con dition of certain parts of the central nervous system, the so-called vaso-motor centres, and on the other by the passage of certain afferent sensory impulses to those vaso-motor centres from sensory surfaces such as the skin. Similarly the general blood-pressure is dependent on the condition, patent or narrowed, of the small arteries generally, this being likewise governed by the vaso-motor system and on the coincident work done by the heart in driving blood into the great blood-vessels, this work being also governed by the nervous system. Hence in attacking such a problem as to how any particular event, such as the exposure of skin to the cold, influences the flow of blood through the kidney and thus the secretion of urine, the investigator, without staying to inquire into the nature of nervous impulses, or into the nature of changes taking place in vaso-motor centres, itc., directs his attention to determining what impulses are generated under the circumstances, what paths they take, to what extent they are quantitatively modified, how far they and their effects react upon each other, and so on. His inquiry in fact takes on to a large extent the characters of an attempt to unravel an intricate game, in which the counters are nervous impulses, muscular contractions, and elastic reactions, but in which the moves are determined by topographical distribution and mechanical arrangements. But there are other problems connected with the phy siology of the kidney of quite a different nature. The kidney is, broadly speaking, constructed of living proto plasmic cells so arranged that each cell is on one side bathed with blood and lymph, and on the other forms the boundary of a narrow canal, which, joining with other canals, ultimately opens into the urinary bladder. Here the question arises how it is that these protoplasmic cells, having nothing to draw upon but the common blood, which is distributed to other organs and tissues as well, are able to discharge on the other side of them into the canal the fluid urine, which is absolutely distinct from blood, which contains substances wholly unknown in blood, as well as substances which, though occurring in blood, are found there in minute quantities only, and, moreover, are not found to escape from the blood into any other tissues or organs. In attempting to answer this question we come upon an inquiry of quite a different nature from the pre ceding, an inquiry for the solution of which mechanical suggestions are useless. We have to deal here with the molecular actions of the protoplasmic cell. We must seek for molecular explanations of the questions, why a current sets across the cells from blood -capillary and lymph -space to the hollow canal ; why the substances which emerge on the far side are so wholly unlike those which enter in on the near side ; why, moreover, the in tensity of this current may wax and wane, now flooding the canal with urine, now nearly or quite drying up why not only the intensity of the current but also the absolute and relative amount of the chemical substances carried along it are determined by events taking place in the cell itself, being largely independent of both the quantity and quality of the blood which forms the cell s only source of supply. These and other like questions can only be solved by looking with the mind s eye, by penetrating through careful inferences, into those inner changes which we call molecular, and which no optical aid will ever reveal to the physical eye. These two lines of inquiry, which we may call the mechanical and the molecular, obtain in all parts of physio logy, sometimes the one and sometimes the other being dominant. A study of the special articles dealing with the several parts of physiology (see "Nervous System" below, NUTRITION, REPRODUCTION, RESPIRATION, VASCU LAR SYSTEM) will perhaps sufficiently show this ; but it may be worth while to give a very brief survey of the whole field from this point of view. The master tissues and organs of the body are the Brief nervous and muscular systems, the latter being, however, survey merely the instrument to give effect and expression to the motions of the former. All the rest of the body serves simply either in the way of mechanical aids and protection to the several parts of the muscular and nervous systems, or as a complicated machinery to supply these systems with food and oxygen, i.e., with blood, and to keep them cleansed from waste matters throughout all their varied changes. The physiology of the muscular system is fairly simple. The mechanical problems involved have been long ago for the most part worked out, and the molecular problems which touch on the nature of muscular contractions, their dependence on the blood -supply, and their relations to nervous impulses are being rapidly solved. The physiology of the nervous system, on the other hand, is in its infancy. The mechanical side of the inquiry is here represented, inasmuch as the various actions of the system are condi tioned by the distribution and topographical arrangement of the constituent fibres and cells ; and even these simple problems, as may be seen from the article " Nervous Sys tem " below, are as yet largely unworked. The deeper molecular problems, those which deal with the real nature of the processes taking place in cell and fibre, even the simpler of these, such as the one which asks why the neural protoplasm of one cell, or group of cells, seems quiescent until stirred by some foreign impulse, its own vibrations being otherwise retained and lost within its own substance, while the neural protoplasm of another cell is continually, or from time to time, discharging vibrations, as rhythmic molecular pulses, along adjoining fibres, these, at the present day, can hardly be said to be touched. The physiology of the nervous system is emphatically the physiology of the future. The rest of the body may, from a broad point of view, be regarded as a complex machinery for supplying these master tissues with adequately-prepared food and oxygen, for cleansing them from the waste-products of their activity, and for keeping them at a temperature suitable for the development of their powers. As we have already said, the blood is the agent which not only supplies both food and oxygen but sweeps away all refuse, and, we may add, is the instrument for maintaining an adequate temperature. All the rest of the body may in fact be looked upon as busied in manufacturing food into blood, in keeping up the oxygen supply of the blood, in sifting out from the blood all waste material, and in maintaining the blood at a uniform heat. This work, of which blood is, so to speak, the centre, is, as we have already seen, carried out by protoplasmic cells, many of which are themselves of a mus cular nature, often forming part of complicated mechanical
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