

PHYSIOLOGICAL PHYSICS. [Chap. xv. 



(3) to detect the presence in the tissues of foreign me- 

 tallic bodies, (4) to unmask malingerers, (5) as a final 

 test of death. 



(1) To test irritability of muscle or nerve, use an 

 induction current, and apply well-moistened electrodes 

 to the part, the skin over which is also moist. This 

 ensures the current traversing the skin without affect- 

 ing it. Then graduate the intensity of the current by 

 moving the secondary coil or altering the extent of 

 surface of plates in action in the cell. Begin with 

 the healthy side, and find the feeblest current that 

 will produce a response on the part of the muscle or 

 group being tested. Compare the result obtained 

 with that of a similar experiment on the suspected 

 side, taking care that the experiment is repeated under 

 precisely similar conditions. If both sides are sus- 

 pected, then a healthy standard must be obtained else- 

 where, and the physician must compare his results with 

 an average obtained from healthy individuals. 



For testing sensibility the skin must be acted on, 

 and not the tissues beneath. Therefore the electrodes 

 must be dry (a wire brush), and the skin should be 

 well dried and dusted. Then find what strength of 

 current just begins to be painful on the healthy side 

 of the patient, and compare this with the diseased side. 



(2) For purposes of electrical diagnosis paralysis is 

 considered to be due either to a CENTRAL or to a PERI- 

 PHERAL lesion, and the value of electricity is in the 

 aid it gives in distinguishing between these two. A 

 CENTRAL lesion is, for this purpose, counted one which 

 separates the muscles from the higher centres, a peri- 

 pheral lesion is one which cuts off the muscles from 

 their lower centres. Thus the muscles of the legs are 

 in nervous communication with centres in the spinal 

 cord, their lower centres ; but these lower centres are 

 subservient to centres in the brain, their higher 

 centres. Now these muscles may be cut off from 



