Excretory Work of the Food Canal.—The glands connected with the food canal, other than the liver, while secreting liquids that aid in digestion, also separate waste materials from the blood. These are passed into the canal, whence they leave the body with the undigested portions of the food and the waste from the liver. Though the nature and quantity of the materials removed by these glands have not been fully determined, recent investigations have tended to enhance the importance attached to this mode of excretion.
The Perspiratory Glands.—The perspiratory, or sweat, glands are located in the skin. They belong to the type of simple tubular glands and are very numerous over the[pg 207] entire surface of the body. A typical sweat gland consists of a tube which, starting at the surface of the cuticle, penetrates to the under portion of the true skin and there forms a ball-shaped coil. The coiled extremity, which forms the secreting portion, is lined with secreting cells and surrounded by a network of capillaries. The portion of the tube passing from the coil to the surface serves as a duct (Figs. 91 and 121).
Fig. 91—Diagram of section through a sweat gland. a. Outer layer of skin or cuticle. b. Dermis or true skin. d, e. Sections of the tube forming the coiled portion of the gland. c. Duct passing to the surface. The other structures of the skin not shown.
The sweat glands secrete a thin, colorless fluid, called perspiration, or sweat. This consists chiefly of water, but contains a small per cent of salts and of urea. The excretory work of these glands seems not to be so great as was formerly supposed, but they supplement in a practical way the work of the kidneys and, during diseases of these organs, show an increase in excretory function to a marked degree. The perspiration also aids in the regulation of the temperature of the body (Chapter XVI).
Excretory Work of the Lungs.—While the lungs cannot be regarded as glands, they do a work in the removal of waste from the body which must be considered in the general process of excretion. They are especially adapted to the removal of gaseous substances from the blood, and it is through them that most of the carbon dioxide leaves the body. The lungs[pg 208] remove also a considerable quantity of water. This is of course in the gaseous form, being known as water vapor.
Ductless Glands and Internal Secretion.—Midway in function between the glands that secrete useful liquids and those that remove waste materials from the blood is a class of bodies, found at various places, known as the ductless glands. They are so named from their having the general form of glands and from the fact that they have no external openings or ducts. They prepare special materials which are passed into the blood and which are supposed to exert some beneficial effect either upon the blood or upon the tissues through which the blood circulates. The most important of the ductless glands are the thyroid gland, located in the neck; the suprarenal bodies, situated one just over each kidney; and the thymus gland, a temporary gland in the upper part of the chest. The spleen and the lymphatic glands (page 68) are also classed with the ductless glands. The liver, the pancreas, and (according to some authorities) the kidneys, in addition to their external secretions, produce materials that pass into the blood. They perform in this way a function like that of the ductless glands. The work of glands in preparing substances that enter the blood is known as internal secretion.
Quantity of Excretory Products.—If the weight of the normal body be taken at intervals, after growth has been attained, there will be found to be practically no gain or loss from time to time. This shows that materials are leaving the body as fast as they enter and that the tissues are being torn down as fast as they are built up. It also shows that substances do not remain in the body permanently, but only so long perhaps as is necessary for them to give up their energy, or serve some additional purpose in the ever changing protoplasm. The excretory organs then remove from the body a quantity of material that is equal in weight to the materials absorbed by the organs of digestion and respiration. This is estimated for the average individual to be about five pounds daily. The passage of waste from the body is summarized in Table III.
| TABLE III. THE PASSAGE OF WASTE MATERIALS FROM THE BODY | ||||
|---|---|---|---|---|
| Materials | State | How Formed in the Body | Condition in the Blood | How Removed from the Blood |
| Carbon dioxide | Gas | By the oxidation of the carbon of proteids,carbohydrates, and fats. | Dissolved in the plasma and in loosecombination with salts in the blood. | Separated from the blood at thealveoli of the lungs and then forced through the air passages into theatmosphere. |
| Urea | Solid | By the oxidation in the liver of nitrogenous compounds. | Dissolved in the plasma. | Removed by the uriniferous tubules of thekidneys and to a small extent by the perspiratory glands. |
| Water | Liquid | By the oxidation of the hydrogen of proteids,carbohydrates, and fats. Amount formed in the body is small. | As water. | Removed by all the organs of excretion, but in the largest quantities bythe kidneys and the skin. |
| Salts | Solid | Dissolved in the plasma. | By the kidneys, liver,and skin. | |