water. It exists in a solid, demi-solid, liquid, and gaseous state. Thus in muscle it is in such chemical combination with the tissue that it is solid; in other substances not being in sufficient quantity to effect solution, it is rendered demi-solid; in fluids it is fluid; in the halitus of the lungs it is gaseous. taken into the body in food and drink, and is also supposed to be formed in the system. Without its presence in the body, chemical and vital acts would be impossible.

It is

The Salts are found in every tissue and fluid, dissolved in their water, and thus serve as solvents for certain other immediate principles; and some of them by combining in the state of solution with certain organic immediate principles compose tissues.

Common Salt (Chloride of Sodium, NaCl) is found in every portion of the body, both solid and fluid, except enamel; and exists in greater quantity than any other immediate principle of inorganic origin. It always freely exists in fluids where cells are developing. Its existence in the blood is necessary to endosmosis of dissolved alimentary substances from the digestive tract. It is necessary for the performance of assimilation and dis-assimilation.

Chloride of Potassium (KCl.) is always dissolved in water, like common salt, and is found in the milk, muscles, liver, blood, etc.

Carbonate of Lime (CaO, CO2) is found in all cases combined with the phosphates of lime.

Carbonate of Soda (NaO, Co.) is found in the blood, etc., and is always dissolved in water; it is combined with and dissolves the albumen of the blood.

Sub-phosphate or Basic Phosphate of Lime (8CaO, 3PO,) is found in the bones, teeth, nails and hair, in a solid state, and in a liquid form in the blood and all the other fluids.

OF THE SECOND CLASS.-The immediate principles of organic origin formed within the body by dis-assimilation or waste contain but nine of the simple chemical elements, viz., sodium, potassium, calcium, magnesium, sulphur, carbon, oxygen, hydrogen, and nitrogen, as may be observed by reference to the table. These compounds, although so numerous, constitute by far the smallest part of the body; about two-thirds of them are found in the blood-they are also found in the urine and bile, by which they are for the most part removed from the system.

It is not necessary for our purpose to enter into a detailed description of these immediate principles, or do more than state that it is not so certain that all the fatty principles (which are non-nitrogenized) included in this category are the results of "waste" or dis-assimilation; in a normal condition they all exist in the body in a fluid state, either inclosed in cells forming adipose tissue, in chemical combination with other elements, or as oil-globules, without any envelop, existing in nearly every tissue. Most of these fatty principles are taken into the body already formed in the food, and may also be, to some extent, formed in the body; thus cholesterin is probably a result of "waste" of the liver itself. The uses of the fatty principles are manifold-they serve to nourish the tissues which contain fat, and to be consumed by uniting with oxygen, and thus maintain vital heat. The presence of fat seems to be indispensable to the original development of the tissues, for the nuclei of all primary cells contain fat-granules.

OF THE THIRD CLASS of immediate principles, which. constitute the greater part of the body, all are composed of the four elements, carbon, hydrogen, oxygen and nitrogen, and are called nitrogenized compounds; they are prone to rapid decomposition. The two first divisions form the albuminous compounds, which are found, as is shown in the table, either in a liquid or in a solid or demisolid condition; water, however, being an essential part of their chemical constitution, since, if they are completely dessicated, their vital properties are com pletely lost.

Albumen, fibrin and casein have been called protein compounds; since they may be supposed to be formed by the combination of small proportions of either sulphur or phosphorus, or both, with a compound radical, called protein (poTEVW, I am first) by Mulder. Protein, however, does not exist in nature, and is only obtained by the destructive decomposition of the substances which it is supposed to aid in forming. The formula of Mulder for protein is, C6H25N,O10+2HO, which represents, according to him, perfectly non-sulphurous protein.

36

Albumen (Pro+PS2) is found in the serum of the blood, chyle, and lymph; like all other of these organic immediate principles, its chemical composition is not definite. Sulphur and phosphorus are always found aiding to form it. It is the essential pabulum of the tissues, for from it they are all developed and nourished. It is itself probably formed from Albuminose, which is formed in the alimentary canal by the digestion of albuminous compounds.

Fibrin (Pro+PS) is found in the blood, chyle and lymph. It is probably not a simple substance as has been supposed, but is formed from albumen, fat, and some saline materials. It has the property of spontaneous coagulability or fibrillation, which is readily witnessed a few minutes after venesection; and by this property exercised within the body affords a matrix for new growth, but is not the only plastic element which is called into requisition. Fibrin may be formed directly from albuminose, or from the albumen in the blood.

Casein (Pro+S) is found in the milk, both as the investing membrane of the milk-globules and in solution; its properties generally resemble albumen. Pancreatin is found in the pancreatic juice.

Ptyalin is found in the saliva.

Mucosin is found in mucus, and varies according to its source.

Globulin constitutes the principal part of the red corpuscles of the blood, and is no doubt developed from albumen by their cell-membranes.

Crystallin exists in the crystalline lens, and is similar to globulin.

Musculin exists in the striated and non-striated muscular tissue, and is its main constituent; it is probably formed from albumen; that which has been called fibrin in the muscles is musculin.

Ostein exists in bone and white-fibrous tissue; in bone it is chemically combined with phosphate of lime.

Cartilagein is found in cartilage and fibro-cartilage.

Elasticin is found in yellow-fibrous tissue.

Keratin is peculiar to nails, hair, and epidermis.

Hæmatin, Biliverdin, Melanin, and Urrosacin are the coloring matters of the blood, bile, pigment-cells and urine.

HISTOLOGY.

This is the description of the simple structural elements, and the tissues which are formed from them.

THE SIMPLE STRUCTURAL ELEMENTS.

The simple structural elements, of which the proper tissues are formed, are 1. Homogeneous substance,

2. Simple membrane,

3. Simple fibre,

4. Cells.

1. Homogeneous substance, homogeneous, matrix or hyaline substance, is a structureless material, varying in density, found in several tissues, often filling up the spaces between fibres and cells, and sometimes constituting a great part of their bulk; sometimes it is granular. It is developed from albumen, which

ap

has thereby assumed a higher stage, and with it a low grade of vital properties. It is used to connect other structural elements, and is found in bone and cartilage.

2. Simple membrane is a thin structureless layer of coagulated albumen, occasionally being granular; it is not traversed by either vessels or nerves, but receives its nourishment from neighboring capillaries; it forms limitary membrane and the walls of every variety of cell.

3. Simple fibre consists of threads of coagulated fibrin, and it is supposed a rete or net thus formed furnishes a matrix for the development of tissues, and the nidus for repair of solutions of continuity, being absorbed after the higher structural elements are developed.

4. Cells. The cell is the most important of the simple structural elements; for from cells almost all the tissues proper are formed - the three just described structural elements being inferior in grade, and of course not derived from them. Vegetable tissues are likewise formed from cells.

1

FIG. 3.

A cell is a vesicle, which may be from too of an inch in diameter, formed from a plasma, blastema, or cytoblastema, which is the liquor sanguinis : a granule first appears in the clear plasma, and is called the nucleus or cytoblast (xvros, cell, 32ast, to produce), this increases in size and becomes vesicular, and there is then perceived in its interior other granules, one of which being larger than the others is called the nucleolus; from the nucleus is developed the cellwall, and the space between the nucleus and the cell-wall is filled with a fluid containing granules, which constitute the cell-contents.

CELLS SHOWING THE CELL-MEMBRANE, THE CONTAINED GRANULES, THE NUCLEUS, AND THE NUCLEOlus.

The cell-wall is simple membrane, formed from albumen, and possessing great endosmotic properties. The contained fluid varies in composition according to the character of the cell, that is, it differs in epithelial, epidermic, or fat cells, etc. ; the granules contained, which are very numerous or may be altogether absent, may be fatglobules devoid of investment, or pigment-granules, etc. The nucleus is generally attached to some part of the wall of the cell, and contains a fluid, and, as stated, the nucleolus, supposed to be formed of fat with an albuminous envelop, and which may itself become vesicular; sometimes several nucleoli are found in one nucleus.

Cells are developed in several ways:

1 and 2. The typical spherical form. The rest as changed by pressure.

1st. Directly from the plasma (by virtue of the vital force communicated by

its contact with living tissues), in the manner already described, which is called free cell-development.

2dly. In the interior of other cells, which is the endogenous method. By this method the nucleus of the primary cell, after enlarging, and becoming possessed of two nucleoli, becomes constricted in the middle, and finally divides into two distinct portions, each of which becomes the nucleus of a new cell.

ENDOGENOUS CELL-DEVELOPMENT. A, B, C, D. Early stages of the process, showing duplication of cells (ovum of Ascaris dentata). E, F, G, H. More advanced stages (ovum of Cucullanus elegans).

The original cell may now burst, and set free the secondary cells, or enlarge until several generations have been formed in the same manner within it; the nucleoli becoming the nuclei of the new cells, and developing new nucleoli within themselves.

3dly. Cells may multiply by division. Here the original cell elongates, and

MULTIPLICATION OF CARTILAGE-CELLS BY DIVISION: A, original cell; B, the same beginning to divide; c, the same showing complete division of the nucleus; D, the same with the halves of the nucleus separated, and the cavity of the cell subdivided; E, continuation of the same process, with cleavage in contrary direction, to form a cluster of four cells; F, G, H, production of a longitudinal series of cells, by continuation of cleavage in the same direction.

the nucleus divides into two. The cell then, after being constricted, divides into two distinct nucleated cells, and this process may be continued.

Different cells have different functions, thus some are Formative, producing the solids of the body, by absorbing from the surrounding blastema their pabulum, which they alter by a vital process.

Others are Secreting, and may receive their pabulum without altering it, or but slightly, where they are intended simply to separate formed materials from the blood, or they may produce substances in their interior from the pabulum furnished, as in the bile-cells.

FIG. 6.

1

2

3

A peculiar structural element, called the Cytoid corpuscle (xvτoś, εidos, cellresembling), is found in some of the fluids of the body, viz., in the lymph, chyle, blood, etc., and has been called the lymph-, chyle-, and colorless blood-corpuscle, etc.; from this element the red blood-corpuscle is formed, probably by being converted into a non-nucleated cell by alteration and solidification of the cell-contents, and change of color, and the assumption of a bi-concave discoid form; the colorless blood-corpuscles originating by free cell-formation in the lymph and chyle, and perhaps in the liquor sanguinis itself. These cytoid corpuscles also exist in mucus, pus, and exudations. In exudations they form the basis of new tissues, but in mucus and pus never arrive at a higher organization.

FIG. 7.

CYTOID CORPUSCLES OF

BLOOD. 1. Natural ap

pearance.

2

and 3.

Changed by dilute acetio

acid.

DEVELOPMENT OF THE RED FROM THE COLORLESS CORPUSCLES OF THE BLOOD. A. Cytoid corpuscle. B. Same, being converted into a red corpuscle. c. Cytoid corpuscle with its membrane raised by the action of water. D. Same, having lost most of its granules. E. Same, acquiring color; a single granule remaining like a nucleus. F. Perfect red corpuscle.

THE TISSUES.

The tissues are formed from the simple structural or histological elements, and of these the cells are of chief importance. In some tissues the cellular structure is retained as a permanent characteristic, as in the adipose; whilst in others the cells are converted into other forms, as in the striped muscular tissue. The following table, from Todd and Bowman, will present a general view of the various tissues which enter into the construction of the organs of which the body is composed.