ACTION OP ELECTRICITY UPON THE NERVES. 529 



msil rate in the motor nerves. As compared with the sensory nerves, the 

 cord conducts tactile impressions a little faster and painful impressions less 

 than one half as fast. 



Attempts have been made to estimate the duration of acts involving the 

 central nervous system, such as the reflex phenomena of the spinal cord or 

 the operations of the cerebral hemispheres. These have been partially suc- 

 cessful, or, at least, they have shown that the reflex and the cerebral acts 

 require a distinctly appreciable period of time. This in itself is an impor- 

 tant fact ; although the duration of these acts has not been measured with 

 absolute accuracy. As the general result of experiments upon these points, 

 it has been found that the reflex action of the spinal cord occupies more than 

 twelve times the period required for the transmission of stimulus or impres- 

 sions through the nerves. Donders found, in experiments upon his own 

 person, that an act of volition required ^ of a second, and one of simple 

 distinction Or recognition of an impression, ^ of a second. These esti- 

 mates, however, are merely approximate, and until they attain greater cer- 

 tainty, it is unnecessary to describe in detail the apparatus employed. 



Personal Equation. In recording astronomical observations, it has been 

 found that a certain time elapsed between the actual observation of a phe- 

 nomenon and the moment of its record. This error, which is equal to the 

 interval of time between the impression made upon the retina and the mus- 

 cular act by which a record is made, is not the same in different persons or 

 even in the same person at all times. It may amount to of a second or 

 even more, and it may be as low as -5^ of a second. If this difference be due 

 to different rates of nervous conduction, and not entirely to variations in the 

 rapidity of mental operations, it is evident that the velocity of the nerve-cur- 

 rent must vary very considerably in different individuals. 



Action of Electricity upon the Nerves. So long as the nerves retain their 

 excitability and anatomical integrity, they will respond to properly regulated 

 electric stimulus. Experiments may be made upon the exposed nerves in 

 living animals or in animals just killed ; and of all classes, the cold-blooded 

 animals present the most favorable conditions, on account of the persistence 

 of nervous and muscular excitability for a considerable time after death. 

 Experimenters most commonly use frogs, on account of the long persistence 

 of the physiological properties of their tissues and the facility with which 

 certain parts of the nervous system can be exposed. For ordinary experi- 

 ments upon nervous conduction, the parts are prepared by detaching the 

 posterior extremities, removing the skin, and cutting away the bone and 

 muscles of the thigh, so as to leave the leg with the sciatic nerve attached. 

 A frog's leg thus isolated presents a nervous trunk one or two inches (25 or 

 50 mm.) in length, attached to the muscles, which will respond to a feeble 

 electric stimulus. It is by experiments made upon frogs prepared in this 

 way that most of the important facts with regard to the action of electricity 

 upon the nervous system have been developed. 



In physiological experiments it is sometimes necessary to use different 

 forms of electrical apparatus in order to study different properties and 



