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:—
- Cerebral hemispheres, a pair of elongated lobes, the anterior enlargements.
- Mid brain, or optic lobes, a pair of large ovoid structures, projecting diagonally forward and sidewise.
- Cerebellum, a slender, transverse ridge, close behind the midbrain.
- 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.