all the tissues from cells, have arrived at the conclusion, that as regards the areolar and other simple fibrous tissues, no other explanation of their production need be looked for than the known tendency of the particles of fibrin to arrange themselves in a linear manner so as to form fibres, a tendency which "manifests itself much more decidedly, when the consolidation takes place upon a living surface than upon a dead one."

Nor can ideas be esteemed more fixed in regard to the character of the matrix or blastema. M. Mandl1 affirms that we know not whether it is the albumen or fibrin of the blood. Others, and perhaps the majority of the present day, ascribe it to fibrin, between which, as we have elsewhere seen, and albumen, there is, according to Mulder, Liebig, and others, an almost identity of chemical composition. Fibrin, however, is considered to possess much higher vital properties; and the change of albumen into fibrin has been esteemed the first important step in the process of assimilation. In the chyliferous vessels, the proportion of fibrin increases as the chyle and lymph proceed onwards in the vessels; whilst that of the albumen diminishes. Such, however, is not rigorously the fact, for on referring to the table slightly modified from that of Gerber, which has been given elsewhere (vol. i. p. 653), it will be seen, that in the afferent lacteals between the intestines and mesenteric glands, the albumen has been found in minimum quantity; in the efferent or central lacteals, from the mesenteric glands to the thoracic duct, in maximum quantity; and in the thoracic duct in medium quantity; whilst the fibrin goes on progressively increasing as the chyle and lymph proceed onwards. On the other hand, the fat was found to diminish progressively; so that there appears to be more probability that the fibrin is formed from the fat, directly or indirectly, than from the albumen. It would seem not improbable, that some nitrogenized material like pepsin, or diastase in plants, is secreted from the parietes of the chyliferous vessels, which occasions a change in the elements of the constituents of the chyle, and is the earliest step in animalization: and the view is somewhat confirmed by the fact to which attention has been drawn by Mr. G. Ross, that the constituents of fatty matter, added to those of uric acid, would very nearly give the atomic constituents of albumen; whence, as Dr. Carpenter has remarked, it might be surmised, that when there is a demand for proteinaceous compounds in the system, nitrogenized matter, which would otherwise be thrown out of the system, may be united with non-nitrogenized compounds taken as food, in order to supply its wants. That there is an essential physiological difference, however, between fibrin and albumen, notwithstanding their affirmed similarity in chemical composition, is shown by the fact, that effused fibrin has a tendency to spontaneous coagulation, whilst albumen requires the agency of heat; and that, as we have seen, there is an appearance of distinct organization in coagulated fibrin. This difference in properties would necessarily induce the belief, that the two substances differ more perhaps in chemical composition than the results of the analyses of Mulder, Liebig, 1 Op. cit., p. 548. 3 Op. cit., p. 492.

2 Lancet, 1842-3, vol. i.

and others, would seem to indicate; and such appears to be proved by those of MM. Dumas and Cahours, which have been conducted on a very extensive scale; and show, that the proportion of carbon is seven per cent. less in fibrin than in albumen; whilst that of azote is from eight to nine per cent. more. A correct idea, these gentlemen think, may be formed of the elementary composition of fibrin by considering it a compound of casein, albumen, and ammonia.1

A view is entertained by many, that nothing but proteinaceous compounds can serve for the nutrition of the tissues; and that, as before remarked, gelatin is not adapted for this purpose. Liebig suggests, that it may be inservient to the nutrition of the gelatinous tissues; and Dr. Carpenter says, there is no doubt, that it is incapable of being applied to the reconstruction of any but the gelatinous tissues; and that it seems questionable, whether, even in these, it exists in a condition that can rightly be termed organized: yet it appears to the author, that great doubt may be entertained on this subject. The inconclusiveness of the experiments made on gelatin as an article of food has been animadverted on elsewhere (vol. i. p. 547). Although not a proteinaceous compound, it is one that is highly nitrogenized. When used as an aliment, it is not capable of being detected in the chyle or blood, and hence must have undergone a metamorphosis, probably into an albuminous compound; and it is certainly as difficult to comprehend how, under such circumstances, gelatin can be inservient to the nutrition of gelatinous tissues when no gelatin is present in the blood, as to comprehend that it may be converted first into albumen, and afterwards into fibrin. How gelatinous aliment, in other words, is formed into chyle and blood in which gelatin is not discoverable, and from these again gelatinous tissues are re-formed, is as incomprehensible as that any of the proteinaceous tissues should be constituted from the same pabulum; or that oleaginous aliments-as is admitted by some, who deny the same power to the gelatinous-should be convertible into proteinaceous compounds.

Such is the state of uncertainty in which we are compelled to rest in regard to this important function. None of the views can be esteemed established. They are in a state of transition; and all, perhaps, that we are justified in deducing is, that the vital property, which exists in organizable matters-in the fibrinous portion of the blood, and in the blastema furnished by the parents at a fecundating union-gives occasion to the formation of cells, in some cases-of fibres in others; and that the tissues are farther developed through the agency of cell-life or fibre-life, so as to constitute all the textures of which the body is composed.

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 developement or growth of organs and of the body generally, as well as of the limit accurately assigned to such developement, according to the animal or vegetable species, is dependent upon vital laws that are unfathomable. Nor are we able to

1 Med. Examiner, October 14, 1843, p. 232.

2 Principles of Human Physiology, 2d edit., p. 476, London, 1844.

detect the precise mode in which the growth of parts is effected. It cannot be simple extension, for the obvious reason that the body and its various compartments augment in weight as well as in dimension. The rapidity with which certain growths are effected is astonishing. The Bovista giganteum has been known to increase, in a single night, from a mere point to the size of a large gourd, estimated to contain 48,000,000,000 of cellules; and supposing twelve hours to have been necessary for its growth, the cells in it must have been produced at the rate of 4,000,000,000 an hour, or more than 66,000,000 a minute,— the greater part of the elements necessary for this astonishing formation being obtained from the air. But these rapid growths possess little vitality, and their decay is almost as rapid as their production. Analogous growths-but not to the like extent-occur in the human body, and the same remark applies to them.

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 developement; 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.2 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 shots 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 them. The flat bones also increase by a deposition at their margins; and the long bones by a similar deposition at their periphery,-additional circumstances strongly exhibiting the analogy between the successive developement of animals and vegetables. Exercise or rest; freedom from, or the existence of, pressure, produces augmentation of the size of organs, or the contrary; and there are certain medicines, as iodine, which are said to occasion emaciation of particular organs only-as of the female mammæ. The effect of disease is likewise, in this respect, familiar and striking.3

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 might be accomplished, so that not one of its quondam constituents should be present. By some, this was held to be seven years; by others, 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 its parts varies, indeed, according to numerous circumstances. It is not the same during the period of growth as subsequently, when 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 developement, at which time the nutrition of such parts must necessarily be

1 Truman, Food and its Influence on Health and Disease, &c., p. 229, Lond., 1842.

2 Observations on Certain Parts of the Animal Economy, with notes, by Prof. Owen, Amer. edit., p. 321, Philad., 1840.

The author's General Therapeutics and Mat. Med., 4th edit., ii. 285, Philad., 1850; and his Practice of Medicine, 3d edit., Philad., 1848.

more active, the organs of generation, for example, at the period of puberty; the enlargement of the mamme in the female; the appearance of the beard and the amplification of the larynx in the male, &c. All these changes occur after a determinate plan.

The activity of nutrition appears to be increased by exercise, at least in 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 the inactive male may not be very apparent.

Fig. 314.

The most active parts in their nutrition are the glands, muscles, and skin, which alter their character-as to size, colour, and consistencewith 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 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 are often covered with various designs from head to foot; subjecting themselves to a 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 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. M. Magendie2 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 sub

1 Lawrence, Lectures on Physiology, &c., p. 411, Lond., 1819.

2 Précis, &c., edit. cit., ii. 483.

VOL. II.-14

stance, which takes no part in the changes constantly going on in the tissue in which it is embedded; and the circumstance seems to afford a 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 remain free from all absorbing influence.

CHAPTER VI.

CALORIFICATION.

THE function we have now to consider is one of the most important to organized existence, and one of the most curious in its causes and results. It has, consequently, been an object of interesting examination with the physiologist, both in animals and plants; and as it has been presumed to be greatly owing to respiration, it has been a favourite topic with the chemist also. Most of the hypotheses, devised for its explanation, have, indeed, been of a chemical character; and hence it will be advisable to premise a few observations regarding the physical relations of caloric or the matter of heat,-an imponderable body, according to common belief, which is generally distributed throughout nature. It is this that constitutes the temperature of bodies,-by which is meant, the sensation of heat or cold we experience when they are touched by us; or the height at which the mercury is raised or depressed by them, in the instrument called the thermometer;-the elevation of the mercury being caused by the caloric entering between its particles, and thus adding to its bulk; and the depression produced by the abstraction of caloric.

Caloric exists in bodies in two states ;-in the free, uncombined or sensible; and in the latent or combined. In the latter case, it is intimately united with the other elementary constituents of bodies, and is neither indicated by the feelings nor thermometer. It has, consequently, no agency in the temperature of bodies; but, by its proportion to the force of cohesion, it determines their condition ;-whether solid, liquid or gaseous. In the former case, caloric is simply interposed between the molecules; and is incessantly disengaged, or abstracted from surrounding bodies; and, by impressing the surface of the body or by acting upon the thermometer, indicates to us their temperature. Equal weights of the same body, at the same temperature, contain the same quantities of caloric; but equal weights of different bodies at the same temperature have by no means the same. The quantity, which one body contains, compared with another, is called its specific caloric, or specific heat; and the power or property, which enables bodies to retain different quantities of caloric, is called capacity for caloric. If a pound of water heated to 156° be mixed with a pound of quicksilver at 40°, the resulting temperature is 1520,-instead of 98°, the exact mean. The water, consequently, must have lost four degrees of temperature, and the quicksilver gained one hundred and twelve; from which we· deduce, that the quantity of caloric, capable of raising one pound of