LESSON XXXIII 



THE NERVOUS SYSTEM (Continued) 

 REFLEX ACTION. REMOVAL OF CEREBRUM 



1. Inhibition of Reflexes by Higher Centers. Etherize a frog. 

 Make a median incision through the skin covering the skull-cap. Per- 

 forate with the point of a knife and enlarge the opening by means of a 

 pair of forceps. Identify the cerebral hemispheres and remove them. 

 After an interval suspend the frog and determine the reflex time in the 

 usual way. 



Sprinkle a few crystals of sodium chlorid upon the upper surface 

 of the optic lobes, and again ascertain the reflex time. Since these 

 bodies possess an inhibitor action upon spinal reflex action, this means 

 of stimulation will tend to lengthen this period. 



Remove both optic lobes, and again determine the reflex time. 

 Obviously, their removal must destroy this inhibitor influence and 

 intensify spinal reflex action. 



2. Exaggeration of Reflexes by Means of Strychnin. Inject a drop 

 or two of a 0.5 per cent, solution of sulphate of strychnin into the dorsal 

 lymph-sac of a frog. After a few minutes stimulate the foot of this frog 

 mechanically. Repeat at brief intervals, noting the progressive char- 

 acter of the muscular seizures. Blow your breath at the frog or tap 

 upon the table upon which it is resting. Upon which elements of the 

 reflex circuit does the strychnin exert its action? 



3. Reflexes in Man, Let the subject open his mouth. Touch the 

 uvula with the end of an aseptic glass rod. It will rise. Touch the 

 fauces. The response may be either a movement concerned with the 

 act of swallowing or the gagging reflex, an act tending to protect the 

 digestive tract. 



Make a sudden movement in front of the eyes of the subject as if 

 you were going to strike him in the face. The eyelids are closed. 

 Touch the outer surface of the cornea of the subject with a cotton fiber. 

 An immediate closure of the eyelids is the result. In all these instances, 

 however, the character of the reflex may be modified volitionally (in- 

 hibition by the cerebrum). 



Shield the eye of the subject for a few seconds with your hand. 

 Suddenly withdraw the latter, allowing light to enter the pupil. Ob- 

 serve the decrease in the size of this orifice, brought about by the con- 

 traction of the circular muscle cells of the iris (light reflex). Request 

 the subject to accommodate alternately for far and near objects. The 

 pupil is enlarged during far vision and constricted during near vision 

 (accommodation reflex). Pinch the skin of the neck. The pupil will 

 dilate (ciliospinal reflex). 



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