CHEMICAL CONSTITUENTS:-FIBRIN, GELATIN.

35 distinctly fibrous structure, that the mouths of divided bloodvessels are closed up, when the flow of blood from them spontaneously stops. In all such cases, the fibrous network, if formed out of connexion with a living body, passes after a time into decay; but if it be formed in apposition with living parts, blood-vessels gradually extend into it from these, its nutrition is maintained and improved, and it progressively comes to present the ordinary characters of the simple fibrous tissues (§ 22).

19. Although the tissues most actively concerned in carrying on the vital operations, retain for the most part the composition of albumen, yet that very large proportion of the fabric of the higher animals whose offices are essentially mechanical, has a very different chemical constitution. If we boil down either their bones, their skin, or their internal membranes, we shall get a considerable quantity of the substance scientifically termed Gelatin, familiarly glue. Though consisting of the same elements as albumen, its composition is simpler, because these elements are united in smaller proportions; the atom or combining equivalent of gelatin being made up of 13 Carbon, 10 Hydrogen, 5 Oxygen, 2 Nitrogen. The distinctive character of gelatin consists in its solubility in warm water, its coagulation on cooling into a uniform jelly which can be liquefied again by warmth, and its formation of a peculiar insoluble compound with tannin. Gelatin is very

The re

sparingly soluble in cold water, though made to swell up and soften by prolonged contact with it. A solution of only one part of gelatin in 100 of hot water is sufficiently strong for the whole to form a consistent jelly on cooling. action of gelatin with tannin is so decided, that the presence of only one part in 5000 of water is at once detected by infusion of galls; and it is in this action that the process of tanning consists,-the gelatinous fibre of the skin, which would speedily pass into decay, being converted into a comparatively unchangeable substance. The different tissues which have gelatin for their base, yield it to boiling water with different degrees of facility; this diversity apparently depending in some degree upon the definiteness of their organization. Thus the "sound" or air-bladder of the cod, sturgeon, and other fish, which, when dried and cut into strips, is known as isinglass, is very readily acted on; the

36

CHEMICAL CONSTITUENTS :-GELATIN, CHONDRIN.

same is the case with the animal substance of bones from which the earthy matter has been removed; and in each case the fibrous texture of the living tissue is but very imperfectly developed. For the extraction of gelatin from the skin, the ligaments, the tendons, and various internal membranes, whose fibrous texture is more pronounced (§ 29), a much longer action of boiling water is required.

20. A peculiar modification of gelatin, which presents itself in Cartilage (or gristle), is distinguished as Chondrin. This requires longer boiling than gelatin for its solution in water; as is seen when a knuckle of veal or of mutton is cooked, the tendons and ligaments about the joint being almost reduced to pulp, whilst the cartilages are scarcely at all softened. The essential properties of chondrin are nearly the same as those of gelatin, and its composition seems nearly identical; but it is thrown down from its solution by muriatic and acetic acids and some other reagents, which do not disturb a solution of gelatin.

21. It is not yet fully known how the material of the gelatinous tissues is produced in the animal body. There can be no doubt of its being producible from albumen; since we find it in large proportion in the tissues of animals that have never received gelatin into their bodies in any shape. And although carnivorous animals will receive it as part of their aliment, yet there is strong reason to believe that the gelatin which is thus supplied to them does not really serve to nourish their bodies, but that it is speedily decomposed and got rid of (§ 159). It may be considered as quite certain that the albuminous tissues cannot be formed by the metamorphosis of gelatin; whilst conversely, looking to the fact that in the egg and in milk no gelatin is provided for the young animal, although the gelatinous tissues form a yet larger proportion of its body than they do in the adult, we seem entitled to question whether it is possible that these tissues can be formed in any other way than at the expense of the albuminous constituents of the blood.

Structure of the Primary Tissues,

22. In considering the structure of the "primary tissues," of which the various organs of animals are composed, it will be convenient first to treat of those which are subservient

PRIMARY TISSUES:-SIMPLE FIBROUS TISSUES.

37

merely to the physical actions of the framework; as, for example, by holding its parts together, by communicating motion, or by giving them mechanical support and protection.-The several parts of the body, even to the very minute divisions of its organs, are held together by what may be termed, in contradistinction to Muscular and Nervous fibre, the simple fibrous tissues; and these are merely endowed, like ordinary cords, with the power of resisting tension or strain, either without themselves yielding to it at all, or with a certain amount of elasticity, which enables them first to yield to a certain degree, and then to recover their previous state. These two qualities are characteristic of two distinct forms of simple fibrous tissue, the white and the yellow.

23. The White fibrous tissue presents itself under various forms, being sometimes composed of fibres so minute as to be scarcely distinguishable, but

more commonly presenting itself under the aspect of flattened bands, which are but imperfectly divided into fibres, and have more or less of a wavy aspect (fig. 1). This tissue is resolved, by long boiling, into gelatine; and when treated with acetic acid, it swells up and becomes

Fig. 1.-WHITE FIBROUS TISSUE.

transparent, by which peculiarity it can be readily distinguished from the other kind, to be next described. The Yellow fibrous tissue presents

itself in the form of long, separate,clearly defined fibres, which sometimes branch, and which break short off when overstrained, their extremities being disposed to curl up (fig. 2). They are, for the most part, between 1-5,000th and 1-10,000th of an inch in diameter; but they are often

met with both larger and smaller. This kind of tissue undergoes but very little change from long boiling, and it is

38

PRIMARY TISSUES:-AREOLAR TISSUE.

not acted on by acetic acid. It is but little prone to decomposition, and will exhibit its peculiar elasticity long after it has been separated from the body, provided it be kept moist. -These two forms of tissue exist separately in certain parts of the fabric, but they are much more frequently combined; and the proportion of the yellow elastic tissue which exists in any such combination, may be readily determined under the microscope by the use of acetic acid, which renders all the white fibrous structure so transparent, that the yellow fibres are seen completely isolated in the midst of it.

24. One of the tissues which is composed of such an admixture of white and yellow (or non-elastic and elastic) fibres, is the one which was formerly called "cellular," but which is now more correctly designated as Areolar.1

This

is composed of a mesh-work of fibres, and of bands of fibrous membrane, which are interwoven in such a manner as to leave very numerous interstices and cavities amongst them, having a tolerably free communication with each other (fig. 3). These

cavities are filled during life with a serous fluid; 2 and it is a necessary result of the communication between them, that if an accumulation of this fluid takes place to an undue extent,

1 From the Latin areola, a small open space.

2 A fluid resembling the serum of the blood, diluted with water (§ 236).

PRIMARY TISSUES:-AREOLAR TISSUE.

39

as in dropsy, it descends by gravity to the lowest situation. Hence, the legs swell more frequently than any other parts. In its natural state, this tissue possesses considerable elasticity; hence, when we press upon any soft part, and force out the fluid beneath into the tissue around, the original state returns as soon as the pressure is removed. But in dropsy, it appears as if the elasticity of the fibres were impaired or destroyed by their being over-stretched; for when we press with the finger upon a dropsical part, a pit remains for some time after the finger has been removed.

25. This Areolar tissue is diffused through almost the whole fabric of the adult animal, and enters into the composition of almost every organ. It binds together the minute parts of which the muscles are composed; it lies amongst the muscles themselves, connecting them together, but yet permitting them sufficient freedom of motion; it exists in large amount between the muscles and the skin; it forms sheaths to the blood-vessels and nerves, and so connects them with the muscles that they shall not be strained or suddenly bent by the movements of the latter; and it enters into the structure of almost every one of the organs which are contained in the cavity of the trunk, uniting its parts to each other, and keeping the whole in its place. But it is a great mistake to assert, as it was formerly common to do, that it penetrates the harder organs, such as bone, teeth, and cartilage. Its purpose obviously is to allow a certain amount of motion among the parts it unites; and we find that the more free this motion is required to be, the larger is the proportion borne by the yellow or elastic fibres, to the white or non-elastic.

26. Although the Areolar tissue contains a very large number of blood-vessels and nerves, yet it does so merely because it furnishes the bed or channel in which they are conducted to the parts where they are really wanted. Its own vitality is low, and its sensibility very slight. It is quickly reproduced after injury; and it is by its means that losses of substance are repaired in tissues of a more elaborate kind, which are not so easily regenerated.

27. The continuity or connectedness of this tissue over the whole surface of the body, admits air to pass readily from one part to another; and the inflation or blowing-up of its cavities with air, which has sometimes happened accidentally,