240 EXPERIMENTAL PHYSIOLOGY. 



1. THE FLEXION REFLEX 'by pricking the skin of the paw 

 with a pin or applying a moderately strong electric shock. The 

 flexion at knee and hip is accompanied by extension of the leg of the 

 opposite side, THE CROSSED EXTENSION REFLEX. By taking simul- 

 taneous tracings of the two legs after attaching the feet to suitable 

 levers by threads, the correspondence in time between the two 

 reflexes is demonstrated. 



2. THE KNEE JERK by giving a sharp blow with a ruler or 

 the handle of a scalpel to the ligamentum patellae. Note that the 

 leg swings limply back to its flexed position after the tap (compare 

 with the behaviour of the jerk in a decerebrate animal (p. 103 ). 



3. THE SCRATCH REFLEX by moving the finger or a pencil 

 backwards and forwards on the skin of the body. The skin area 

 from which the reflex can be elicited is very extensive and the paw 

 usually is directed approximately to the place of stimulation 

 ('local sign'). Sometimes this property of 'local sign', however, is 

 very imperfect. An electrical stimulus (through the stigmatic 

 electrode) will also elicit the reflex, but much less satisfactorily than 

 the mechanical one. 



4. THE EXTENSOR THRUST by pushing the blunt end of a 

 pencil between the pads of the paws. The corresponding leg makes 

 a sudden extension movement. 



The following properties of reflex action may now be studied 

 using one or other of the foregoing reflexes. 



1. THE LATENT PERIOD OR UNCORRECTED REFLEX TIME. To 

 determine this it is necessary to employ electrical stimulation and to 

 insert a signal magnet in the primary circuit. A fast rate of drum 



to be attached to the long arm of a right angled lever, from the short arm of which 

 a second thread runs to a straight muscle lever. The threads are adjusted so as 

 suitably to diminish the amplitude of movement at the writing points. 



A large electrode made out of a strip of copper covered with a pad of surgica 

 gauze, which is moistened with salt solution, is tied on to the middle of the back 

 (indifferent electrode) and a much smaller electrode of the same type is tied to the 

 foot of one side (stimulating electrode). A third (stigmatic) electrode composed 

 of a stout copper wire covered at its tip by a piece of surgical gauze is also required 

 The wires leading from the electrodes are connected with the secondary coil of an 

 inductorium, the indifferent electrode with one pole and the stimulating and 

 stigmatic electrodes with the other. (A well insulated simple key should be 

 inserted in the wire leading from the attached foot electrode). 



