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 principal 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 pass 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?

3. If folds or convolutions in the surface of the cerebrum indicate intelligence, thoughtfulness, or mind, what do you infer as to the frog's mental condition and power to think?

4. Has the frog brain or "brains"? Explain.

5. Name several things done by an animal's brain.

6. Which of the spinal nerves are specially large? Why should they be large?

7. What advantages are there in the frog's having a dorsal nerve cord instead of a ventral one, as the earthworm has?

What would be the effect of cutting or breaking the dorsal root of a spinal nerve? the ventral root? the entire spinal cord? How do you explain the convulsions of the brainless (beheaded) chicken or frog?

8. What kinds of impulses originate outside of the nerve center, and what kinds in the center?

9. Sensations reach the cord and brain through the dorsal root of the spinal nerve. What kinds of messages travel through the ventral root, and in what direction do they go?

_Suggested drawings._

a. The brain.

b. The spinal cord and its nerves.

c. The nervous system.