How To Study Living Animals


The Organs of Digestion, Absorption, and Excretion
Materials.

Freshly killed or preserved frogs in dishes or shallow pans of water, forceps, and, if the pupil is to do any dissecting, scissors.

Directions and Observations.

The specimens may have been opened by the teacher, or may be dissected by the pupil as follows:—

Placing the frog on its back, with forceps firmly grasp the skin of the abdomen and the muscles beneath, just in front of the hind legs, and with the scissors cut straight forward in the middle line until the floor of the mouth is reached; this will separate the arms herunterladen. Care must be taken not to cut too deeply, but this may be avoided by keeping the skin uplifted. Now cut sidewise in front of each hind leg in order that the body wall may be laid aside. Under the arms the heart will be seen; it will be studied as a part of the circulatory system.


Identify the following organs:—

Liver,

the large red or brown mass, consisting of several divisions and lying close up under the arms herunterladen.

Bile sac,

small, green, and between the liver lobes.

Alimentary canal.

1. Mouth.

2. Esophagus.

3. Stomach, the elongated, light-colored, firm, and muscular portion.

4. Small intestine, a slender, more or less closely coiled, tubular portion microsoft office kostenlos downloaden vollversion deutsch chip.

5. Large intestine, a thin-walled enlargement at the posterior end of the canal.

6. Duodenum. This is a muscular portion of the small intestine immediately following the stomach, against which it is folded.

Pancreas,

a yellowish, pulpy mass lying in the fold between the stomach and the duodenum.

Spleen,

a dark red globule, usually smaller than a pea, lying nearly free among the folds of the small intestine.

Fat bodies,

yellow fringe-like structures, sometimes found near the stomach.

Kidneys,

a pair of elongated dark red organs, behind the spleen and against the back. Note their numerous blood vessels. Possibly theureters, or urinal ducts, can be discovered and traced to their junction with the bladder, a clear membranous sac in the posterior extremity of the body cavity.

Peritoneum,

a thin membrane lining the body cavity and attaching the vital organs to the backbone.

Note.—Specimens secured in late fall, winter, or early spring may contain, if female, a large number of dark-colored eggs; or if male, two white testes, located near the kidneys and similar to them in form, though smaller.

Questions.

1. Name the parts of the frog’s alimentary canal.

2. Name the glands or organs which are accessory to the canal.

3. How long is the esophagus? How does the presence or absence of a neck affect the esophagus?

4. How does the thickness of the stomach wall compare with that of the intestine, and how do you account for the difference?

5. Measure the length of the trunk of the frog’s body and that of the outstretched alimentary canal. How many times the length of the one is that of the other? How does this ratio compare with that of an herbivorous animal? (The sheep’s food canal is about thirty-two times the length of its body.)

6. What is the color of the bile, as seen through the walls of the bile sac? This color is characteristic of carnivorous animals; in herbivorous forms it is yellow. Find its color in some omnivorous form, as man.

7. Name the organs concerned in excretion.

8. What holds the internal organs in place, and from what are they suspended?

9. The spleen is called a “ductless gland.” Give its function, and explain why a duct is not necessary to it.

10. Since the frog swallows its food alive and entire, what work must the stomach do? What digestive organs would be absent from the mouth, or else poorly developed?

11. Since the frog is carnivorous, what digestive ferments are probably present, and what ones absent from the alimentary canal?

12. Fat bodies are largest in the fall, and are rarely found in the spring. How can you account for this?

13. When through with the general study of the alimentary canal, you may open the stomach by cutting it lengthwise. Describe the character of the stomach lining as to folds and villi, stating the advantage of each being present and the reason for the direction of the folds. Tell how the food is propelled onward through the alimentary canal. Give the scientific name for this action.

Suggested drawings.

a. The viscera (internal organs) undisturbed.

b. The alimentary canal extended.

c. The excretory system.