Seminal fluid, containing spermatozoids. Fluid of the Graafian follicles.

The Essential Apparatus for secretion is a delicate, homogeneous, structureless membrane, on one side of which, in close contact, is a capillary plexus of blood-vessels, and on the other side a layer of cells whose physiological function varies in different situations.

Secreting organs may be divided into membranes and glands.

Serous membranes usually exist as closed sacs, the inner surface of which is covered by pale, nucleated epithelium, containing a small amount of

secretion.

The serous membranes are the pleura, peritoneum, pericardium, synovial

sacs, etc.

The serous fluids are of a pale amber color, somewhat viscid, alkaline, coagulable by heat, and resemble the serum of the blood; their amount is but small; the pleural varies from 4 to 7 drams; the peritoneal from I to 4 ounces; the pericardial from 1 to 3 drams.

The synovial fluid is colorless, alkaline, and extremely viscid, from the presence of synovin.

The function of serous fluids is to moisten the opposing surfaces, so as to prevent friction during the play of the viscera.

The mucous membranes are soft and velvety in character, and line the cavities and passages leading to the exterior of the body, e.g., the gastrointestinal, pulmonary, and genito-urinary. They consist of a primary basement membrane covered with epithelial cells, which in some situations are tessellated, in others, columnar.

Mucus is a pale, semi-transparent, alkaline fluid, containing epithelial cells and leucocytes. It is composed, chemically, of water, an albuminous

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and hydrations taking place during the general process of nutrition, and the combustion of the carbonaceous compounds by the oxygen of the inspired air; the amount of its production is in proportion to the activity of the internal changes.

Every contraction of a muscle, every act of secretion, each exhibition of nerve force, is accompanied by a change in the chemical composition of the tissues and an evolution of heat. The reduction of the disintegrated tissues to their simplest form by oxidation, the combination of the oxygen of the inspired air with the carbon and hydrogen of the blood and tissues, results in the formation of carbonic acid and water and the generation of a large amount of heat.

Certain elements of the food, particularly the non-nitrogenized substances, undergo oxidation without taking part in the formation of the tissues, being transformed into carbonic acid and water, and thus increase the sum of heat in the body.

Heat-producing Tissues.-All the tissues of the body add to the general amount of heat, according to the degree of their activity. But special structures, on account of their mass and the large amount of blood they receive, are particularly to be regarded as heat producers; e. g. :

1. During mental activity the brain receives nearly one-fifth of the entire volume of blood, and the venous blood returning from it is charged with waste matters, and its temperature is increased.

2. The muscular tissue, on account of the many chemical changes occurring during active contractions, must be regarded as the chief heatproducing tissue.

3. The secreting glands, during their functional activity, add largely to the amount of heat.

The entire quantity of heat generated within the body has been demonstrated experimentally to be about 2300 calories, a calorie or heat unit being that amount of heat required to raise the temperature of one kilo. of water (2.2 lbs.) one degree Centigrade. This quantity of heat, if not utilized and retained within the body, would elevate its temperature in 24 hours about 60° F. That this volume of heat depends very largely upon the oxidation of the food stuffs can be shown experimentally.

The normal temperature of the body is maintained by a constant expenditure of the heat in several directions:

1. In warming the food, drink, and air that are consumed in 24 hours. For this purpose about 157 heat units are required.