PRACTICAL WORK

*Suggested Experiments.*-1. The change of kinetic into potential energy may be shown by stretching a piece of rubber, by lifting a weight, and by separating the armature from a magnet.

2. The change of potential into kinetic energy may be shown by letting weights fall to the ground, by releasing the end of a piece of stretched rubber, and by burning substances.

3. The change of one form of kinetic energy to another may be ill.u.s.trated by rubbing together two pieces of wood until they are heated, by ringing a bell, and by causing motion in air or in water by heating them. If suitable apparatus is at hand, the transformation of electrical energy into heat, light, sound, and mechanical motion can easily be shown.

4. A weight connected by a cord with some small machine and made to run it, will help the pupil to grasp the general principles in the storage of energy through gravity. A vessel of water on a high support from which the water is siphoned on to a small water wheel will serve the same purpose.

5. The storing of energy by chemical means may be ill.u.s.trated by decomposing pota.s.sium chlorate with heat or by decomposing water by means of a current of electricity.

6. Study the transfer of energy from the body to surrounding objects, as in moving substances and lifting weights.

Fill a half gallon jar two thirds full of water and carefully take the temperature with a chemical thermometer. Hold the hand in the water for four or five minutes and take the temperature again. Inference.

CHAPTER XIII - GLANDS AND THE WORK OF EXCRETION

In our study so far we have been concerned mainly with the introduction of materials into the body. We are now to consider the removal of materials from the body. The structures most directly concerned in this work are known as

*Glands.*-As generally understood, glands are organs that prepare special liquids in the body and pour them out upon free surfaces. These liquids, known as _secretions_, are used for protecting exposed parts, lubricating surfaces that rub against each other, digesting food, and for other purposes. They differ widely in properties as well as in function, but are all alike in being composed chiefly of water. The water, in addition to being necessary to the work of particular fluids, serves in all cases as a carrier of solid substances which are dissolved in it.

*General Structure of Glands.*-While the various glands differ greatly in size, form, and purpose, they present striking similarities in structure.

All glands contain the following parts:

1. Gland, or secreting, cells. These are _specialized_ cells for the work of secretion and are the active agents in the work of the gland. They are usually cubical in shape.

2. A bas.e.m.e.nt membrane. This is a thin, connective tissue support upon which the secreting cells rest.

3. A network of capillary and lymph vessels. These penetrate the tissues immediately beneath the secreting cells.

4. A system of nerve fibers which terminate in the secreting cells and in the walls of the blood vessels pa.s.sing to the glands.

These structures-secreting cells, bas.e.m.e.nt membrane, capillary and lymph vessels, and nerve fibers-form the essential parts of all glands. The capillaries and the lymph vessels supply the secreting cells with fluid, and the nerves control their activities.

*Kinds of Glands.*-Glands differ from one another chiefly in the arrangement of their essential parts.(73) The most common plan is that of arranging the parts around a central cavity formed by the folding or pitting of an exposed surface. Many such glands are found in the mucous membrane, especially that lining the alimentary ca.n.a.l, and are most numerous in the stomach, where they supply the gastric juice. If these glands have the general form of tubes, they are called _tubular_ glands; if sac-like in shape, they are called _saccular_ glands. Both the tubular and the saccular glands may, by branching, form a great number of similar divisions which are connected with one another, and which communicate by a common opening with the place where the secretion is used. This forms a _compound_ gland which, depending on the structure of the minute parts, may be either a _compound tubular_ or a _compound saccular_ gland. The larger of the compound saccular glands are also called _racemose_ glands, on account of their having the general form of a cl.u.s.ter, or raceme, similar to that of a bunch of grapes. The general structure of the different kinds of glands is shown in Fig. 85.

[Fig. 85]

Fig. 85-*Diagram ill.u.s.trating evolution of glands.* _A._ Simple secreting surface. 1. Gland cells. 2. Bas.e.m.e.nt membrane. 3. Blood vessel. 4. Nerve.

_B._ Simple tubular gland. _C._ Simple saccular gland. _D._ Compound tubular gland. _E._ Compound saccular gland. _F._ A compound racemose gland with duct pa.s.sing to a free surface. _G._ Relation of food ca.n.a.l to different forms of glands. The serous coat has a secreting surface.

*Nature of the Secretory Process.*-At one time the gland was regarded merely as a kind of filter which separated from the blood the ingredients found in its secretions. Recent study, however, of several facts relating to secretion has led to important modifications of this view. The secretions of many glands are known to contain substances that are not found in the blood, or, if present, are there in exceedingly small amounts. Then again the cells of certain glands have been found to undergo marked changes during the process of secretion. If, for example, the cells of the pancreas be examined after a period of rest, they are found to contain small granular bodies. On the other hand, if they are examined after a period of activity, the granules have disappeared and the cells themselves have become smaller (Fig. 86). The granules have no doubt been used up in forming the secretion. These and other facts have led to the conclusion that secretion is, in part, the separation of materials without change from the blood, and, in part, a process by which special substances are prepared and added to the secretion. According to this view the gland plays the double role of a _filtering apparatus_ and of a _manufacturing organ_.

[Fig. 86]

Fig. 86-*Secreting cells from the pancreas* (after Langley). _A._ After a period of rest. _B._ After a short period of activity. C. After a period of prolonged activity. In _A_ and _B_ the nuclei are concealed by the granules that acc.u.mulate during the resting period.

*Kinds of Secretion.*-In a general way all the liquids produced by glands may be considered as belonging to one or the other of two cla.s.ses, known as the _useful_ and the _useless_ secretions. To the first cla.s.s belong all the secretions that serve some purpose in the body, while the second includes all those liquids that are separated as waste from the blood. The first are usually called _true secretions_, or secretions proper, while the second are called _excretions_. The most important glands producing liquids of the first cla.s.s are those of digestion (Chapter X).

*Excretory Work of Glands.*-The process of removing wastes from the body is called _excretion_. While in theory excretion may be regarded as a distinct physiological act, it is, in fact, leaving out the work of the lungs, but a phase of the work of glands. From the cells where they are formed, the waste materials pa.s.s into the lymph and from the lymph they find their way into the blood. They are removed from the blood by glands and then pa.s.sed to the exterior of the body.

*The Necessity for Excretion* is found in the results attending oxidation and other chemical changes at the cells (page 107). Through these changes large quant.i.ties of materials are produced that can no longer take any part in the vital processes. They correspond to the ashes and gases of ordinary combustion and form wastes that must be removed. The most important of these substances, as already noted (page 110), are carbon dioxide, water, and urea.(74) A number of mineral salts are also to be included with the waste materials. Some of these are formed in the body, while others, like common salt, enter as a part of the food. They are solids, but, like the urea, leave the body dissolved in water.

Waste products, if left in the body, interfere with its work (some of, them being poisons), and if allowed to acc.u.mulate, cause death. Their removal, therefore, is as important as the introduction of food and oxygen into the body. The most important of the excretory glands are

*The Kidneys.*-The kidneys are two bean-shaped glands, situated in the back and upper portion of the abdominal cavity, one on each side of the spinal column. They weigh from four to six ounces each, and lie between the abdominal wall and the peritoneum. Two large arteries from the aorta, called the _renal arteries_, supply them with blood, and they are connected with the inferior vena cava by the _renal veins_. They remove from the blood an exceedingly complex liquid, called the _urine_, the princ.i.p.al const.i.tuents of which are water, salts of different kinds, coloring matter, and urea. The kidneys pa.s.s their secretion by two slender tubes, the _ureters_, to a reservoir called the _bladder_ (Fig. 87).

[Fig. 87]

Fig. 87-*Relations of the kidneys.* (Back view.) 1. The kidneys. 2.

Ureters. 3. Bladder. 4. Aorta. 5. Inferior vena cava. 6. Renal arteries.

7. Renal veins.

*Structure of the Kidneys.*-Each kidney is a compound tubular gland and is composed chiefly of the parts concerned in secretion. The ureter serves as a duct for removing the secretion, while the blood supplies the materials from which the secretion is formed. On making a longitudinal section of the kidney, the upper end of the ureter is found to expand into a basin-like enlargement which is embedded in the concave side of the kidney. The cavity within this enlargement is called the _pelvis of the kidney_, and into it project a number of cone-shaped elevations from the kidney substance, called the _pyramids_ (Fig. 88).

From the summits of the pyramids extend great numbers of very small tubes which, by branching, penetrate to all parts of the kidneys. These are the _uriniferous tubules_, and they have their beginnings at the outer margin of the kidney in many small, rounded bodies called the _Malpighian capsules_ (_A_, Fig. 88). Each capsule incloses a cl.u.s.ter of looped capillaries and connects with a single tubule (Fig. 89). From the capsule the tubule extends toward the concave side of the kidney and, after uniting with similar tubules from other parts, finally terminates at the pyramid. Between its origin and termination, however, are several convolutions and one or more loops or turns. After pa.s.sing a distance many times greater than from the surface to the center of the kidney, the tubule empties its contents into the expanded portion of the ureter.

[Fig. 88]

Fig. 88-*Sectional view of kidney.* 1. Outer portion or cortex. 2.

Medullary portion. 3. Pyramids. 4. Pelvis. 5. Ureter. _A._ Small section enlarged to show the tubules and their connection with the capsules.

[Fig. 89]

Fig. 89-*Malpighian capsule* highly magnified (Landois). _a._ Small artery entering capsule and forming cl.u.s.ter of capillaries within. _e._ Small vein leaving capsule and branching into _c_, a second set of capillaries, _h._ Beginning of uriniferous tubule.

The uriniferous tubules are lined with secreting cells. These differ greatly at different places, but they all rest upon a bas.e.m.e.nt membrane and are well supplied with capillaries. These cells provide one means of separating wastes from the blood (Fig. 90).

[Fig. 90]

Fig. 90-*Diagram ill.u.s.trating renal circulation.* 1. Branch from renal artery. 2. Branch from renal vein. 3. Small artery branches, one of which enters a Malpighian capsule (5). 6. Small vein leaving the capsule and branching into the capillaries (7) which surround the uriniferous tubules.