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. 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 ma.s.s, consisting of several divisions and lying close up under the arms.

_Bile sac_, small, green, and between the liver lobes.

_Alimentary ca.n.a.l._

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.

5. _Large intestine_, a thin-walled enlargement at the posterior end of the ca.n.a.l.

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

_Pancreas_, a yellowish, pulpy ma.s.s 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 the _ureters_, 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 ca.n.a.l.

2. Name the glands or organs which are accessory to the ca.n.a.l.

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 ca.n.a.l. 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 ca.n.a.l 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 ca.n.a.l?

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 ca.n.a.l, 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 ca.n.a.l. Give the scientific name for this action.

_Suggested drawings._

a. The viscera (internal organs) undisturbed.

b. The alimentary ca.n.a.l extended.

c. The excretory system.

The Organs of Circulation and Respiration of the Frog

_Materials._

The materials used in this exercise are the same as those used in the preceding exercise.

_Observations._

The pupil should identify the following structures:--

_Pericardium_, a membrane that surrounds the heart and, in the case of the frog, separates the body cavity into two portions, the _abdomen_ and the _pericardial_ chamber.

_Heart_, lying between the shoulders and in front of the false diaphragm. It is conical in form and composed of three chambers.

_Ventricle_, the pink, conical, and muscular portion of the heart, pointing backward and outward.

_Auricles_, right and left. These are anterior and dorsal to the ventricle, thin, membranous, and dark-colored.

_Arterial trunk_, the single large blood vessel, usually empty of blood, and white. Note its origin and trace it as far as possible, at least until you see it divide to encircle the throat.

_Sinus venosus_, a large membranous sac dorsal to the heart and connected with the right auricle.