The growth of C. requires some notice. Growth probably occurs in all C., although not in all to the same extent. It is most obvious in those which are formed directly round a nucleus, since in these the membranes which at first closely invest the nucleus, in time become distended and enlarged, and merely remain in contact with the nucleus at one point. Growth may take place either in surface or in thickness. The former is most commonly general-viz., in all those cases where C. increase without altering their form; but is sometimes partial-viz., in those cases in which the cell deviates considerably from the primary globular form. The latter occurs to a certain degree in all C., but in some kinds to a far greater extent than in others. In fig. 6, two cartilage-C., magnified 350 diameters, are shewn, in which the walls are much thickened; in addition to their nucleus, each contains a clear drop of fat. The nuclei and nucleoli also take part to a certain extent in the growth of the cells. Schwann gives the following general explanation of the process of growth. He considers that the molecules of the cell-membrane exert an attractive influence on the fluid which surrounds them, and deposit its newly formed particles amongst themselves. If the deposition take place between the molecules already present in the substance of the membrane, the cell becomes distended; if it take place only in one or more definite directions, the membrane becomes thickened.

00

Fig. 6.

Having now traced the cell to the period of its full growth, we are prepared to consider the processes which occur in the interior of this minute organic structure, or, in other words, the physiology of cells. To enter satisfactorily into this subject, we ought to have an exact knowledge of the chemical composition of the contents of different cells. All that we know of the contents of C. generally is, as we have already stated, that they usually consist of a moderately concentrated solution of protein matters, with alkaline and earthy salts, and dissolved or suspended fat-particles; and that besides these ingredients many C contain either a great preponderance of one of these constituents, to the almost entire exclusion of others, or are found to contain altogether new substances. Thus, there are C. with much protein matters, as the nerve-C., and with much fat, like the fat-C.; while there are other C. which specially contain hæmatine (the red colouring matter of the blood), pigment, biliary and urinary constituents, mucus, milk, sugar, &c.

The main cell-processes occurring in these variously constituted C. are absorption, secretion, and excretion. These depend principally, if not entirely, upon chemical and physical laws, and are to a great extent amenable to micro chemical observation.

Absorption, or the appropriation of matters from withort, is most manifest in those C. which at first have little or no contents save the nucleus. Although endosmose must be taken into account as a condition of absorption, C. must not be regarded merely as vesicles provided with indifferent porous membranes; for the filling of C. does not take place by their admitting every kind of matter

The cell having thus become filled from without, we have next to inquire into the changes which take place in the membrane and in the contents. As regards the former, the membranes of most C. not only become denser and more solid with age, but they undergo changes in their chemical constitution. Thus, in the horny tissues, the young C. are easily soluble in alkalies and acids, while older C. of the same nature are scarcely affected by these re-agents; again, in cartilage-C., the membrane not only becomes firmer with age, and thickens as ossification proceeds, but is changed into a tissue yielding gelatine or glue on boiling, which subsequently becomes impregnated with salts of lime (phosphate and carbonate). See BONE. The function of secretion is mainly carried on by changes in the contents of the cells. Thus, mucus is formed in the epithelial C. of the mucous membranes, pepsin in those of the gastric glands, bile in the C. of the liver, and sepia in the C. of the ink-bag of the cuttle fish. In these cases, the C. do not separate mucus, pepsin, &c., from the blood, but merely the materials from which they elaborate these substances. In other cases, as for instance, in the C. of the kidney, the function of these minute organisms is not to manufacture new products, but merely to separate certain substances (urea, uric acid, &c.) from the blood, which, if not immediately removed from the general circulation, would speedily accumulate, and act as a deadly poison. That these C. merely separate the urea from the blood, and do not form it in their interior, is proved by the fact that, if the kidneys of an animal were extirpated, the urea and other urinary constituents may speedily be found in large quantity in the blood.

Excretion takes place by the bursting or solution of the distended secreting cell, usually into the duct of a secreting gland. The reader who desires further information on the functions of the C. in relation to secretion and excretion, is especially referred to an admirable memoir by Professor Goodsir, On Secreting Structures,' published in John and Harry D. S. Goodsir's Anatomical and Pathological Researches, 1845.

In conclusion, we must notice the metamorphoses of cells. The ovum itself is, as we have already mentioned, merely a nucleated cell; after impreg nation, a number of secondary C. are formed within it, by a process of cleavage or segmentation. See articles GENERATION and OVUM. Some of the C. which occur in the ovum in its early stages soon coalesce with others to form the higher elementary parts, which we shall shortly enumerate; others, without entering into combinations, more or less change their previous nature, as the horny plates of the epidermis and nails; while others, again, undergo no change of form throughout the period of their existence.

The permanent C. are arranged by Kölliker (Manual of Human Histology, translated by Busk and Huxley, 1853, vol. i. p. 47) under the following heads:

1. True Cells, which have in no essential respect altered their cellular character. These occur in the epidermis and the epithelium; in the blood, chyle, and lymph; in the glandular secretions, in the fatty tissue, in the gray nervous substance, in the glands (liver, spleen, &c.), and the cartilages, Their varieties of form and contents have been already noticed. Regarding their modes of occur rence, some are either isolated in fluids or in solid tissues; others are united by apposition, without any intervening structure, into a cellular parenchyma:

while others, again, are conjoined by an intercellular substance of some kind.

2. Metamorphosed Cells. To these belong-The horny scales flattened, polygonal, or fusiform; their membrane being fused into one mass with their contents. They occur in the epidermis, the laminated pavement epithelium, and the hair and nails. The contractile fibre-C.: fusiform, slightly flattened, considerably elongated C., whose membrane, with its soft, solid contents, is changed into a contractile substance. They occur in the smooth or involuntary muscles. The tubules of the crystalline lens of the eye: very elongated C., with viscid, albuminous contents. The prisms of the enamel of the teeth greatly elongated, prismatic, and strongly calcified ceнs. The bone-C.: thickened C. (with canaliculi, or minute branching canals) which have coalesced with the matrix of the bones. The transversely striated muscular C.: large polygonal C. whose contents have become metamorphosed into a transversely striated or striped tissue, such as is found in voluntary muscular fibre. From these C. are formed all the different fibres, net-works, membranes, tubes, &c.; in short, all the higher elementary parts of which the animal body is composed.

For further information on C. and cell-development, the reader is referred, in addition to the works quoted in this article, to Leydig, Lehrbuch der Histologie des Menschen und der Thiere, 1857; and to Frey, Histologie und Histochemie des Menschen, 1859; while he will find full details on morbid cell-development (the development and growth of C. in tubercle, cancer, and other morbid deposits) in Vogel's Pathological Anatomy of the Human Body, translated by Day, 1847; and in Wedl's Pathological Histology, translated (for the Sydenham Society) by Busk, 1855.

II. VEGETABLE CELLS.-In the vegetable, as in the animal kingdom, the primary form of the cell is that of a sphere. They are, however, interfering influences, which usually alter or modify the primary form, of which the most important are, (1.) Special directions assumed in the development, in obedience to a law regulating the structure of the tissue in which the cell occurs; and (2.) Obstructions to the expansion of the cell in certain directions from the pressure of surrounding cells.

The most common forms referrible to the law of development are, (1.) the spherical or fundamental form; (2.) The cylindrical, in which there is a tendency to elongation in the direction of a vertical axis; and (3.) The tubular, in which there is an excess of development in the direction of the two

transverse axes.

The secondary modifications of these forms are numerous. Thus, in lax tissues, the spherical form may become an irregular spheroid, running out into lobed, and even stellate forms, as may be seen in the pith of rushes and the stems of various aquatic plants. Again, in seeds, the hard part of fruits, &c., the mutual pressure of the C. converts the spherical into polyhedral forms, of which the dodecahedron giving a hexagonal section, and arising from equal pressure in all directions-is the most common, although cubic and many other forms occasionally

Both the cell-wall and the contents differ from the corresponding parts in animal cells. In all young C. the wall is membranous, freely permeable by water, elastic, and flexible. In many cases it retains these properties, whilst in others it becomes much modified, as the cell grows older. It consists mainly of CELLULOSE (q. v.). As the vital and chemical phenomena exhibited by plants depend primarily upon operations in the interior of the cell, the careful study of the cell contents is of the highest importance. Of these contents, the most important are the primordial utricle, with the protoplasm, the nucleus, chlorophyll corpuscles, and starch granules. The primordial utricle is a layer of substance of mucilaginous consistence (coloured yellow by iodine), lining the entire wall of the young cell, but often disappearing at a comparatively early period. The protoplasm is a tough mucilaginous and frequently granular fluid, which fills up the space in the interior of the cell not occupied by the nucleus. The nucleus or cytoblast is a globular or lenticular body, identical in its character with the substance of the primordial utricle, and occurring in the protoplasm of most young cells. Little is known with certainty regarding the chlorophyll corpuscles, except that, under the influence of solar light, green colouring matter is developed from them. Of the starch granules, which are very commonly found in the cell contents, we need not speak, as they are sufficiently described in the article STARCH.

In addition to the above organised structures, we must mention as frequent constituents of the cellcontents, fluid colouring matters, essential and fixed oils, resins, sugar, dextrine, gum, alkaloids, and mineral or organic salts, which are not unfrequently found in a crystalline form, when they are termed raphides.

There are two modes of cell-development in the vegetable kingdom--viz. (1.) Cell-division, where two or more new cells fill the cavity of the parent cell, and adhere to its membranes, appearing to divide it into compartments; and (2.) Free cell-formationnot to be confounded with a process of the same name which is supposed to occur in the animal kingdom-in which the whole or part of the cellcontents become detached from the cell-wall and resolved into new loose C., which ultimately escape from the parent cell. The former mode universally occurs in the formation of the C. by which growth is effected; the latter occurs only in the production of C. connected with reproduction. For further information, we must refer the reader to Von Mohl's Principles of the Anatomy and Physiology of the Vegetable Cell, translated by Henfrey, London, 1852.

Occur.

entire organs, it at the same time performs a similar function in regard to the minute parts of which each organ is made up. Thus, for instance, in muscular tissue, it enters between the fibres of the muscle, uniting them into bundles; and similarly, it enters

into glands, &c. This is termed penetrating or parenchymal cellular tissue.

It is not only one of the most general and most extensively distributed of the tissues, but it is continuous through the whole organism, and may be traced without interruption from any one region of the body to any other. It is in consequence of this continuity that dropsical fluids, air, &c., effused into the C. T., may spread far from the spot where they were first introduced.

On examining a fragment of this tissue, when stretched out, we see with the naked eye that it presents the appearance of a multitude of fine, soft, colourless, elastic threads, like spun glass; intermixed with these are delicate films or laminæ, crossing one another in all directions, and leaving open spaces, or areola; hence the name of areolar tissue. A small quantity of colourless transparent fluid is always present in this tissue; when abnormally increased, it gives rise to the form of general dropsy known as anasarca. The microscopic characters of C. T. are briefly noticed in the article TISSUES,

ANIMAL.

CELLULAR TISSUE, in Botany, is any vegetable tissue formed of cohering cells alone, and in which there are no vessels. It is often called parenchyma (Gr. something spread out), although an attempt has been made to restrict that term to one kind of it, with cells of a particular form, and terms of Greek derivation have been multiplied for other kinds. The cells of C. T. vary much, both in form and size (see CELLS); but particular forms and sizes are characteristic of particular kinds or particular parts of plants. The products of the vital activity

of plants are formed in the interior of cells, or by secretion from the inner side of their walls. Vessels being formed from cells, it is not easy to fix the limits between C. T. and Vascular Tissue (q. v.). Some kinds of plants, however, are entirely composed of C. T. (see next article); all consist of it in the earliest stages of their growth; none are at any time destitute of it. Fluids are transmitted from cell to cell, through the mass of C. T., passing through the walls of the cells where there are no openings that can be detected by the microscope. The soft and succulent parts of plants, which it is the care of the gardener to cherish and increase, consist chiefly of cellular tissue.

CELLULA'RÉS, in Botany, a designation applied to those plants which consist entirely of Cellular Tissue (q. v.), without proper vessels of any kind. C., thus defined, are a sub-class of acotyledonous plants, containing the orders of Lichens, Fungi, and Algae. In the system of De Candolle, however, the name C. was given to the second grand division of plants, the first being called Vasculares, and the distinction between them being the presence or absence of vessels, the C. including all acotyledonous or cryptogamous plants. But ferns and mosses are not destitute of vessels; so that this system is not strictly accurate with regard to them: whilst, as all

vessels are now known to be formed by the elongation and union of cells, the distinction between vascular and cellular tissue is not generally regarded as affording a good basis for primary divisions in the classification of plants.

CELLULOSE is the term applied to the carbohydrate, C12H10010, which forms the mass of the cell-membranes of all plants. It is one of a class of compounds intimately connected in their chemical constitution, but presenting remarkable physical differences. Without entering into chemical details, we may mention the following points of difference between it and the chemically allied substances-sugar, dextrine, and starch. Sugar and dextrine are soluble in cold water, and occur in the cell sap in solution; starch is insoluble in cold water, but softens into a mucilage in boiling water, and is found in granules in the cell-contents; while C. is insoluble in cold or boiling water, and, as far as is at present known, is very slightly soluble in the strong mineral acids, its only perfect solvent being a solution of oxide of copper in ammonia.

The occurrence of C. in an organism was formerly regarded as a certain proof that the latter belonged to the vegetable kingdom. It has, however, been shewn to be a constituent of the lower animals.

Although C. forms a large proportion of the food of herbivorous animals, it is supposed to pass through the intestinal canal unchanged, and not to contribute directly to nutrition.

CELSUS, an Epicurean philosopher, but tinged with Platonism, lived in the 2d c. after Christ, and wrote, after 150 A. D., under the title Logos Alethes (the True World), the first considerable polemic against Christianity. The book itself has perished; but considerable fragments have been preserved as quotations given by Origen in his answer, Contra Celsum, in eight books. In the fragments-which are very interesting, as shewing the views of a heathen philosopher in regard to Christianity-C., with wit and acuteness, but without depth or earnestness of thought, prefers against the new religion charges of unphilosophicalness and blind credulity; and especially endeavours to convict Christians of self-contradiction in their spiritual doctrine contrasted with their anthropomorphic representations of Deity; in their religious arrogance contrasted with their confession of sinfulness; and in their views of the necessity of redemption. He also reproaches Christians with their party divisions and ever-varying opinions. With regard to his own positive doctrines, he speaks of evil as necessary and eternal, as an essential property of the material world (hyle); sin as something that can never be entirely removed, and least of all, through a vicarious sacrifice. He charges Christians with having wilfully altered their sacred writings.

CELSUS, AULUS CORNELIUS, a Latin physician and writer, who flourished probably in the reign of Augustus. He was called the Roman Hippocrates, because he generally followed the great Father of Medicine,' and introduced the Hippocratic system among the Romans. C. wrote not only on medicine, but also on rhetoric, history, philosophy, the art of war, and agriculture. His style is succinct and clear, but full of Græcisms. The only great work of his which survives, is the De Medicina, which is divided into eight books. The portions relating to surgery are exceedingly interesting and valuable, because C. has there given an account of the opinions and observations of the Alexandrian school of medicine. The first edition of the De Medicind appeared at Florence in 1478. C.'s works have been translated into several modern languages. A translation into English was made by Dr. Grieve, London, 1756.