LESSON XXXII 



THE NERVOUS SYSTEM 



REFLEX ACTION 



1. Histologic Study of Different Neurons. Examine under the 

 microscope motor neurons from the cerebral cortex, cells of Purkinje 

 from the cerebellum, motor cells from the anterior horn of the spinal 

 gray matter, and sensory cells from the spinal ganglion. Draw a dia- 

 gram of each. 



Orient yourself regarding the principal tracts of the spinal cord, 

 the formation of the spinal roots and their function, and other data of 

 general interest. 



2. Dissection of the Nervous System of the Frog. Kill a frog with 

 ether. Make a median incision through the skin covering the skull 

 cap. Hold the scalpel slantingly and perforate the bone about midway 

 between the eyes, taking care not to penetrate too deeply. With a 

 pair of small forceps cut away the bone around the perforation, enlarg- 

 ing the opening considerably. Having uncovered the white cerebral 

 hemispheres and olfactory lobes, dissect backward until you have 

 brought into view the rounded, grayish optic lobes or corpora bigemini. 

 The cerebellum is rudimentary and occupies a position in front of these 

 bodies. Identify the optic nerves. 



Expose the entire spinal cord by breaking away the vertebrae along 

 the dorsal aspect of the animal. Identify the spinal nerves and spinal 

 roots. 



3. Reflex Action. Pith a frog and destroy its brain (not the spinal 

 cord). Suspend the animal from a stand over a plate. Pinch the toes 

 of one foot with the forceps. Observe that the foot is withdrawn from 

 the seat of the stimulation by muscular activity. 



Produce this reaction by immersing the foot in a weak solution of 

 acetic acid. Repeat by applying the electrodes to the sole of the foot 

 and stimulating with a brief tetanic current of moderate strength. 



Destroy the spinal cord with a wire. Repeat the electric excitation 

 and observe whether or no the different impulses so generated still 

 induce motor responses. Draw conclusions regarding the part played 

 by the spinal cord in this reaction. 



Annotation. The solution of acetic acid should be weak, possibly 3 drops of 

 glacial acetic acid to about 20 c.c. of water. Strengthen it in case it should fail to 

 stimulate. Immerse the foot in clean water after every stimulation. When the 

 electric current is employed as a means of inducing reflex action, differentiate sharply 

 between the local muscular contractions and those co-ordinated general contractions 

 which eventually cause the removal of the foot from the seat of the stimulation. 



In order to save material the student may omit destroying the spinal cord 

 with the wire. This procedure, as may be surmised, destroys the spinal reflex ac- 



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