40

PRIMARY TISSUES:-SEROUS MEMBRANES.

and has sometimes been purposely effected, does not produce any disorder in the general functions of the body. In blowing the nose violently, some part of the membrane lining its cavity has occasionally given way, so as to allow air to pass into the areolar tissue of the face, and especially into that contained in the eyelids, which is particularly loose; an enormous swelling of these parts then takes place, presenting a very frightful appearance, but not attended with the least danger, and subsiding of itself in a few days. This swelling presents a character to the touch quite different from that which would be occasioned by a similar distension with liquid; for it gives somewhat of the crackling feel that is occasioned by pressing on a blown bladder. A similar inflation of the areolar tissue of the body has sometimes occurred from the formation of an aperture, by disease or injury, in the walls of the lungs or air-passages, and the consequent escape of air during the act of breathing: in one remarkable case of this kind, the skin of the whole body was so tightly distended with air as to resemble a drum. It is intentionally practised by butchers, who "blow up" the areolar tissue of their veal, in order to increase its plumpness of aspect; and the inflation of the areolar tissue of the head, in the living state, has been sometimes practised by impostors, in order to excite commiseration.

28. Fibres and shreds of fibro-membrane, resembling those of which areolar tissue is composed, may be so interwoven as to form a continuous sheet of membrane, having a smooth and glistening surface; and in this manner are produced the Serous Membranes that line the different cavities in which the viscera (or organs contained within the skull, the chest, and the abdomen) are lodged. The peculiar manner in which these membranes are arranged, will be explained hereafter (§ 43). One of their surfaces is always free or unattached, whilst the other is in contact with the outer wall of the cavity; and from the free surface, which is covered with a layer of flattened epithelium-cells (fig. 10), a serous fluid is exhaled, which adds to its smoothness. It is by an accumulation of this fluid, that dropsies of the cavities are produced,such as water on the brain, or in the chest.

29. By the union of fibres of a stronger kind, those firmer tissues are produced, which are employed wherever a greater

FIBROUS MEMBRANES AND LIGAMENTS.

41

strain has to be borne. This is the case with the Ligaments, which bind together the bones at the joints, the Tendons, by which the muscles are usually attached to the bones, and the tough Fibrous Membranes that envelope and protect many of the most important viscera. In these any considerable amount of elasticity would be misplaced; and we consequently find that they are chiefly or entirely composed of the white fibrous tissue. Whenever an elastic ligament is required, however, we find the white replaced by yellow. One of the best examples of this is seen in the ligament of the neck of many quadrupeds, commonly known as the paxywaxy; which is given to the large herbivorous quadrupeds, such as the ox, to assist them in supporting their heavy heads with as little exertion as possible; whilst carnivorous quadrupeds, such as the lion and tiger, are endowed with it to give them additional power of carrying away heavy burdens in their mouths. In Man we scarcely find a trace of it. This yellow fibrous tissue is found, moreover, in the walls of the arteries (§ 248), to which it gives their peculiar elasticity; and it also forms the vocal cords of the larynx (§ 681). It is by the same kind of elastic ligament that the claws of the Feline tribe are drawn back into their sheaths when not in use, being projected (when required) by muscular action; and that the two pieces of the shell of Bivalve Mollusks are united at the hinge, and are at the same time kept apart for the admission of water between them, except when the animal forcibly draws them together by its adductor muscle (§ 113).

30. All these fibrous tissues, then, are concerned in actions purely mechanical; and there is nothing in their properties which is so distinct from those of inorganic substances, as to require to be considered as vital. We may consider them, therefore, as among the lowest forms of animal tissue; and accordingly we find that, when the higher forms degenerate or waste away, these appear in their place. Such a degeneration may take place simply from want of use. Thus if, from palsy or want of power of the nerves, the muscles of the legs are disused for several years, they will lose their peculiar property of contractility (§ 5); and it will be found that scarcely any true muscular structure remains, but that it is replaced by some form of fibrous tissue. Or again, if the

42

BASEMENT MEMBRANE-CELLS.

front of the eye be so injured by accident or disease, that light cannot pass through it to make its impression on the nerve, that nerve, being thrown into disuse, will gradually degenerate into fibrous tissue. Moreover, this change may take place as a part of the regular actions of life; for there are certain

organs in the young animal previous to birth, which are not required afterwards; and these degenerate in like manner, gradually wasting away, and leaving only traces behind them, -tubes shrivelling into fibrous ligaments, and glandular structures remaining only as areolar tissue.

31. Along every free surface of the body, both external and internal, is spread out a delicate structureless layer, which is termed the Basement or Primary Membrane. This forms the outer layer of the True Skin, lying between it and the Epidermis or scarf-skin (§ 37); in the same manner it underlies the Epithelial layer of the Mucous membranes which line the open cavities of the body (§ 39), and of the Serous membranes which line its closed cavities (§ 43); and it occupies the same position in the walls of the bloodvessels, gland-ducts, and other tubes. It is difficult to separate it, in any of these parts, from the tissues with which it is in contact; and its characters may be well studied by dissolving the calcareous part of an oyster or mussel-shell in dilute acid, when it will be found that layers of a thin transparent membrane are left, which have been thrown off at each act of shell-formation, from the surface of the mantle. This elementary membrane, like that which forms the walls of cells (§ 32), is remarkable for the readiness with which

less membrane, and having its cavity filled with fluid of some kind. In some part of its interior, most commonly adhering

CELLS; THEIR MODE OF MULTIPLICATION.

43

to its wall, there is usually to be observed a solid collection of granular matter, which is termed the nucleus (fig. 4, a a). The typical form of the cell is globular or oval (fig. 5); but when a number of cells are in contact with each other, and are pressed together, their sides become flattened; so that when they are cut across no intervals are seen between them, but their walls are everywhere in contact (fig. 6), just as in

the section of a vegetable pith. The chemical composition of the nucleus differs from that of the cell-wall; for whilst the latter is dissolved by acetic acid, the former (like the yellow elastic tissue, with which its substance appears to have some relationship) is unchanged by it. When the formation of a cell is complete, and it is not destined to reproduce its kind, the nucleus frequently disappears; this is the case, for example, with the red corpuscles of the blood of Mammalia (§ 229), and also with Fat-cells (§ 46).

33. New cells may originate in one of two very distinct modes; either from a pre-existing cell, or by an entirely new production in the midst of an organizable fluid or blastema. The most remarkable example of the first process is presented in the early development of the germ, which entirely consists of an aggregation of cells, every one of which undergoes successive subdivisions into two, so that the total number in the germ-mass is repeatedly doubled (Chap. xv.). The same method of multiplication by binary subdivision may be seen to continue throughout life in Cartilage-cells (§ 47), the growth of which almost exactly repeats the history of the growth of the lowest forms of Sea-weeds. The process of subdivision seems to commence in the nucleus, which begins to separate itself into two equal parts, and each of these draws

44

MULTIPLICATION AND NEW PRODUCTION OF CELLS.

around it a portion of the contents of the cell; so that the cell-wall, which is at first merely doubled inwards by a sort of hour-glass contraction, at last forms a complete partition between the two halves of the original cavity. The process may be repeated either in the same or in a transverse direction, so as to produce four cells, which may be either arranged in a single line o o or may form a cluster 88; and another subdivision of each cell will, of course, again double the entire number. In other cases, however, the nucleus appears to break up at once into several fragments, each of which may draw around it a portion of the contents of the parent-cell, which becomes invested by a cell-wall of its own; and thus the cavity of the parent-cell may at once become filled with a whole brood of young cells, without any successive subdivision. Generally speaking, the former method seems to prevail in structures which, like Cartilage, have a comparatively permanent destination; whilst the latter is followed in cases in which the cells thus formed are destined only for a transitory existence. This is the case especially in Cancerous structures, which are particularly distinguished by their proneness to the rapid production of cells within cells.

34. The production of new cells in the midst of an organizable blastema or formative fluid, such as is poured out from the blood for the reparation of an injury, is a very different process. This blastema, when first effused, is an apparently homogeneous semi-fluid substance; as it solidifies, however, it becomes dimly shaded by minute dots, and as it is acquiring further consistence, some of these dots seem to aggregate, so as to form little round or oval clusters, bearing a strong resemblance to cell-nuclei. These bodies appear to be the centres of the further changes which take place in the blastema; for if it be about to undergo development into a fibrous tissue (§ 18), they seem to be the centres from which the fibrillation spreads; whilst, if a cellular structure is to be generated, it is from them that the cells take their origin. The first stage of the latter process appears to consist in the accumulation of the substance which the cell is to include, about each nucleus, and around this the cell-membrane is subsequently developed. It is in this mode that the development of new structures, for the filling up of losses of substance, is provided for; and it appears, from recent