Experiment 76. _To show the action of pancreatic juice upon the alb.u.minous ingredients (casein) of milk_. Into a four-ounce bottle put two tablespoonfuls of cold water; add one grain of Fairchild's extract of pancreas, and as much baking soda as can be taken up on the point of a penknife. Shake well, and add four tablespoonfuls of cold, fresh milk.

Shake again.

Now set the bottle into a basin of hot water (as hot as one can bear the hand in), and let it stand for about forty-five minutes. While the milk is digesting, take a small quant.i.ty of milk in a goblet, and stir in ten drops or more of vinegar. A thick curd of casein will be seen.

Upon applying the same test to the digested milk, no curd will be made.

This is because the pancreatic ferment (trypsin) has digested the casein into "peptone," which does not curdle. This digested milk is therefore called "peptonized milk."

Experiment 77. _To show the action of bile_. Obtain from the butcher some ox bile. Note its bitter taste, peculiar odor, and greenish color. It is alkaline or neutral to litmus paper. Pour it from one vessel to another, and note that strings of mucin (from the lining membrane of the gall bladder) connect one vessel with the other. It is best to precipitate the mucin by acetic acid before making experiments; and to dilute the clear liquid with a little distilled water.

Experiment 78. _Test for bile pigments_. Place a few drops of bile on a white porcelain slab. With a gla.s.s rod place a drop or two of strong nitric acid containing nitrous acid near the drop of bile; bring the acid and bile into contact. Notice the succession of colors, beginning with green and pa.s.sing into blue, red, and yellow.

Experiment 79. _To show the action of bile on fats_. Mix three teaspoonfuls of bile with one-half a teaspoonful of almond oil, to which some oleic acid is added. Shake well, and keep the tube in a water-bath at about 100 F. A very good emulsion is obtained.

Experiment 80. _To show that bile favors filtration and the absorption of fats_. Place two small funnels of exactly the same size in a filter stand, and under each a beaker. Into each funnel put a filter paper; moisten the one with water (_A_) and the other with bile (_B_).

Pour into each an equal volume of almond oil; cover with a slip of gla.s.s to prevent evaporation. Set aside for twelve hours, and note that the oil pa.s.ses through _B_, but scarcely any through _A_. The oil filters much more readily through the one moistened with bile, than through the one moistened with water.

Experiments with the Fats.

Experiment 81. Use olive oil or lard. Show by experiment that they are soluble in ether, chloroform and hot water, but insoluble in water alone.

Experiment 82. Dissolve a few drops of oil or fat in a teaspoonful of ether. Let a drop of the solution fall on a piece of tissue or rice paper. Note the greasy stain, which does not disappear with the heat.

Experiment 83. Pour a little cod-liver oil into a test tube; add a few drops of a dilute solution of sodium carbonate. The whole ma.s.s becomes white, making an emulsion.

Experiment 84. Shake up olive oil with a solution of alb.u.men in a test tube. Note that an emulsion is formed.

Chapter VII.

The Blood and Its Circulation.

177. The Circulation. All the tissues of the body are traversed by exceedingly minute tubes called capillaries, which receive the blood from the arteries, and convey it to the veins. These capillaries form a great system of networks, the meshes of which are filled with the elements of the various tissues. That is, the capillaries are closed vessels, and the tissues lie outside of them, as asbestos packing may be used to envelop hot-water pipes. The s.p.a.ce between the walls of the capillaries and the cells of the tissues is filled with lymph. As the blood flows along the capillaries, certain parts of the plasma of the blood filter through their walls into the lymph, and certain parts of the lymph filter through the cell walls of the tissues and mingle with the blood current. The lymph thus acts as a medium of exchange, in which a transfer of material takes place between the blood in the capillaries and the lymph around them. A similar exchange of material is constantly going on between the lymph and the tissues themselves.

This, then, we must remember,--that in every tissue, so long as the blood flows, and life lasts, this exchange takes place between the blood within the capillaries and the tissues without.

The stream of blood _to_ the tissues carries to them the material, including the all-important oxygen, with which they build themselves up and do their work. The stream _from_ the tissues carries into the blood the products of certain chemical changes which have taken place in these tissues. These products may represent simple waste matter to be cast out or material which may be of use to some other tissue.

In brief, the tissues by the help of the lymph live on the blood.

Just as our bodies, as a whole, live on the things around us, the food and the air, so do the bodily tissues live on the blood which bathes them in an unceasing current, and which is their immediate air and food.

178. Physical Properties of Blood. The blood has been called the life of the body from the fact that upon it depends our bodily existence.

The blood is so essentially the nutrient element that it is called sometimes very aptly "liquid flesh." It is a red, warm, heavy, alkaline fluid, slightly salt in taste, and has a somewhat fetid odor. Its color varies from bright red in the arteries and when exposed to the air, to various tints from dark purple to red in the veins. The color of the blood is due to the coloring const.i.tuent of the red corpuscles, _haemoglobin_, which is brighter or darker as it contains more or less oxygen.

[Ill.u.s.tration: Fig. 65.--Blood Corpuscles of Various Animals. (Magnified to the same scale.)

A, from proteus, a kind of newt; B, salamander; C, frog; D, frog after addition of acetic acid, showing the central nucleus; E, bird; F, camel; G, fish; H, crab or other invertebrate animal ]

The temperature of the blood varies slightly in different parts of the circulation. Its average heat near the surface is in health about the same, _viz_. 98 F. Blood is alkaline, but outside of the body it soon becomes neutral, then acid. The chloride of sodium, or common salt, which the blood contains, gives it a salty taste. In a hemorrhage from the lungs, the sufferer is quick to notice in the mouth the warm and saltish taste. The total amount of the blood in the body was formerly greatly overestimated. It is about 1/13 of the total weight of the body, and in a person weighing 156 pounds would amount to about 12 pounds.

179. Blood Corpuscles. If we put a drop of blood upon a gla.s.s slide, and place upon it a cover of thin gla.s.s, we can flatten it out until the color almost disappears. If we examine this thin film with a microscope, we see that the blood is not altogether fluid. We find that the liquid part, or plasma, is of a light straw color, and has floating in it a mult.i.tude of very minute bodies, called corpuscles. These are of two kinds, the red and the colorless. The former are much more numerous, and have been compared somewhat fancifully to countless myriads of tiny fishes in a swiftly flowing stream.

180. Red Corpuscles. The red corpuscles are circular disks about 1/3200 of an inch in diameter, and double concave in shape. They tend to adhere in long rolls like piles of coins. They are soft, flexible, and elastic, readily squeezing through openings and pa.s.sages narrower than their own diameter, then at once resuming their own shape.

The red corpuscles are so very small, that rather more than ten millions of them will lie on a surface one inch square. Their number is so enormous that, if all the red corpuscles in a healthy person could be arranged in a continuous line, it is estimated that they would reach four times around the earth! The princ.i.p.al const.i.tuent of these corpuscles, next to water, and that which gives them color is _haemoglobin_, a compound containing iron. As all the tissues are constantly absorbing oxygen, and giving off carbon dioxid, a very important office of the red corpuscles is to carry oxygen to all parts of the body.

181. Colorless Corpuscles. The colorless corpuscles are larger than the red, their average diameter being about 1/2500 of an inch. While the red corpuscles are regular in shape, and float about, and tumble freely over one another, the colorless are of irregular shape, and stick close to the gla.s.s slide on which they are placed. Again, while the red corpuscles are changed only by some influence from without, as pressure and the like, the colorless corpuscles spontaneously undergo active and very curious changes of form, resembling those of the amba, a very minute organism found in stagnant water (Fig. 2).

The number of both red and colorless corpuscles varies a great deal from time to time. For instance, the number of the latter increases after meals, and quickly diminishes. There is reason to think both kinds of corpuscles are continually being destroyed, their place being supplied by new ones. While the action of the colorless corpuscles is important to the lymph and the chyle, and in the coagulation of the blood, their real function has not been ascertained.

[Ill.u.s.tration: Fig. 66.--Blood Corpuscles of Man.

A, red corpuscles; B, the same seen edgeways; C, the same arranged in rows; D, white corpuscles with nuclei.

Experiment 85. _To show the blood corpuscles_. A moderately powerful microscope is necessary to examine blood corpuscles. Let a small drop of blood (easily obtained by p.r.i.c.king the finger with a needle) be placed upon a clean slip of gla.s.s, and covered with thin gla.s.s, such as is ordinarily used for microscopic purposes.

The blood is thus spread out into a film and may be readily examined. At first the red corpuscles will be seen as pale, disk-like bodies floating in the clear fluid. Soon they will be observed to stick to each other by their flattened faces, so as to form rows. The colorless corpuscles are to be seen among the red ones, but are much less numerous.

182. The Coagulation of the Blood. Blood when shed from the living body is as fluid as water. But it soon becomes viscid, and flows less readily from one vessel to another. Soon the whole ma.s.s becomes a nearly solid jelly called a clot. The vessel containing it even can be turned upside down, without a drop of blood being spilled. If carefully shaken out, the ma.s.s will form a complete mould of the vessel.

At first the clot includes the whole ma.s.s of blood, takes the shape of the vessel in which it is contained, and is of a uniform color. But in a short time a pale yellowish fluid begins to ooze out, and to collect on the surface. The clot gradually shrinks, until at the end of a few hours it is much firmer, and floats in the yellowish fluid. The white corpuscles become entangled in the upper portion of clot, giving it a pale yellow look on the top, known as the _buffy coat_. As the clot is attached to the sides of the vessel, the shrinkage is more p.r.o.nounced toward the center, and thus the surface of the clot is hollowed or _cupped_, as it is called.

This remarkable process is known as coagulation, or the clotting of blood; and the liquid which separates from the clot is called serum.

The serum is almost entirely free from corpuscles, these being entangled in the fibrin.

[Ill.u.s.tration: Fig. 67.--Diagram of Clot with Buffy Coat.

A, serum; B, cupped upper surface of clot; C, white corpuscles in upper layer of clot; D, lower portion of clot with red corpuscles.

This clotting of the blood is due to the formation in the blood, after it is withdrawn from the living body, of a substance called fibrin.[31]

It is made up of a network of fine white threads, running in every direction through the plasma, and is a proteid substance. The coagulation of the blood may be r.e.t.a.r.ded, and even prevented, by a temperature below 40 F., or a temperature above 120 F. The addition of common salt also prevents coagulation. The clotting of the blood may be hastened by free access to air, by contact with roughened surfaces, or by keeping it at perfect rest.

This power of coagulation is of the most vital importance. But for this, a very small cut might cause bleeding sufficient to empty the blood-vessels, and death would speedily follow. In slight cuts, Nature plugs up the wound with clots of blood, and thus prevents excessive bleeding. The unfavorable effects of the want of clotting are ill.u.s.trated in some persons in whom bleeding from even the slightest wounds continues till life is in danger. Such persons are called "bleeders," and surgeons hesitate to perform on them any operation, however trivial, even the extraction of a tooth being often followed by an alarming loss of blood.

Experiment 86. A few drops of fresh blood may be easily obtained to ill.u.s.trate important points in the physiology of blood, by tying a string tight around the finger, and piercing it with a clean needle. The blood runs freely, is red and opaque. Put two or three drops of fresh blood on a sheet of white paper, and observe that it looks yellowish.

Experiment 87. Put two or three drops of fresh blood on a white individual b.u.t.ter plate inverted in a saucer of water. Cover it with an inverted goblet. Take off the cover in five minutes, and the drop has set into a jelly-like ma.s.s. Take it off in half an hour, and a little clot will be seen in the watery serum.

Experiment 88. _To show the blood-clot._ Carry to the slaughter house a clean, six or eight ounce, wide-mouthed bottle. Fill it with fresh blood. Carry it home with great care, and let it stand over night.

The next day the clot will be seen floating in the nearly colorless serum.

Experiment 89. Obtain a pint of fresh blood; put it into a bowl, and whip it briskly for five minutes, with a bunch of dry twigs. Fine white threads of fibrin collect on the twigs, the blood remaining fluid.

This is "whipped" or defibrinated blood, which has lost the power of coagulating spontaneously.