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being very thin and aqueous-as the fluid of the serous membranes ; and others of more consistence-as the secretion of the mucous membranes, animal oils, &c.

The physical and chemical properties of the fluids will engage attention when they fall individually under consideration; and we shall find that one of them at least-the blood-exhibits certain phenomena analogous to those of the living solid.

The fluids have been differently classed, according to the particular views that have, from time to time, prevailed in the schools. The ancients referred them all to four,-blood, bile, phlegm or pituita, and atrabilis; each of which was conceived to abound in one of the four ages, seasons, climates, or temperaments. Blood predominated in youth, in the spring, in cold mountainous regions, and in the sanguine or inflammatory temperament. Pituita or phlegm had the mastery in old age, in winter, in low and moist countries, and in the lymphatic temperament. Bile predominated in mature age, in summer, in hot climates, and in the bilious temperament; and atrabilis was the characteristic of middle age, of autumn, of equatorial climes, and of the melancholic temperament. This was their grand humoral system, which has vanished before a better observation of facts, and more improved methods of physical and metaphysical investigation. atrabilis was a creature of the imagination; the pituitous condition is unintelligible to us; and the doctrine of the influence of the humours on the ages, temperaments, &c., irrational.

The

Subsequently, the humours were classed according to their physical and chemical properties: they were divided, for instance, into liquids, vapours, and gases; into acid, alkaline, and neutral; into thick and thin; into aqueous, mucilaginous, gelatinous, and oily; into saline, oily, saponaceous, mucous, albuminous, and fibrinous, &c. In more modern times, endeavours have been made to arrange them according to their uses in the economy into-1, recrementitial fluids, or those intended to be again absorbed; 2, excrementitial, those that have to be expelled from the body; and 3, those which participate in both purposes, and are hence termed excremento-recrementitial. Blumenbach1 divided them into crude humours, blood, and secreted humours, a division which has been partly adopted by M. Adelon; and Chaussier, whose anatomical arrangements and nomenclature have rendered him justly celebrated, reckoned five classes:-1, those produced by the act of digestion,chyme and chyle; 2, the circulating fluids,-lymph and blood; 3, the perspired fluids; 4, the follicular; and 5, the glandular. This arrangement has been adopted by M. Magendie,3 and, with slight modification, is perhaps as satisfactory as any that has been proposed. All these will have to engage attention under SECRETION.

e. Physical Properties of the Tissues.

The tissues of the body possess the physical properties of matter in

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'Institutiones Physiologica, Sect. ii., § 4. Gotting., 1798.

Physiologie de l'Homme, 2de édit., i. 124. Paris, 1829.

3 Précis Elémentaire de Physiol., 2de édit., i. 20. Paris, 1825.

general. They are found to vary in consistence,-some being hard, and others soft; as well as in colour, transparency, &c. They have, also, physical properties, analogous, indeed, to what are met with in certain inorganic substances, but generally superior in degree. These are flexibility, extensibility, and elasticity, which are variously combined and modified in the different forms of animal matter, but exist to a greater or less extent in every tissue. Elasticity is only exerted under particular circumstances: when the part, for example, is put upon the stretch or compressed, the force of elasticity restores it to its primitive state, as soon as the distending or compressing cause is withdrawn. The tissues, in which elasticity is inherent, are so disposed through the body, as to be kept in a state of distension by the mechanical circumstances of situation; but, as soon as these circumstances are modified, elasticity comes into play, and produces shrinking of the substance. It is easy to see, that these circumstances, owing to the constant alteration in the relative situation of parts, must be ever varying. Elasticity is, therefore, constantly called into operation, and in many cases acts upon the tissues as a new power. The cartilages of the ribs, joints, &c., are in this manner valuable agents in particular functions.

We have other examples of the mode in which elasticity exhibits itself, when the contents of hollow parts are withdrawn, and whenever muscles are divided transversely. The gaping wound, produced by a cut across a shoulder of mutton, is familiar to all. Previous to the division, the force of elasticity is kept neutralized by the mechanical circumstances of situation, or by the continuity of the parts; but as soon as this continuity is disturbed, in other words, as soon as the mechanical circumstances are altered, the force of elasticity is exerted, and produces recession of the edges. This property has been described under various names, tone or tonicity, contractilité de tissu, contractilité par défaut d'extension, &c.

The other properties, flexibility and extensibility, vary greatly according to the structure of parts. The tendons, which are composed of areolar tissue, exhibit very little extensibility; and this for wise purposes. They are the conductors of force developed by muscle, and were they to yield, it would be at the expense of the muscular efforts; but they possess great flexibility. The articular ligaments are very flexible, and somewhat more extensible. On the other hand, the fibrous or ligamentous structures, which are employed to support weights, or are antagonists to muscular action, as the ligamentum nucha, which passes from the spine to the head of the quadruped,-are very extensible and elastic.

Another physical property, possessed by animal substances, is a kind. of contractility, accompanied with sudden corrugation and curling. This effect, which Bichat terms racornissement, is produced by heat, and by chemical agents, especially the strong mineral acids. The property is exhibited by leather when thrown into the fire.

An effect, in some measure resembling this, is caused by the evaporation of the water that is united to animal substances. This constitutes what has been called the hygrometric property of animal mem

branes. It is characteristic of dry, membranous structures; all of which are found to contract, more or less, by the evaporation of moisture, and to expand again by its re-absorption; hence the employment of such substances as hygrometers. According to M. Chevreul, many of the tissues are indebted for their physical properties to the water they contain, or with which they are imbibed. When deprived of this fluid, they become unfit for the purposes for which they are destined in life, and resume them as soon as they have recovered it.

A most important property possessed by the tissues of organized bodies is imbibition; a property to which attention has been chiefly directed of late years. If a liquid be put in contact with any organ or tissue, in process of time the liquid will be found to have passed into the areola of the organ or tissue, as it would enter the cells of a sponge. The length of time occupied in this imbibition will depend upon the nature of the liquid and the kind of tissue. Some parts of the body, as the serous membranes and small vessels, act as true sponges, absorbing with great promptitude; others resist imbibition for a considerable time, as the epidermis.

Liquids penetrate equally from within to without: the process is then called transudation.

Some singular facts have been observed regarding the imbibition of fluids and gases. On filling membranous expansions, as the intestine of a chicken, with milk or some dense fluid, and immersing it in water, M. Dutrochet3 observed, that the milk left the intestine, and the water entered it; hence he concluded, that whenever an organized cavity, containing a fluid, is immersed in another fluid, less dense than that which is in the cavity, there is a tendency in the cavity to expel the denser and absorb the rarer fluid. This M. Dutrochet termed endosmose, or "inward impulsion;" and he conceived it to be a new power, a "physico-organic or vital action." Subsequent experiments showed, that a reverse operation could take place. If the internal fluid was rarer than the external, the transmission occurred in the opposite direction. To this reverse process, he gave the name exosmose, or "outward impulsion." At times, the term endosmose is applied to the mutual action of two liquids when separated by a membrane; at others, to the passage of the liquid, that permeates the membrane in greatest quantity."

Soon after the appearance of M. Dutrochet's essay, the experiments were repeated, with some modifications, by Dr. Faust, and by Dr.

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1 Roget, art. Physiology, in Supplement to Encyclopædia Britannica; and Outlines of Physiology, with an Appendix on Phrenology. First American edition, with notes by the author of this work, p. 73, Philad., 1839.

"Magendie, Précis Elémentaire de Physiologie, 2de édit., 1825, i. 13.

• Mém. pour servir à l'Histoire Anatom. et Physiol. des Animaux et des Végétaux, Paris, 1837; art. Endosmosis, in Cyclopædia of Anatomy and Physiology, part x. p. 98, June, 1837, See, also, Vierordt, art. Transudation und Endosmose, in Wagner's Handwörterbuch der Physiologie, s. 631, Braunschweig, 1848.

* Matteucci, Lectures on the Physical Phenomena of Living Beings; translated by Pereira, p. 45, Amer. edit., Philad., 1848.

5 Poiseuille, Comptes Rendus, xix. 944, Paris, 1844.

Amer. Journal of the Med. Sciences, vii. 23, Philad., 1830.

VOL. I.-5

Togno,' of Philadelphia; and with like results. The fact of this imbibition and transudation was singular and impressive; and, with so enthusiastic an individual as M. Dutrochet, could not fail to give birth to numerous and novel conceptions. The energy of the action of both endosmose and exosmose is in proportion, he asserted, to the difference between the specific gravities of the two fluids; and, independently of their gravity, their chemical nature affects their power of transmission. These effects-he at once decided-must be owing to electricity. The cavities, in which the changes take place, he conceived to be like Leyden jars having their two surfaces charged with opposite electricities,the ultimate effect or direction of the current being determined by the excess of the one over the other.

In an interesting and valuable communication by Dr. J. K. Mitchell," of Philadelphia, "on the penetrativeness of fluids," many of the visionary speculations of M. Dutrochet are sensibly animadverted upon. It is there shown, that he had asserted, in the teeth of some of his most striking facts, that the current was from a less dense to a more dense fluid; and that it was from positive to negative, dependent not on an inherent power of filtration,-a power always the same when the same membrane is concerned, but modified at pleasure by supposed electrical agencies. This view was subsequently abandoned by M. Dutrochet, in favour of the following principle. It is well known that porous bodies, as sugar, wood, or sponge, are capable of imbibing liquids, with which they are in contact. In such case the liquid is not merely introduced into the pores of the solid, as it would be into an empty space; but is forcibly absorbed, so that it will rise to a height considerably above its former level. This force is molecular, and is the same that we witness in the phenomena presented by the capillary tube, which affords us the simplest case of the insinuation of a liquid into a porous body. It cannot alone, however, cause the liquid to pass entirely through the body. If a capillary tube, capable of raising water to the height of six inches, be depressed, so that one inch only be above the surface, the water will rise to the top of the tube; but no part of it will escape. Even if the tube be inserted horizontally into the side of the vessel containing water, the water will only pass to the end of the tube. The same thing occurs when a liquid is placed in contact with one side of a porous membrane: it enters the pores; passes to the opposite side, and is there arrested. But if this membrane communicates with a second vessel containing a different liquid-as a saline solution, capable of mixing with the first, and affected to a different degree by capillary attraction-a new phenomenon will be presented. It will be found, that both liquids enter the pores, and pass through to the opposite side. They will not, however, be carried through with the same force that which has the greatest power of capillary ascension, has the greatest affinity for the membrane, or will wet it more readily, -in other words, that which will rise the highest in a capillary tube, will pass through in greater quantity, and cause an accumulation of liquid on the opposite side. The action is well shown by the simple

'Amer. Journal of the Med. Sciences, iv. 73, Philad., 1829.
ง Ibid., vii. 23, Philad., 1830.

Fig. 1.

instrument figured in the margin. It consists of a glass tube, the lower extremity of which, covered by bladder, is funnel-shaped. This M. Dutrochet termed an endosmometer. If an aqueous solution of either gum or sugar be poured into it, and the closed extremity be immersed in pure water, the water is found to pass continually into the tube by filtration through the membrane, so that the liquid will rise in the tube, and may even flow out at the upper aperture. At the same time, a portion of the mucilaginous or saccharine solution will escape from the tube through the bladder, and become mixed with the water, but the quantity will be much less than that of the water which entered. The facts and arguments adduced by Dr. Mitchell clearly exhibit, that imbibition and transudation are dependent upon the penetrativeness of the liquid, and the penetrability of the membrane; that if two liquids, of different rates of penetrativeness, be placed on opposite sides of an animal membrane, "they will in time present the greater accumulation on the side of the less penetrant liquid, whether more or less dense; but will, finally, thoroughly, and uniformly mix on both sides; and at length, if any pressure exist on either side, yield to that, and pass to the other side." In all such cases, there are both endosmose and exosmose—or double imbibition; in other words, a certain quantity of one fluid passes in, and a certain quantity of the other passes out.2 As a general rule, imbibition takes place from the rarer to the denser medium; from pure water or dilute solutions towards those that are more concentrated. It would appear, again, that the stronger current is always from the medium which has the strongest affinity for the substance of the septum. It is well known, that in the case of a mixture of dilute alcohol covered over by a piece of bladder, the alcohol becomes concentrated, owing to the water-a denser fluid-passing more rapidly through the septum or bladder than the alcohol; but if the same mixture be tied over with elastic gum, the contrary effect will be produced the alcohol escaping in greater quantity.3 The general conditions of the phenomena of endosmose are:-first, that the two liquids shall have an affinity for the septum or interposed membrane; and, secondly, that they shall have an affinity for, and be miscible with, each other.

Endosmo

meter.

A portion of the communication of Dr. Mitchell relates to an analogous subject, to which, as M. Magendie' has observed, little or no attention had been paid by physiologists, the permeability of membranes by gases. "The laminæ," M. Magendie remarks, "of which membranes are constituted, are so arranged that gases can penetrate

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* Amer. Journal of the Medical Sciences for November, 1833, p. 100.

* Magendie, Leçons sur les Phénomènes Physiques de la Vie, tom. i. p. 99, Paris, 1836-38.

3 Henle, Allgem. Anat., or Jourdan's French translat., p. 210, Paris, 1843; and Wagner, Elements of Physiology, by Willis, p. 438, Lond., 1842.

▲ Précis Elémentaire de Physiologie, 2de édit., 1825, i. 13; and Lecons, &c., tom. i. p.

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