substance and a nucleus.

3.

Cells possessing the cell substance only. They vary in size, from the go to the of an inch in diameter; when young and free to move in a fluid medium they assume the spherical form; but when subjected to pressure, may become flattened, cylindrical, fusiform or stellate.

Structure of Cells. The cell wall is not an essential structure, as many cells are entirely devoid of it. It is a thin, structureless, transparent membrane, permeable to fluids.

The Cell Substance in young cells is a soft, viscid, albuminous matter, unstable, insoluble in water, and known as protoplasm, bioplasm, sarcode, etc.; in older cells the original cell substance undergoes various transformations, and is partly replaced by fat globules, pigment and crystals.

The Nucleus is a small vesicular body in the interior of the cell substance, and frequently contains smaller bodies, the nucleoli.

MANIFESTATIONS OF CELL LIFE.

Growth. Cells when newly formed are exceedingly small, but as they approach maturity they increase in size, by the capability which the cells possess of selecting and appropriating new material as food, vitalizing and organizing it. The extent of cell growth varies in different tissues; in some the cells remain exceedingly small, in others they attain considerable size. In many instances the cell substance undergoes transformation into new compounds destined for some ulterior purpose.

Reproduction. Like all organic structures cells have a limited period of life; their continual decay and death necessitates a capability of reproduction. Cells reproduce themselves in the higher animals mainly by fission. This is seen in the white blood corpuscles of the young embryos of animals; the corpuscle here consists of a cell substance and nucleus. When division of the cell is about to take place, the nucleus elongates, the cell substance assumes the oval form, a constriction occurs, which gradually deepens, until the original cell is completely divided and two new cells are formed, each of which soon grows to the size of the parent cell.

In cells provided with a cell membrane the process is somewhat different. In the ova of the inferior animals, after fertilization has taken place, a furrow appears on the opposite sides of the cell substance, which deepens until the cell is divided into two equal halves, each containing a nucleus; this process is again repeated until there are four cells, then eight, and so on until the entire cell substance is divided into a mulberry mass of cells,

4

completely occupying the interior of the cell membrane. The whole process of segmentation takes place with great rapidity, occupying not more than a few minutes, in all probability.

Motion. Spontaneous movement has been observed in many of the cells of the body. It may be studied, for example, in the movements of the spermatozoids, the waving of the cilia covering the cells of the bronchial mucous membrane, the white corpuscles of the blood, etc.

By a combination and transformation of these original structural elements, and material derived from them, all the tissues are formed which enter into the structure of the human body.

CLASSIFICATION OF TISSUES.

I. Homogeneous Substance, a more or less solid, albuminous structure, filling the spaces between the cells and fibres of various tissues, e. g., cartilage, bone, dentine, etc.

II. Limiting Membrane, a thin, homogeneous membrane, structureless, composed of coagulated albumen, and often not more than the of an inch in thickness, found lining the blood vessels and lymphatics, forming the basement membrane of the skin and mucous membranes, the posterior layer of the cornea, the capsule of the crystalline lens, etc.

III. Simple fibrous or filamentous tissue-the elements of which are real or apparent filaments.

(a) Connective or areolar; white fibrous tissue; constituting tendons, ligaments, aponeuroses, periosteum, dura mater, synovial membranes, vascular tunics, etc.

(b) Yellow elastic tissue, found in the middle coats of arteries, veins, lymphatics, ligamentum nuchæ, vocal cords, ligamenta subflava, etc.

IV. Compound membranes (membrano-cellular or fibro-cellular tissues), cells aggregated into laminæ.

(a) Epidermic tissue; (b) epithelial tissue; (c) glandular tissue; (d)

cornea.

V. Cells containing coloring matter, or pigment cells, e. g., skin, choroid membrane, etc.

VI. Cells coalesced or consolidated by internal deposits, e. g., hair, nails, bone, teeth, etc.

VII. Cells imbedded in an intercellular substance, e. g., cartilage, crystalline lens, etc.

VIII. Cells aggregated in clusters, forming tissues more or less solid, e. g., adipose tissue, lymphatic glands.

IX. Cells imbedded in a matrix of capillaries, e. g., gray or vesicular

nervous matter.

X. Cells whose coalesced cavities form tubes containing liquids or secondary solid deposits, e. g., vascular tissue, dentine.

XI. Cells free, isolated, or floating-fluid tissue-e. g., red and white blood corpuscles, lymph and chyle corpuscles.

FOOD.

A Food may be defined to be any substance capable of playing a part in the nutrition of the body.

Food is required for the repair of the waste of the tissues consequent on their functional activity, for the generation of heat and the evolution of force.

Hunger and Thirst are sensations which indicate the necessity for taking food; they arise in the tissues at large, and are referred to the stomach and fauces, respectively, through the sympathetic nervous system.

Inanition or Starvation results from an insufficiency or absence of food, the physiological effects of which are hunger, intense thirst, intestinal uneasiness, weakness and emaciation; the quantity of carbonic acid exhaled diminishes and the urine is lessened in amount; the volume of the blood diminishes; a fetid odor is exhaled from the body; vertigo, stupor followed by delirium, and at times convulsions, result from a disturbance of the nerve centres; a marked fall of the bodily temperature occurs, from a diminished activity of the nutritive process. Death usually takes place, from exhaustion.

During starvation the loss of different tissues, before death occurs, averages, or 40 per cent. of their weight.

Those tissues which lose more than 40 per cent. are fat, 93.3; blood, 75; spleen, 71.4; pancreas, 64.1; liver, 52; heart, 44.8; intestines, 42.4; muscles, 42.3. Those which lose less than 40 per cent. are the muscular coat of the stomach, 39.7; pharynx and oesophagus, 34.2; skin, 33.3; kidneys, 31.9; respiratory apparatus, 22.2; bones, 16.7; eyes, 10; nervous system, 1.9.

The Fat entirely disappears, with the exception of a small quantity which remains in the posterior portion of the orbits and around the kidneys. The Blood diminishes in volume and loses its nutritive properties. The Muscles undergo a marked diminution in volume and become soft and flabby. The Nervous system is last to suffer, not more than two per cent. disappearing before death occurs.

The appearances presented by the body after death from starvation are those of anæmia and great emaciation; almost total absence of fat; bloodlessness; a diminution in the volume of the organs; an empty condition of the stomach and bowels, the coats of which are thin and transparent. There is a marked disposition of the body to undergo decomposition, giving rise to a very fetid odor.

The duration of life after a complete deprivation of food varies from eight to thirteen days, though life can be maintained much longer if a quantity of water be obtained. The water is more essential under these circumstances than the solid matters, which can be supplied by the organism itself.

The different alimentary principles which are appropriated by the system are combined in different proportions in the various articles of food, and are separated from the innutritious substances during the process of digestion. They belong to the organic and inorganic worlds, and may be classified, according to their chemical composition, as follows::

CLASSIFICATION OF ALIMENTARY PRINCIPLES.

1. Albuminous group-nitrogenized, C. O. H. N. S. P.

2. Saccharine group-non-nitrogenized, C. O. H.

Inosite, liver sugar, glycogen...... Muscles, liver, etc.

Starch

Cereals, tuberous roots and leguminous plants.

3. Oleaginous group-non-nitrogenized, C. O. H.

Animal fats and oils.......
Stearin, olein........

Palmatin, fatty acids...........

Found in the adipose tissue of ani

mals, seeds, grains, nuts, fruits,

and other vegetable tissues.

4. Inorganic group. Water, sodium and potassium chlorides, sodium, calcium, magnesium and potassium phosphates, calcium carbonate and iron.

5. Vegetable acid group. Malic, citric, tartaric and other acids, found principally in fruits.

6. Accessory foods. Tea, coffee, alcohol, cocoa, etc.

The Albuminous principles enter largely into the composition of the body, and constitute the organic bases of the different tissues; they are mainly required for the growth and repair of the tissues. There is good reason to believe that the albuminous principles are decomposed in the body into fat and urea, and the former when oxidized gives rise to the evolution of heat and force, while the latter is eliminated by the kidneys. Muscular work, however, does not result from a destruction of the albuminous compounds. The oxidation of the carbonaceous compounds, sugars and oils, furnishing the force which is transformed by the muscular system into motor power. When employed exclusively as food for any length of time, the albuminous substances are incapable of supporting life.

The Saccharine principles are important to the process of nutrition, but the changes which they undergo are not fully understood; they form but a small proportion of the animal tissues, and by oxidation generate heat and force. Starch undergoes conversion into dextrin and grape sugar.

The Oleaginous principles form a large part of the tissues of the body. They are introduced into the system as food, and are formed also from a transformation of albuminous matter during the nutritive process; they enter into the composition of nervous and muscular tissue, and are stored up as adipose tissue in the visceral cavities and subcutaneous connective tissue, thus giving roundness to the form and preventing, to some extent, the radiation of heat. While they aid in the reconstruction of tissue, they mainly undergo oxidation, giving rise to the production of heat and the evolution of muscular and nervous force.

The Inorganic principles constitute an essential part of all animal tissues, and are introduced with the food.

Water is present in all fluids and solids of the body, holding their ingredients in solution, promoting the absorption of new material into the blood and tissues, and the removal of waste ingredients.

Sodium chloride is an essential constituent of all tissues, regulating the passage of fluids through animal membranes (endosmosis and exosmosis).