How To Study Living Animals


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 herunterladen.

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 herunterladen. 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 microsoft office kostenlos downloaden vollversion deutsch chip.

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?