2. In evaporating water from the skin and lungs; 619 heat units being utilized for this purpose.

3. In radiation and conduction. By these processes the body loses at least 50 per cent. of its heat, or 1156 heat units.

4. In the production of work; the work of the circulatory, respiratory, muscular, and nervous apparatus being performed by the transformation of 369 heat units into units of work.

The nervous system influences the production of heat in a part by increasing the amount of blood going through it by its action upon the vasomotor nerves. Whether there exists a special heat center has not been satisfactorily determined, though this is probable.

The Process of Secretion consists in the separation of materials from the blood which are either to be again utilized to fulfil some special purpose in the economy or are to be removed from the body as excrementitious matter; in the former case they constitute the secretions, in the latter, the excretions.

The materials which enter into the composition of the secretions are derived from the nutritive principles of the blood, and require special organs, e. g., gastric glands, mammary glands, etc. for their proper elaboration.

The materials which compose the excretions preëxist in the blood, and are the results of the activities of the nutritive process; if retained within the body they exert a deleterious influence upon the composition of the blood.

Destruction of a secreting gland abolishes the secretion peculiar to it, and it cannot be formed by any other gland; but among the excreting organs their exists a complementary relation, so that if the function of one organ be interfered with, another performs it to a certain extent.

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

principle, mucosin, and mineral salts; the principal varieties are nasal, bronchial, vaginal, and urinary.

Secreting Glands are formed of the same elements as the secreting membranes ; but instead of presenting flat surfaces, are involuted, forming tubules, which may be simple follicles, e g., mucous, uterine, or intestinal; or compound follicles, e. g., gastric glands, mammary glands; or racemose glands, e. g., salivary glands and pancreas. They are composed of a basement membrane, enveloped by a plexus of blood-vessels, and are lined by epithelial and true secreting cells, which in different glands possess the capability of elaborating elements characteristic of their secretions.

In the Production of the Secretion two essentially different processes are concerned :-

1. Chemical.-The formation and elaboration of the characteristic organic ingredients of the secreted fluids, e.g., pepsin, pancreatin, takes place during the intervals of glandular activity, as a part of the general function of nutrition. They are formed by the cells lining the glands, and can often be seen in their interior with the aid of the microscope, e.g., bile in the liver cells, fat in the cells of the mammary gland.

2. Physical. Consisting of a transudation of water and mineral salts from the blood into the interior of the gland.

During the intervals of glandular activity, only that amount of blood passes through the gland sufficient for proper nutrition; when the gland begins to secrete, under the influence of an appropriate stimulus, the bloodvessels dilate and the quantity of blood becomes greatly increased beyond that flowing through the gland during its repose.

Under these Conditions a transudation of water and salt takes place, washing out the characteristic ingredients, which are discharged by the gland ducts. The discharge of the secretions is intermittent; they are retained in the glands until they receive the appropriate stimulus, when they pass into the larger ducts by the vis-a-tergo, and are then discharged by the contraction of the muscular walls of the ducts.

The activity of glandular secretion is hastened by an increase in the blood pressure and retarded by a diminution.

The nervous centers in the medulla oblongata influence secretion, (1) by increasing or diminishing the amount of blood entering a gland; (2) by exerting a direct influence upon the secreting cells themselves, the centers being excited by reflex irritation, mental emotion, etc.