This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. Secretion is one of the most obscure and mysterious functions in the animal economy. To secrete means to sepa rate ; but most of the fluids formed by this process, did not previously exist in the blood, but only the elements out of which they are made. It is purely a vital and not a mechanical process, like straining through a seive, to which some have compared it: and the vessels by which it is accomplished may well be called the architects and chemists of the system; for out of the same material, the blood, they construct a variety of wonderful fabrics, and chemical compounds.
2. We see the same wonderful power possessed also by vegetables-for, out of the same materials, the olive prepares its oil; the cocoa nut its milk ; the cane its sugar ; the poppy its narcotic juice ; the henbane its poison ; the oak its green pulpy leaves, its light pith, and its dense woody fibre : all composed of the same, few simple elements, only arranged in different order and proportions.
3. In like manner, we find the vessels in animal bodies, capable of forming all the various textures and substances Which make up the frame; the cellular tissue ; the membranes ; the ligaments; the cartilages ; the bones ; the marrow ; the muscles, with their tendons ; the lubricating fluid of the joints ; the pulp of the brain ; the transparent jelly of the eye ; in short, all the textures of the various organs of which the body is composed; and still all are made out of the same blood; and consist of the same ultimate elements.
4. The most simple form of secretion, however, seems to be, the mere separation of some principle, which previously existed in the blood ; as serum is deposited in most of the cavities, by a kind of exhalation. Some other principles also exist in the blood, which are found in the secretions, as fibrin, a fatty substance; and some of the elements of the bile. Some physiologists believe, that many of the secretions do exist ready formed in the blood, but cannot be detected by analysis. In proof of this, two ounces of bile have been injected into the veins of a dog, but the blood of the animal, which was analyzed a few moments after, exhibited not a trace of bile.
5. That secretion is a vital process and not a chemical one, is evident from the fact, that it is so much under the influence of the nervous system. For example, sorrow and grief change the qualities of the bile: a fit of anger sometimes causes an excessive flow of it: it will also change the nature of milk to such a degree, as to produce colic, vomiting, and even convulsions in the infant that swallows it. Grief will also suspend the secretion of the gastric fluid, and thus destroy the appetite ; while fear causes a cold sweat to break out, all over the surface of the body. If the nerves going to any organ are divided, the function of secretion will be suspended. It is doubtless owing in a great degree to the changeful states of the nervous system, that the secretions vary so much in quality as well as quantity at different periods.
6. Though we are able to understand but little of the true nature of secretion, we know that there are three kinds of secretory organs, viz., exhalent vessels; follicles ; and glands. The exhalents are believed to be the termination of the arteries, or capillaries ; and they are of two kinds, internal and external; the former terminating on all the surfaces within the body; and the latter on the outside. Their use is to soften and lubricate these parts.
7. The fluid, which is thrown out by the serous membrane which surrounds the brain, the lungs, and the contents of the abdomen, as well as into the cavities of the joints, is very similar to the water of the blood, and its use is, to keep the parts in a moist state, and enable the organs to move easily on each other. Fat is also a secretion, which is thrown out in a fluid state, from the cellular tissue, into little cells; and it is mostly found immediately under the skin, between that and the flesh. Its use seems to be, to lubricate the solids and facilitate their movements ; to form a cushion around the body, and protect it from external injuries as well as the extremes of heat and cold.
8. During sickness, when we take little or no food into the stomach, life is supported by the absorption of the fat, which is taken up by little vessels and again poured into the blood, to nourish the body; also in animals which lie in their burrows, in a half torpid state during the winter, their nourishment is derived from the same source.
9. Marrow which fills all the cavities of the long bones, is very much like fat, and this also is a secretion from a thin, delicate membrane, that lines the cavities of the bones. These are the principal internal exhalations, or secretions.
10. The external secretions are two in number; the first and most important from the skin is called, when insensible, perspiration; and when it is visible, is called sweat; the second is from the lungs, and can be seen in the form of a vapour in a frosty morning. The fluid which escapes from the skin, is chiefly water, containing a little acid and some salts, with a small quantity of animal matter.
11. The skin is covered with an innumerable number of pores, so small indeed, as to be invisible to the naked eye, through which the insensible perspiration is constantly pouring, amounting in weight to more than one half of all the food and drink taken into the stomach, and much exceeding that lost by all the other excretions.
12. From a vast number of experiments performed by different persons, it appears that the largest amount of insensible perspiration amounts to five pounds in twenty four hours, and the smallest to thirty two ounces. Now, when we consider that the skin, serves as one of the chief outlets, by which the old and useless particles are got rid of, out of the blood, and that checked perspiration is one of the most powerful causes of disease and death, we shall see how important it is to keep the pores of the skin free, by frequent washing and bathing at all seasons of the year.
13. When the surface also is chilled by cold, and reaction does not follow, the blood vessels become contracted all over the skin, and the blood retreats to the inner parts of the body; the 29 ounces of waste and noxious matter, which ought to be discharged, are kept in the system, and sickness is the consequence. Accordingly, we find that it is a good remedy, in most cases of disease, to excite a perspiration and keep it up for some time.
14. The uses of transpiration by the skin, are, not only to carry out of the body the noxious properties of the blood, but also to render the skin soft and pliable, the sense of touch delicate, and also to cool the body when exposed to great heat.
15. The exhalation from the lungs closely resembles that from the skin. It was once supposed to be formed in the lungs, by the union of hydrogen from the blood, and oxygen from the air, thus forming water ; but it is the opinion of most physiologists at present, that it is either the watery part of the mucus secreted by the mucous coat of the lungs and air passages, or that it is given off directly from the blood.
16. The Follicles, are small sacks or bags, found in the skin, and the mucous membranes. The pores which we see on the skin are only the outlets of these follicles. Their use is to secrete an oily matter to mix with the perspiration, and help to keep the skin soft and moist; when these outlets are closed, sometimes there appear small black specks on the skin, sometimes called worms, but they are nothing more than hardened mucus. Every hair has a follicle at its root, and the wax which collects in the ears, is secreted by follicles.
17. The chief agents of secretion in the body, are the glands, which are bodies of various size, generally of a rounded form; and the fluids they secrete, are very different from each other, and also from the blood which furnishes the same materials to all. For instance, the liver secretes the yellow, ropy fluid, called bile ; the kidneys, secrete urine; the lachrymal gland, which is placed immediately over the eye in the orbit, secretes the tears ; and the spittle or saliva is poured out from the salivary glands.
18. Glands are formed of a large number of arteries, veins, nerves, and lymphatics, disposed in a peculiar manner, and connected together by a tissue of cellular membrane. When in a cavity, they are covered on their external surface by a coat, derived from the membrane that lines the cavity, and they are also provided with a canal, called excretory duct, which is lined with mucous membrane. There are seven kinds of glandular secretions.
19. The secreted fluids have been divided into serous or watery, albuminous mucous, oily, and the mixed ; such as saliva, bile, tears, etc. They have also been divided into recre. mentitious and the excrementitious, or those which are destined to be absorbed and returned into the mass of the blood, and which are deposited in cavities that have no external opening; and the second, those which are designed, after their formation, to be expelled from the system.
20. When any substance is taken up by the absorbents, and carried into the blood, which cannot be converted to any useful purpose in the system, it is immediately discharged by means of the secretions. Not many years ago, a man was carried into a London hospital, who was picked up drunk in the streets. He lived but a short time, and on examining his brain, nearly a half a gill of fluid strongly impregnated with gin, was found in the ventricles. This was secreted.
21. Unless the secretions all go on, we cannot enjoy good health. If that from the skin is stopped, fever, or some internal inflammation is the consequence. If the bile ceases to flow, we cannot properly digest our food ; and so if any of the others are interrupted, some serious disease will be the result. The use of ardent spirits deranges all the secretions, and this is one reason why they shorten life so much.
What is Secretion ? Is it a vital process ? Have vegetables the same property of secretion ? Illustrate this. What is the use of this function ? What is the most simple form of secretion ? How is it proved that secretion is a vital process ? How many kinds of secretory organs are there ? What are exhalents ?-their use ? What is fat? How is life supported in sickness ? What is marrow ? What are the external secretions ? How much does the insensible perspiration amount to in 24 hours ? What important inference do we derive from this in relation to health ? What is use of transpiration by the skin ? What is the exhalation from the lungs owing to ? What are follicles ? What are the chief agents of secretion ? What are glands composed of ?-their structure ? How are the secreted fluids divided ? What becomes of useless substances carried by absorption into the blood ? Can health be maintained if the secretions be checked ?