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 consider ably from the primary globular form. The latter occurs to a certain degree in all C., i but in some kinds to a far greater extent than in others. 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 mem brane becomes thickened.
Having now traced the cell to the period of its full growth, we are prepared to con sider the processes which occur in the anterior 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. A]1 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 earthly salts, and dissolved or suspended fat-particles; and that besides these ingredients many C. contain either a great preponderance of one of these constitu ents, 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-cells, and with much fat, like the fat-cells; while there are other cells which specially contain hwmatine (the red coloring matter of the blood), pigment, biliary and urinary constituents, mucus, ;milk, sugar, etc.
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 without, is most manifest to those C. which at first have little or no contents save the nucleus. Although endosmosc 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 indiscriminately; but they have the power of taking up one constituent, and rejecting another, and thus exhibit a selective faculty.
The cell having thus become filled from without, we have next to inquire into the chan ges 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 C. Thus, mucous 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 cuttlefish. In these cases, the C. do not separate mucus, pepsin, etc., 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, etc.) 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 iu their interior, is proved by the fact that, if the kidneys of an animal are 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 prof. Goodsir, "On Secreting Structures," published in John and Harry D. S. Goodsir's Anatomkal and Pathological 1?esearches, 1845.