_Lungs_, two small oblong, pink, spongy sacs, lying between and behind the shoulders.

The pupil may also identify the following structures if a specimen is available which has the blood vessels injected.

_Conus arteriosus_, or "arterial trunk," a large artery pa.s.sing obliquely forward from the ventricle, and dividing into three branches on each side.

_Carotid arteries_, the first branches of the conus, to the head and neck.

_Aortas_, the second branches of the conus, to the dorsal region.

_Pulmocutaneous arteries_, the third branches of the conus, to the lungs and skin.

_Dorsal aorta_, the large artery along the back, formed by the union of the two aortas.

_Iliac_, or _femoral arteries_, the two posterior divisions of the dorsal aorta, supplying the legs.

_Posterior_ (ascending) _vena cava_, a large vein close to the dorsal aorta, pa.s.sing forward from the kidneys.

_Hepatic veins_, large veins connecting the liver with the posterior vena cava.

_Anterior_ (descending) _venae cavae_, large veins formed by the junction of the veins from the arm, neck, and head on the right and left sides.

_Subclavian veins_, from the arms.

_Jugular veins_, from the neck.

_Questions._

1. Of how many chambers does the frog's heart consist? Name them, and describe them as to size, color, and structure.

2. Which chamber receives blood from the body, and which receives blood from the lungs?

3. Name the large arteries and give the regions which they supply.

Name the large veins and give the regions from which they come.

4. Describe the lungs as to size, both when inflated and when uninflated. Describe their color; entirety or subdivision; texture.

5. In the inflated lung, notice the interior part.i.tions or chambers, which are called _vesicles_. How do they affect the amount of surface exposed for gas exchange in breathing?

6. Measuring the lung collapsed and again when inflated, calculate its approximate volume in each case and state how much air it may take in during an inspiration.

7. Describe the diaphragm and state its probable use as a factor in respiration or as a part.i.tion.

_Advanced questions._

8. Apparently the pure and the impure blood must commingle upon entering the single ventricle, but by a simple device this scarcely occurs. How would such commingling affect the purity of the blood as it reached the tissues, and hence affect waste removal, oxidation, body temperature, activity, and intelligence?

9. What are the chambers of the fish's heart? of the mammalian (human) heart? How may the heart of the frog be regarded as intermediate between these others? How and where might the growth of a part.i.tion within it bring about the higher structure? (This actually occurs in certain reptiles.)

10. The lungs are said to be outgrowths of the alimentary ca.n.a.l.

Explain how their connection would tend to show this.

_Suggested drawings._

a. The heart and lungs in their normal position.

b. The circulatory system, as seen in the injected specimen.

c. A copy of the diagram or model of the circulatory system.

d. A diagram of the respiratory tract from the nostrils to the lungs, by arrows showing the course of the air.

The Nervous System of the Frog

_Materials._

Specimens which have the brain exposed and other specimens whose viscera have been removed so that the spinal nerves can be seen; pans or shallow dishes of water and forceps.

_Observations._

The nerve tissues are generally white in color unless they have been specially treated and stained. The pupil should identify the following structures:--

_A. Brain_, those enlargements of nerve tissue situated in the head and composed of four princ.i.p.al parts, as follows:--

1. _Cerebral hemispheres_, a pair of elongated lobes, the anterior enlargements.

2. _Mid brain_, or _optic lobes_, a pair of large ovoid structures, projecting diagonally forward and sidewise.

3. _Cerebellum_, a slender, transverse ridge, close behind the midbrain.

4. _Medulla_, the anterior end of the spinal cord, widest in front and containing a triangular depression.

(Frequently a pair of smaller enlargements is to be seen in front of the cerebral hemispheres; they are the _olfactory lobes_, and from them nerves pa.s.s forward to the nasal chamber.)

_B. Spinal cord_, extending along the spine, giving rise to nerves.

_C. Spinal nerves_, ten pairs of nerves which are connected with the cord through dorsal (sensory) and ventral (motor) roots, and which penetrate the body and its appendages. The first enters the neck; the second and third join and enter the arm; the fourth to sixth penetrate the skin and muscles of the trunk; the seventh to ninth join by a plexus to form the sciatic nerve which supplies the leg, and the tenth enters the posterior portion of the body.

_Questions._

1. Which lobes of the brain are paired? Give at least two possible causes or reasons for their double structure.

2. The optic lobes are connected with the eyes. Compare their size with that of the other parts. Of how much use do you think they are to the frog?