NUTRITION.

THE investigation of the phenomena of the circulation has exhibited the mode in which the arterial blood is distributed over the body, in minute vessels, not appreciable by the naked eye, or even with the microscope, but so numerous, that it is impossible for the finest-pointed instrument to be forced through the skin without penetrating one and perhaps several. We have seen, likewise, that, in the capillary system of vessels, this arterial blood is changed into venous; and it was observed, that, in the same system, parts are deposited or separated from the blood, and certain phenomena accomplished, into the nature of which we have now to inquire; beginning with those of nutrition, which comprise the incessant changes, that are taking place in the body, both of absorption and deposition, and which effect the decomposition and renovation of each organ. Nutrition is well-defined by Adelon as the action, by which every part of the body, on the one hand, appropriates or assimilates to itself a portion of the blood distributed to it; and, on the other, yields to the absorbing vessels a portion of the materials that previously composed it.

The precise character of the apparatus by which this important function is accomplished, we have no means of knowing. All admit, however, that the old matter must be taken up by absorbents, and the new be deposited by arteries. As the precise arrangement of these minute arteries is not perceptible by the eye, even when aided by powerful instruments, their arrangement has given rise to much controversy. Whilst some have imagined lateral pores for the transudation of the nutritive deposits, others have presumed that inconceivably small vessels are given off from the capillary system, which constitute a distinct order, and whose function it is to exhale the nutritive substance. Hence, they have been termed exhalants or nutritive exhalants; but the physiological student must bear in mind, that whenever the term is used by writers, they do not always pledge themselves to the existence of any distinct set of vessels, but merely mean the capillary vessel, whatever may be its nature, which is the agent of nutrition, and separates, from the blood, bone where bone is needed; muscle, tendon, ligament, &c. as the case may be.

In investigating the physiology of nutrition, two topics necessarily divide our attention; 1st, The action of decomposition, by which the organ yields to the absorbing vessels a portion of its constituents; and 2dly, The action of composition, by which the organ assimilates a part of the arterial blood, that enters it, and

supplies the loss, which it has sustained by the previous action of decomposition.

The former of these actions obviously belongs to the function of absorption; but its physiology, it will be recollected, was deferred, in consequence of its close application to the function we are now considering.

It comprises what is meant by interstitial, organic, or decomposing absorption, and does not require many comments, after the long investigation of the phenomena of absorption, into which we have entered. The conclusion, at which we then arrived, was, —that the chyliferous and lymphatic vessels form only chyle and lymph respectively, refusing the admission of all other substances; that the veins admit every liquid which possesses the necessary tenuity; and that, whilst all the absorptions,—which require the substance acted upon to be decomposed and transformed,—are effected by the chyliferous and lymphatic vessels, those that demand no alteration are accomplished through the coats of the veins directly by imbibition.

It is easy, then, to deduce the agents to which we refer the absorption of decomposition. As it is exerted on solids, and as these cannot pass through the coats of the vessels in their solid condition, it follows, that other agents than the veins must accomplish the process; and, again, as we never find in the lymphatic vessels any thing but lymph, and as we have every reason to believe, that an action of selection is exerted at their extremities, similar to that of the chyliferous vessels on the heterogeneous substances exposed to them, we naturally look to the lymphatics as the sole organs concerned in the absorption of solids.

In making this affirmation, we leave unexplained the mysterious operation by which these vessels are enabled to reduce to their elements, bone, muscle, tendon, &c., and to recompose them into the form of lymph. Dr. Bostock fancifully suggests, that the first step in this series of operations, is the death of the part, by which expression he means, that it is no longer under the influence of arterial action. "It therefore ceases to receive the supply of matter which is essential to the support of all vital parts, and the process of decomposition necessarily commences." The whole of his remarks on this subject are eminently gratuitous, and are manifestly suggested by his extreme unwillingness to ascribe the process to any thing but physical causes. If there is, however, any one phenomenon of the animal economy, which is more manifestly referable to vital action than another, it is the whole function of nutrition, both as regards the absorption of parts already deposited, and the exhalation of new; and it is wise to confess our utter ignorance of the mode in which it is accomplished. We know that the blood contains most of the principles that are necessary for the nutrition of organs, and that it must contain the elements of all. Fibrine, albumen, fat, osmazome, salts, &c. exist in it, and these are depo

sited, as the blood traverses the tissues; but why one should be selected by one set of vessels, as by the exhalants of bone, and another by another set, and in what manner the elements of those, not ready formed in the blood, are brought together, is totally unknown to us. Blood has been designated as "liquid flesh,"chair coulante,—but something more than simple transudation through vessels is necessary to form it into flesh, and to give it the compound organization of fibrine, gelatine, osmazome, &c. in the form of muscular fibre and cellular membrane, which we observe in the muscle.

Nothing perhaps more clearly exhibits our want of knowledge on the subject than the following vague attempt at solving the mystery by one of the most distinguished physiologists of the age: "Some immediate principles, that enter into the composition of the organs or of the fluids, are not found in the blood,—such as gelatine, uric acid, &c. They are consequently formed at the expense of other principles, in the parenchyma of the organs, and by a chymical action, the nature of which is unknown to us, but which is not the less real, and must necessarily have the effect of developing heat and electricity."

It is the action of nutrition, that occasions the constant fluctuations in the weight and size of the body, from the earliest embryo condition till advanced life. The cause of the development or growth of organs and of the body generally, as well as of the limit, accurately assigned to such development, according to the animal or vegetable species, is dependent upon vital laws that are unfathomable, and which the endosmose of DutroCHET is little calculated to explain. Nor are we able to detect the precise mode in which the growth of parts is effected. It cannot be simply extension, for the obvious reason that the body and its various compartments augment in weight as well as in dimension. In the large trees of our forests, we find a fresh layer or ring added each year to the stem, until the full period of development; and it has been supposed that the growth of the animal body may be effected in a similar manner, both as regards its soft and harder materials, that is, by layers deposited externally. That the long bones lengthen at their extremities is proved by an experiment of Mr. Hunter. Having exposed the tibia of a pig, he bored a hole into each extremity of the shaft, and inserted a shot. The distance between the shot was then accurately taken. Some months afterwards, the same bone was examined, and the shots were found at precisely their original distance from each other; but the extremities of the bone had extended much beyond their first distance from the shot.

The flat bones also increase by a deposition at their margins, and the long bones by a similar deposition at their periphery,—circumstances strongly exhibiting the analogy between the successive development of animals and vegetables.

Exercise or rest, freedom from or presence of pressure, produce augmentation of the size of organs or the contrary; and there are certain medicines, as iodine, which occasion the emaciation of particular organs only—as of the female mammæ. The effect of disease is likewise, in this respect, familiar and striking.

The ancients had noticed the changes effected upon the body by the function we are considering, and attempted to estimate the period at which a thorough conversion must be accomplished, so that not one of its quondam constituents shall be present. By some, this was supposed to be seven years; but Bernouilli reduced it to three. It is hardly necessary to say, that, in such a calculation, we have nothing but conjecture to guide us. The nutrition of the body and of its parts varies, indeed, according to numerous circumstances. It is not the same during the period of growth as subsequently, when the absorption and deposition are balanced, so far at least as concerns the augmentation of the body in one direction. Particular organs have, likewise, their period of development, at which time the nutrition of such parts must necessarily be more active,—the organs of generation, for example, at the period of puberty; the enlargement of the mammæ in the female; the appearance of the beard and the amplification of the larynx in the male, &c.,—and all these changes occur after a determinate plan.

The activity of nutrition appears to be increased by exercise, at least in the muscular organs; hence the well-marked muscles of the arm in the prize-fighter, of the legs in the dancer, &c. The muscles of the male are, in general, much more clearly defined; but the difference between those of the hard-working female and of the inactive male may not be very apparent.

There are several textures of the body that do not experience nutrition, but, when once deposited, appear to remain permanently, such as the epidermis, the nails, the teeth, the colouring matter of the skin, and, it is presumed, the cartilages,—especially the interarticular. The most active in their nutrition are the glands, muscles, and skin, which alter their character—as to size, colour and consistence—with great rapidity; whilst the tendons, fibrous membranes, bones, &c. are much less so, and are altered more slowly by the effect of disease.

A practice which prevails amongst certain professions and people, would seem at first sight to show, that the nutrition of the skin cannot be energetic. Sailors are frequently in the habit of forcing gunpowder through the cuticle with a pointed instrument, and of figuring the initials of their names upon the arm in this manner; the particles of the gunpowder are thus driven into the cutis vera and remain for life. The operation of tattooing, or of puncturing and staining the skin, prevails in many parts of the globe and especially in Polynesia, where it is looked upon as greatly ornamental. The art is said to be carried to its greatest perfection in the Washington or New Marquesas Islands; where the wealthy arc often covered

with various designs from head to foot; subjecting themselves to a

Fig. 119.

most painful operation for this strange kind of personal decoration. The operation consists in puncturing the skin with some rude instrument, according to figures previously traced upon it, and then rubbing into the punctures a thick dye, frequently composed of the ashes of the plant that furnishes the colouring matter. The marks, thus made, are indelible. Magendie asks: "How can we reconcile this phenomenon with the renovation, which, according to authors," (and, he might have added, according to himself,) "happens to the skin?" It does not seem to us to be in any manner connected with the nutrition of the skin. The colouring matter is an extraneous substance, which takes no part in the changes, constantly going on in the tissue in which it is imbedded; and the circumstance seems to afford a powerful negative argument in favour of venous absorption. Had the substance possessed the necessary tenuity it would have entered the veins like other colouring matters; but the particles are too gross for this, and hence they remain free from all absorbent influence.