THE NERVOUS SYSTEM 161 



of the foot. Apply the same stimulus to the trunk of the sciatic nerve. 

 The intensity of the stimulus required to evoke reflex action is usually 

 less when applied to the sense organs. 



Make a median incision through the skin covering the dorsal aspect 

 of this frog. Raise the skin and identify one of the many nerve-fibers 

 crossing the dorsal lymph-space to innervate the skin overlying. Cut 

 out a piece of skin about 1 cm. square, containing the terminals of this 

 nerve. Raise this skin-flap, but allow it to remain in connection with 

 the body by means of the nerve. Determine the least strength of 

 stimulus required to cause a reflex movement when applied to the sur- 

 face of this flap of skin Repeat, applying the electrodes to the afore- 

 said nerve. As a rule, the threshold value of the current will be found 

 to be lower in the former instance. 



7. Effect of Removal of the Cerebrum. Etherize a male frog under 

 a bell-jar. When completely insensitive make a median incision through 

 the skin covering the skull-cap and perforate the skull with the point 

 of a scalpel. Enlarge the opening and remove the cerebral hemispheres 

 and olfactory lobes. Note that the junction between the cerebrum and 

 the optic lobes is indicated externally by an imaginary line drawn 

 through the anterior margins of the ear drums. Work rapidly but care- 

 fully. Bring the edges of the wound together by means of two or three 

 sutures, and moisten the skin repeatedly with fresh water. Allow the 

 animal to recover fully. Carefully study its behavior: 



(a) What is its posture? Pass your hand in front of its eyes. Is it, 

 made to move thereby? Repeat this test upon a normal frog. Com- 

 pare. 



(6) Gently pinch the toe of the decerebrated frog. Do you note 

 any abnormality in its manner of jumping? Place the animal in water. 

 Is its power of swimming affected in any way? Does it retain its 

 upright position? 



(c) Place the frog upon its back. Does it right itself ? 



(d) Place the frog upon a somewhat roughened flat surface. Tilt 

 the board gradually and note how well it adapts the axis of its body 

 to the surface. Repeat this test by placing the frog upon a small 

 Ferris wheel. Gradually turn the wheel, noting that the frog attempts 

 to reach the top by moving against the direction of the rotation. Repeat 

 this test by placing the frog upon a rotating surface arranged horizon- 

 tally. The long axis of its body will then be bent against the direction 

 of the rotation (compensatory movements of equilibration). 



(e) Hold a small tuft of cotton moistened with a few drops of acetic 

 acid in front of its nostrils. The frog will make protective movements 

 with its forelimbs and move away from the seat of the stimulation 

 (trigeminal reflex). 



(/) Place a narrow board between the frog and an incandescent 

 light. Force the frog to jump toward the light. It will avoid the 

 object casting the shadow (retinal reflex). 



(g) Pass your index -finger over the skin covering the dorsal surface 

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