CHAPTER VII.

OF NUTRITION.

1. Selecting Power of the Individual Parts.

612. THE Blood which is carried into the different parts of the system, by the Circulating apparatus, is the source from which all the organs and tissues of the body derive the materials of their growth and development; and, as we have seen, it is distributed by the Capillaries of the several tissues, with a degree of minuteness, which varies according to the activity of the nutrient operations taking place in the individual parts. Thus, in Nerve and Muscle, Mucous Membrane and Skin, a constant decay of the old, and a development of new tissue, are taking place, when these organs are in a state of functional activity; and a copious supply of blood is carried through every part of their substance; whilst in Cartilage and Bone, Tendon and Ligament, the amount of interchange is very small, and is effected by a much less minute reticulation of capillary blood-vessels.

613. The materials of the nutritive process being prepared in the blood, the process of nutrition is the act of each individual part; which grows and developes itself, in virtue of its own inherent powers, as long as the requisite conditions are supplied. The mode in which this takes place, in each individual tissue, has been already explained in the former part of this Treatise. We have seen that, in the great majority of cases, the act of Nutrition is, in fact, a process of cell-growth; and that it takes place under the same conditions as the production of the simple isolated cell, which constitutes the whole of the humblest forms of Cryptogamic Vegetation,-namely, that it grows from a germ, which draws to itself the materials of its nutrition, and gives to some of them a new arrangement, whereby they form the cell-wall, whilst others are introduced into the cell-cavity, and then when it has passed through its regular series of changes, it dies, and sets free its contents. We have seen that, in some cases, the germs are prepared by previouslyexisting cells of the same kind; whilst in others they are furnished by certain "nutritive centres," which seem to be constantly engaged in the preparation of them, deriving their materials from the blood. Frequently it would seem as if the original or parent-cell is able to continue the production of secondary cells to an unlimited extent, even though it may have itself undergone a considerable change of form. Thus the ultimate follicles of Glands seem to be at first closed cells, which subsequently open at the part nearest to the duct, and establish a connexion with it; and having thus changed their condition, they go on developing new generations of secreting cells in their interior, from their own nuclei or germinal centres, to an unlimited extent. In like manner, the parent-cells of Muscular Fibre, which have coalesced to form the tubular Myolemma, seem to continue to develope new fibrille from their nuclei, notwithstanding their change of form.

614. The selecting power, which is possessed by the germs of each kind of tissue, and which enables them to draw from the blood the materials which they severally require for their development, manifests itself also in the mode in which substances, that are abnormally present in the blood, affect the condition and development of the solid tissues. Thus we find that the presence of a certain quantity of Arsenic in the blood, will produce a state of irritation in all the Mucous membranes of the body. The continued introduction of Lead into the circulating system, occasions a modification in the nutrition of the extensor muscles of the fore-arm, producing the form of partial paralysis commonly termed "wrist-drop" and the existence of this modification is shown by the presence of lead in the palsied muscles. Here we have to remark the symmetrical nature of the affection, consequent upon the occurrence of the same disorder in the corresponding parts of the two sides of the body; for these muscles appear to have the same kind of tendency to attract lead from the circulating current, in a degree that is equal on the two sides, as they have to draw from the blood the materials of their regular growth, and to develope themselves in an exactly similar manner. In like manner, the cutaneous eruptions, which are occasionally produced by the internal exhibition of iodide of potassium, are found to be almost precisely symmetrical; the presence of the medicine in the blood being the occasion of a disordered nutrition. of certain parts of the skin; and the selecting power of particular spots being evinced, by the exact correspondence of the parts affected on the two sides.

615. The same appears to be the case with regard to substances, whose presence in the blood is rather the result of a disordered condition of the digestive and assimilating processes, than of their direct introduction from without. Thus in Lepra and Psoriasis,-chronic diseases of the Skin, which seem to have their origin in a disordered state of the blood, rather than in the solid tissues affected,-we find a remarkable tendency to the repetition of the patches on the two sides. of the body, or on the corresponding parts of the limbs; and this we must attribute to the peculiar attraction, existing between the solid tissues of those parts, and the morbid matter circulating through them. So in those chronic forms of Gout and Rheumatism, which modify the nutrition of the joints, producing a deposit of "chalk-stones, or permanent distortion and stiffening, we almost invariably find the corresponding joints of the two sides affected. The chief exceptions to the general principle, that the presence of morbid or extraneous matters in the blood affects corresponding parts alike, are found to exist where there is much febrile disturbance, or where local causes produce a peculiar tendency to disorder of a single part. The nearer the character of the morbid process is to that of the ordinary nutritive operations, the more nearly does it approach these, in the symmetry with which it developes itself.*

* See Dr. W. Budd's valuable Paper on the "Symmetry of Disease," in vol. xxv. of the Medico-Chirurgical Transactions.

2. Varying Activity of the Nutritive Processes.

616. The nutritive operations go on with very great variations in their relative activity, under different circumstances. As a general rule it may be stated that, the greater the demand for the functional activity of the organ or tissue, the more energetic is its nutrition; and vice versa. Now this is readily understood, when it is considered, that the active state of many structures essentially consists in an act of nutrition; thus the energy of the secreting processes is really dependent, as we have seen, upon the growth of the secreting cells, which make up the essential part of the gland, and the energy of the absorbing and assimilating process is dependent upon the development of the cells, which select and elaborate the nutrient matter. This growth is regulated mainly by the supply of blood; being increased by the afflux of blood towards the part, in consequence of the influence of the nerves upon the vessels, or through any other change in the current of the circulation. Thus the secretions are increased in amount, by emotions of the mind, that act (probably through the sympathetic nerve) in regulating the calibre of the arteries supplying their respective glands: or the interruption of the function of one gland shall occasion an increased nutrition, and consequently an augumented secretion, in its fellow, as when one of the kidneys is hypertrophied, through ease in the other, that renders it incapable of performing its office. Still it would appear, that there may be variations in the activity of these organs, resulting from causes inherent in themselves (of the nature of which we know little or nothing); and that here, as elsewhere, active nutritive operations will promote the circulation of blood through the parts, whilst a languid state of the function will retard it.

617. In certain other tissues, however, the functional activity would seem to consist rather in a waste or decay of structures previously developed; this is the case especially in Nerve and Muscle, which are found to undergo disintegration, in exact proportion to the degree in which they are exercised; whilst the degree in which this waste is repaired, depends upon the supply of nutritive material, the quiescent state of the part, and other circumstances. But even here we find that functional activity occasions increased nutrition; in the same manner as burning a lamp with a high flame increases the amount of fluid drawn up by the wick. For neither the nerves nor the muscles can act with energy, without a large supply of arterial blood; and this is drawn to them on the principles already mentioned (§ 60) as increasing the energy of the local circulation. The determination of blood to the parts, thus established, favors their increased nutrition; and thus we find that, under favorable circumstances, any set of Muscles, which is habitually exercised, undergoes a great increase of development; whilst, in like manner the Nervous centres, if too great a demand be made upon their activity, are liable to become hypertrophied (especially in young persons), and may thus become subject to disorders, which temporarily or permanently destroy their powers. In these cases, then, the functional activity determines the increased supply of blood, and occasions the augmented growth; and increased nutrition will

rarely take place in these tissues without an especial stimulus of this kind. Thus we find that, when a larger supply of nutritive matter is introduced into the circulation, than is required to repair the waste of these tissues, they do not undergo an increased development in consequence; but an augmented nutrition is produced, either in the adipose tissue, or in the glandular structures by which the superfluous matter is eliminated from the system.

618. Augmented nutrition, or Hypertrophy, then, may result in cer tain organs, from an excessive supply of their nutrient materials; as in the case of the kidney, just mentioned; or as in the enlargement which we not unfrequently meet with in the livers of those, who have resided long in warm climates, and who have not sufficiently restricted their supply of non-azotized food to the small amount required for respiration at an elevated temperature, thereby sending an over-supply of that par ticular class of bodies, to be separated from the blood by the liver. Or, in other cases, the increase of functional activity may be the immediate cause of the increased nutrition; and this we see, not only in the nervous centres and voluntary muscles, which are put in action by the will, but in parts over which the mind has no control. Thus the heart becomes hypertrophied, when an obstruction exists in the pulmonary or systemic circulation, to overcome which, increased energy of contraction is required; and in the same manner the muscular coats of the urinary and gall-bladder acquire an extraordinary increase of thickness, when long-continued obstruction, by calculi or stricture in the canals issuing from them, impedes the free exit of their contents. Sometimes, however, a local hypertrophy takes place, which cannot be accounted for in either of these modes; as when a single finger is enlarged out of all proportion to the rest, or the whole of one hand increases to a much greater size than the other, by the existence (as it would seem) in the individual part of that tendency to unusual development, which, when it affects the whole body uniformly, produces a gigantic stature.

619. Now a precisely reversed series of conditions diminishes the activity of the nutrient processes, and induces a state of Atrophy. If there be a deficiency in the general amount of nutriment introduced into the system by absorption, a general atrophy results; and the waste being more rapid than the supply, there is a diminution in the volume of all the tissues excepting the nervous, which seems to have its nutrition kept up even to the last, at the expense of all the rest. Such a condition results not merely from the want of food, but also from the want of power to assimilate it; and thus emaciation may take place to an excessive degree, when food of the most nutritive character is copiously supplied, and when the appetite for it is vehement; in conse quence of disorder in the mesenteric glands, or in some other part of the apparatus particularly concerned in the elaboration of fibrine. A partial atrophy may result, in like manner, from a deficiency of the materials required for the formation of an individual tissue or organ; thus the adipose tissue throughout the body, may be atrophied, in consequence of an absence of those materials in the food, which are capable of being converted into fatty matter. Or a particular organ may be atrophied, by a diminution of the circulating current that should flow

to it, either in consequence of obstruction in the trunk or by the partial diversion of the stream of blood in another direction; and thus the liver, which is much more developed in the foetus, relatively to the rest of the body, than it is in the adult, undergoes a partial atrophy immediately after birth, in consequence of the change in the whole course of the circulation which takes place as soon as the lungs are expanded.

620. But partial atrophy may also take place from causes inherent in a particular organ. Thus we occasionally meet with limbs, which are "blighted," never attaining their due size relatively to the remainder of the body, yet not exhibiting any defect of organization. Here there would seem to be, from some unknown cause, a deficient power of growth; analogous to that which, when acting on the body in general, confines it within dwarfish dimensions.-One of the most frequent causes of partial atrophy, however, is want of functional activity in the organ ; and this is particularly the case in regard to the Muscular and Nervous systems. Thus, as already remarked (§ 348), any set of Muscles that is long disused, becomes partially atrophied; which is probably due to the feebleness and languor of the circulation, consequent upon the absence of the demand for arterial blood. As soon as the parts are called into use again, their nutrition improves. So, also, in regard to the Nerves; the nutrition of both the fibrous and vesicular structures appears to be entirely dependent upon the activity of their function; and as the former are inert without the latter, we may say that the due nutrition of the nervous system entirely depends upon the functional activity of the vesicular matter. Of this we have a well-marked illustration in the fact, that when the cornea has been rendered so opaque by disease or accident, as to prevent the penetration of any light to the interior of the eye, the retina and the optic nerve lose after a time their characteristic structure; so that scarcely a trace of the peculiar globules of the former or of the nerve-tubes of the latter can be found in them.

621. In the healthy condition of the organism, however, the nutrition in every part of the body goes on in a degree sufficient to keep it constantly ready for the performance of its appropriate function; a regular supply of the requisite materials being furnished in the aliment, and being prepared by the assimilating processes; and the products of the waste or decay of the tissues, together with such alimentary materials as may be superfluous, being carried off by the excreting operations. When the nutrition and the waste are equal, the weight of the body remains the same; and this is commonly the case in adult age. But during the earlier periods of life, the powers of growth are greater; the demand for food is very large in proportion to the bulk of the body; and though the waste is rapid, and the excreting process very active (as evinced by the large amount of urea and of carbonic acid set free), the growth predominates over the decay, and the development of the whole structure proceeds at a gradually-decreasing rate, until the full stature and bulk are attained. The energy of the nutritive process is particularly manifested, in the rapidity and completeness with which severe injuries, occasioned by disease or accident, are repaired. In advanced life, on the contrary, although the waste is comparatively small, the renewing processes are enfeebled in a still greater degree;