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The heart of the pig, sheep, or calf may be used to show the structure of the mammalian heart. It is best to procure at the meat shop several "plucks," i.e., heart, lungs, and trachea all attached together. Instructions should be given the butcher that the parts are to be left intact, otherwise they will be found to be punctured with knife cuts. Dissect out the blood vessels for some little distance from the heart in order to get their relations. Open some of the hearts lengthwise, others crosswise, to show the internal structure (Fig. 271). Pour water into the cavities to show the action of the valves. The flow of blood through the heart may be illustrated by connecting the aorta with the vena cavæ by means of rubber or glass tubing to represent the systemic circulation, and the pulmonary artery with the pulmonary veins to represent the pulmonary circulation, then filling the heart with water or a colored fluid and compressing the organ with the hand (Fig. 273).

The circulation may be studied in the web of the frog's hind foot. Procure a thin board large enough to lay the frog upon; in one end make a hole about a half-inch in diameter, over which the web may be stretched; anesthetize the frog with ether or chloroform; as soon as the animal becomes insensible lay it on the board, with its body covered with a moist cloth; over the larger toes of the foot to be examined slip nooses of thread, and fasten these in slits around the edge of the board in such positions as to spread the web between two of the toes over the hole in the board. Put a drop of water on the web, lay on the cover glass, place the board on the microscope, and examine with a one fifth or a one sixth objective. The anaesthetic must be renewed from time to time, otherwise the struggles of the animal will interfere with observation (Fig. 263).

CHAPTER XVII

The gross structure of the frog's lung may be studied in specimens which have been removed from the body, inflated with air blown through a small glass tube inserted through the glottis, and placed in alcohol a few hours to harden. When

cut open the lung will be seen to be a hollow sac with corrugated walls (Fig. 282).

"Plucks" obtained from a butcher will illustrate the structure of the mammalian larynx, trachea, bronchial tubes, etc. If fresh and not punctured with the knife they may be inflated. To work well they should be kept moistened (Fig. 283).

The presence of carbon dioxide in the air exhaled from the lungs may be shown by using limewater or baryta water, with either of which carbon dioxide forms an insoluble precipitate, which at first floats as a delicate white film on the surface of the liquid. Pour some of the fluid into a saucer or watch glass, then breathe heavily upon it a few times through the mouth, and the film will be formed.

CHAPTER XVIII

The structure of the kidneys is well illustrated by the kidney of the sheep. Several of these should be procured and opened in various directions to show the structure (Fig. 290).

CHAPTER XIX

With little trouble skeletons of frogs, birds, and mammals with bones connected by flexible attachments may be prepared. Carefully cut away all of the muscles and other soft parts, leaving only the ligaments connecting the bones. Then place the roughly prepared specimen for one or two weeks in Wickersheimer's fluid, which is prepared as follows: In three liters of boiling water dissolve 100 grams of alum, 60 grams of caustic potash, 25 grams of salt, 12 grams of saltpeter, and 10 grams of arsenic. Cool and filter the liquid. Then to each liter of the fluid add 400 cubic centimeters of glycerine and 100 cubic centimeters of alcohol. The ligaments of skeletons soaked in this fluid will remain flexible during many months of exposure to the air. Should the ligaments become stiffened, their flexibility may be restored by a few hours' immersion in the fluid.

CHAPTER XX

Muscle fibers for microscopic examination may be obtained from the leg of a frog, or even from the body of a recently killed animal at the meat shop. Lay a small piece of muscle in a drop of .75 per cent salt solution on a glass slide, and with a pair of dissecting needles carefully pick the muscle to pieces. Some of the smallest shreds, upon examination with a one-fourth or a one-sixth inch objective, will be seen to be single or grouped muscle fibers, which will show the striations and the sarcolemma (Fig. 208).

CHAPTER XXI

Nerve fibers are readily obtained from the sciatic nerve in the frog. This nerve may be found by removing the skin from the back of a frog's thigh and carefully separating the underlying muscles. Among them will be seen the sciatic nerve, covered in places with dark gray or black pigment spots. Remove a quarter to a half inch of the nerve, being careful to stretch it as little as possible; lay it on the glass slide in a few drops of .75 per cent salt solution; cautiously tear it to pieces in the direction of its length with dissecting needles; then put on a cover glass and examine under a high power. The nerve will be found to consist of a number of nerve fibers, some of which will show the primitive sheath (neurilemma), medullary sheath, and axis cylinder (Figs. 210, 211).

The relation between the stimulation of a nerve and the contraction of the muscle to which the nerve runs may be shown as follows: Expose the sciatic nerve as directed above; then with the quick stroke of a sharp scalpel sever the upper end of the nerve as near the body as possible. At the moment of doing this the muscles of the leg and foot will probably contract. Allow the nerve to rest for a few minutes; then pinch its upper end with a pair of forceps. Again the muscles will contract. The stimulation may be repeated at intervals if the nerve be allowed to rest for a few minutes between successive stimula

tions. Try also the effect of touching the nerve with a hot wire and with a drop of dilute acid or alkali. During experimentation the nerve preparation must be kept moistened with the salt solution.

CHAPTER XXIII

The structure of the egg may be studied in the starfish or sea urchin, frog or fowl. Starfish eggs preserved in various stages of segmentation may be purchased from the Department of Laboratory Supply of the Marine Biological Laboratory at Wood's Hole, Mass. Frogs' eggs may be found in ponds and ditches in early spring. If transferred to the laboratory and kept supplied with fresh water they may be watched through their various stages of segmentation to the formation of the tadpole, its liberation from the egg, and its later development. Compare with Fig. 370. To watch the development of a chick, eggs may be incubated by a hen or in an artificial incubator, one egg being removed each day, and opened by breaking away a circular piece of the shell on the upper side. If kept submerged in a dish of .75 per cent salt solution, warmed to the temperature of the body, the embryo chick may be kept alive for several hours to show the beating of the heart, etc. (Figs. 365, 366).

INDEX

In the Index the numbers in Roman type (289) refer to pages; those in bold-faced

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