256 EXPERIMENTAL PHYSIOLOGY. 



severed, if not before. It is advisable to warm the air from the 

 pump, but whether or not this is done, great care must be taken to 

 see that the temperature of the preparation, observed by a clinical 

 thermometer placed in the rectum, does not fall. For some time 

 after the decapitation little reflex activity is shown by the prepara- 

 tion why is this the case? In about one hour, however, many 

 complex reflex movements can readily be elicited. Of these the 

 following should be studied; the movements may be recorded by 

 tying threads to the hind limbs and connecting with reducing levers. 



1. THE FLEXION REFLEX, by applying stimuli (mechanical, 

 electrical) to the skin of the foot or stimulating the central end of 

 one of the sensory nerves (peroneal) with the tetanising current. 

 The latent time, grading of intensity, summation, etc., may he 

 studied by the procedures already described on p. 240. 



2. THE KNEE JERK, by passively flexing the knee joint and 

 tapping the patellar tendon. The prompt and limp-like return 

 of the leg to its original position should be contrasted with the 

 gradual and imperfect return observed in a decerebrate prepara- 

 tion (p. 249) 



3. THE SCRATCH REFLEX, by stroking the skin at the side of the 

 neck. The scratching movement of the homolateral hind limb is 

 not so easily evoked as in a spinal dog that has recovered from shock, 

 and it may not appear until the decapitated animal has been par- 

 tially asphyxiated by discontinuing the artificial respiration for a 

 minute or so. Sometimes the preparation shows a hyperexcitable 

 scratch reflex, but this often depends on inadequate pulmonary 

 ventilation. When it occurs, the respiratory apparatus should be 

 examined and the tracheal cannula cleared of any mucus that may 

 be interfering with the free passage of air into and out of the lungs. 

 If the scratch reflex is marked, its inhibition may readily be demon- 

 strated by stimulating the central end of the peroneal nerves of 

 either leg. 



4. STIMULATION OF THE POSTERIOR COLUMNS OF THE SPINAL 

 CORD, by exposing the upper end of the severed cord and stimulating 

 by the unipolar method. This observation is of value because it 

 shows that stimuli descending by the main sensory pathways of the 

 cord 'because the fibres transmit in both directions (cf. p. 46) flow 

 into the collaterals which are adjacent to the point of entry of the 

 fibres into the cord. To make the observation, place an indifferent 



