than by their influence in trophic (nutritive) or vaso-motor changes, although by many considered probable, is not yet

proven.

Inhibitory heat-centre.--Whether a centre exists which regulates the production of heat in warm-blooded animals, is still undecided. Experiments have shown that exposure to cold at once increases the oxygen taken in, and the carbonic acid given out, indicating an increase in the activity of the metabolism of the tissues, but that in animals poisoned by urari, exposure to cold diminishes both the metabolism and the temperature, and warmblooded animals then re-act to variations of the external temperature just in the same way as cold-blooded. These experiments seem to suggest that there is a centre, to which, under normal circumstances, the impression of cold is conveyed, and from which by efferent nerves impulses pass to the muscles, whereby an increased metabolism is induced, and so an increased amount of heat is generated. The centre is probably situated above the medulla. Thus in urarised animals, as the nerves to the muscles, the metabolism of which is so important in the production of heat, are paralyzed, efferent impulses from the centre cannot induce the necessary metabolism for the production of heat, even though afferent impulses from the skin, stimulated by the alteration of temperature, have conveyed to it the necessity of altering the amount of heat to be produced. The same effect is produced when the medulla is cut.

Influence of Extreme Heat and Cold.—In connection with the regulation of animal temperature, and its maintenance in health at the normal height, may be noted the result of circumstances too powerful, either in raising or lowering the heat of the body, to be controlled by the proper regulating apparatus. Walther found that rabbits and dogs kept exposed to a hot sun, reached a temperature of 114.8° F., and then died. Cases of sunstroke furnish us with several examples in the case of man; for it would seem that here death ensues chiefly or solely from elevation of the temperature. In many febrile diseases the immediate cause of death appears to be the elevation of the temperature to a point inconsistent with the continuance of life.

The effect of mere loss of bodily temperature in man is less well known than the effect of heat. From experiments by Walther, it appears that rabbits can be cooled down to 48° F. (8′9° C.), before

they die, if artificial respiration be kept up. Cooled down to 64° F. (17.8° C.), they cannot recover unless external warmth be applied together with the employment of artificial respiration. Rabbits not cooled below 77° F. (25° C.) recover by external warmth alone.

CHAPTER X.

SECRETION.

Secretion is the process by which materials are separated from the blood by the cells of secreting glands and membranes, and are either elaborated for the purpose of serving some ulterior office in the economy, or are discharged from the body as useless or injurious. In the former case, the separated materials are termed secretions; in the latter, they are termed excretions.

Most of the secretions consist of substances which, probably, do not pre-exist in the same form in the blood, but require special cells and a process of elaboration for their formation, e.g., the liver cells for the formation of bile, the mammary gland-cells for the formation of milk. The excretions, on the other hand, commonly or chiefly consist of substances which exist ready-formed in the blood, and are merely abstracted therefrom. If from any cause, such as extensive disease or extirpation of an excretory organ, the separation of an excretion is prevented, and an accumulation of it in the blood ensues, it frequently escapes through other organs, and may be detected in various fluids of the body. But this is never the case with secretions; at least with those that are most elaborated; for after the removal of the special organ by which each of them is elaborated, the secretion is no longer formed. Cases sometimes occur in which the secretion continues to be formed by the natural organ, but not being able to escape towards the exterior, on account of some obstruction, is re-absorbed into the blood, and afterwards discharged from it by exudation in other ways; but these are not instances of true vicarious secretion, and must not be thus regarded.

These circumstances, and their final destination, are, however,

the only particulars in which secretions and excretions can be distinguished; for, in general, the structure of the parts engaged in eliminating excretions is as complex as that of the parts concerned in the formation of secretions. And since the differences of the two processes of separation, corresponding with those in the several purposes and destinations of the fluids, are not yet ascertained, it will be sufficient to speak in general terms of the process of separation or secretion.

Every secreting apparatus possesses, as essential parts of its structure, a simple and almost textureless membrane, named the primary or basement-membrane; certain cells; and blood-vessels. These three structural elements are arranged together in various ways; but all the varieties may be classed under one or other of two principal divisions, namely, membranes and glands.

ORGANS AND TISSUES OF SECRETION.

The principal secreting membranes are (1) the Serous and Synovial membranes; (2) the Mucous membranes; (3) the Mammary gland; (4) the Lachrymal gland; and (5) the Skin.

(1) Serous Membranes.

The serous membranes are especially distinguished by the characters of the endothelium covering their free surface: it always consists of a single layer of polygonal cells. The ground substance of most serous membranes consists of connective-tissue corpuscles of various forms lying in the branching spaces which constitute the "lymph canalicular system" (p. 343), and interwoven with bundles of white fibrous tissue, and numerous delicate elastic fibrillæ, together with blood-vessels, nerves, and lymphatics. In relation to the process of secretion, the layer of connective tissue serves as a ground-work for the ramification of blood-vessels, lymphatics, and nerves. But in its usual form it is absent in some instances, as in the arachnoid covering the dura mater, and in the interior of the ventricles of the brain. The primary membrane and epithelium are always present, and are concerned in the formation of the fluid by which the free surface of the membrane is moistened.

Serous membranes are of two principal kinds : 1st. Those which line visceral cavities,-the arachnoid, pericardium, pleura, perito

neum, and tunica vaginales. 2nd. The synovial membranes lining the joints, and the sheaths of tendons and ligaments, with which, also, are usually included the synovial burs, or bursa mucosa, whether these be sub

cutaneous, or situated beneath tendons that glide over bones.

The serous membranes form closed sacs, and exist wherever the free surfaces of viscera come into contact with each other or lie in cavities unattached to surrounding parts. The viscera invested by a serous membrane are, as it were, pressed into the shut sac which it forms, carrying before them a portion of the membrane, which serves as their investment. To the law that serous membranes form shut sacs, there is, in the human

subject, one exception, viz.: the opening of the Fallopian tubes into the abdominal cavity,-an arrangement which exists in man and all Vertebrata, with the exception of a few fishes.

Functions.-The principal purpose of the serous and synovial membranes is to furnish a smooth, moist surface, to facilitate the movements of the invested organ, and to prevent the injurious effects of friction. This purpose is especially manifested in joints, in which free and extensive movements take place; and in the stomach and intestines, which, from the varying quantity and movements of their contents, are in almost constant motion upon one another and the walls of the abdomen.

Fluid.-The fluid secreted from the free surface of the serous membranes is, in health, rarely more than sufficient to ensure the maintenance of their moisture. The opposed surfaces of each serous sac are at every point in contact with each other. After

death, a larger quantity of fluid is usually found in each serous sac; but this, if not the product of manifest disease, is probably such as has transuded after death, or in the last hours of life. An excess of such fluid in any of the serous sacs constitutes dropsy of the sac.

The fluid naturally secreted by the serous membranes appears to be identical, in general and chemical characters, with very dilute liquor sanguinis. It is of a pale yellow or straw-colour, slightly viscid, alkaline, and on account of the presence of albumen, coagulable by heat. This similarity of the serous fluid to the liquid part of blood, and to the fluid with which most animal tissues are moistened, renders it probable that it is, in great measure, separated by simple transudation, through the walls of the blood-vessels. The probability is increased by the fact that, in jaundice, the fluid in the serous sacs is, equally with the serum of the blood, coloured with the bile. But there is reason for supposing that the fluid of the cerebral ventricles and of the arachnoid sac are exceptions to this rule; for they differ from the fluids of the other serous sacs not only in being pellucid, colourless, and of much less specific gravity, but in that they seldom receive the tinge of bile when present in the blood, and are not coloured by madder, or other similar substances introduced abundantly into the blood.

It is also probable that the formation of synovial fluid is a process of more genuine and elaborate secretion, by means of the epithelial cells on the surface of the membrane, and especially of those which are accumulated on the edges and processes of the synovial fringes; for, in its peculiar density, viscidity, and abundance of albumen, synovia differs alike from the serum of blood and from the fluid of any of the serous cavities.

(2) Mucous Membranes.

The mucous membranes line all those passages by which internal parts communicate with the exterior, and by which either matters are eliminated from the body or foreign substances taken into it. They are soft and velvety, and extremely vascular. The external surfaces of mucous membranes are attached to various other tissues; in the tongue, for example, to muscle; on cartilaginous parts, to perichondrium; in the cells of the ethmoid bone, in the frontal and sphenoidal sinuses, as well as in the tympanum, to