29 



4. The Knee-jerk. 



One student sits with the right leg crossed on the left, 

 closes his eyes and firmly clasps his hands together. Another 

 student strikes the ligamenturn patellae of the right leg with 

 the edge of the ear-piece of a simple stethoscope or with the 

 side of the hand, and observes the contraction of the quad- 

 riceps extensor femoris and the movement of the leg. 



JII. What Time is taken in Nerve Actions when the 

 Brain is involved? (Visual Stimuli.) 



METHOD. Connect in one circuit, through the terminals 

 marked F, two mercury keys, A and B, separated by a con- 

 siderable length of wire, connected with a time-marker C, so 

 that when both keys are closed the current passes and the 

 lever on the marker is depressed. Bring the lever of the 

 marker C lightly against the smoked surface of a rapidly 

 revolving drum. One student stands beside the drum and 

 lever watching this lever, holding the handle of the closed 

 mercury key, A, which he must open the moment he sees the 

 lever depressed. The other now closes the other mercury key, 

 B, in such a way that the subject can neither see nor hear 

 the closing. The lever is thus suddenly depressed. It is 

 released again when the first student opens A , when the point 

 of the lever will spring up. A time tracing in T^TT sec. is put 

 below the record thus obtained. The tracing is fixed, and 

 the interval between the application of the stimulus and the 

 resulting action is measured and recorded. 



IV. What is the Result of Continuance of Reflex Action ? 

 Fatigue of the Neuro-muscular Mechanism. 



METHOD. Fit the hand and arm in a Mosso's ergograph 

 to the hook of which a weight of 3 kilograms has been 

 attached. Bring the writing point against a very slowly 

 moving drum. Set a metronome beating about 60 times 

 per minute, and as each beat is heard raise and lower the 

 weight with the finger to the fullest extent as long as it is 



