GENERAL PROPERTIES OF THE NERVES. 599 



more than one-tenth as rapid as at 60 or 70. Marey has also noted that the rate is 

 sensibly reduced by fatigue of the muscles. 



The same principle which has led to the determination of the rate of conduction in 

 motor nerves, viz., an estimation of the difference in time of the passage of a stimulus 

 applied to a nerve at two points situated at a known distance from each other, has been 

 applied to the conduction of sensations. Hirsch is quoted as having made the first 

 attempt to resolve this question, in 1851. He employed the delicate chronometric instru- 

 ments used in astronomy, and noted the difference in time between the appreciation of 

 an impression made upon a part of the body far removed from the brain, as the toe, and 

 an impression made upon the cheek. This process admitted of a rough estimate of 

 about one hundred and. eleven feet per second. The later and more elaborate researches 

 of Schelske showed a rapidity of conduction by the sensory nerves of about ninety-seven 

 feet per second. 



Attempts have been made to estimate the duration of acts involving the central ner- 

 vous system, such as the reflex phenomena of the spinal cord or the operations of the 

 cerebral hemispheres. These have been partially successful, 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 important fact ; although the duration of these acts has not 

 yet been measured with all the accuracy that could be desired. As the general result 

 of experiments upon these points, it is found that the reflex action of the spinal cord 

 occupies more than twelve times the period required for the transmission of stimulus 

 or impressions through the nerves. Donders found, in experiments upon his own person, 

 that an act of volition required one-twenty-eighth of a second, and one of simple dis- 

 tinction or recognition of an impression, one-twenty-fifth of a second. These estimates, 

 however, are merely approximative ; and, until they attain greater certainty, it is unne- 

 cessary to describe in detail the apparatus employed. 



The general result of the various observations we have detailed upon the rate of ner- 

 vous conduction as applied to the human subject is, in the first place, that this can be 

 measured with tolerable accuracy ; second, that it is in no wise to be compared with the 

 rate of conduction of light or electricity ; and, finally, that the rate in the human subject 

 is essentially the same in the motor and sensory nerves, being, according to the most 

 reliable estimates, about one hundred and eleven feet per second. 



Action of Electricity upon the Nerves. A great deal has been written with regard to 

 the effects of electricity upon the nervous system, and facts elicited by experiments upon 

 this subject are highly important in their bearing upon physiology and pathology. Still, 

 there are numerous observations upon this subject which have but little importance, in a 

 purely physiological sense, except that they are curious and interesting. These we do 

 not propose to discuss elaborately ; and we shall confine ourselves chiefly to those points 

 which bear directly upon our knowledge of the properties and functions of the nerves. 



The first important fact to which we have already alluded is, that electricity is the 

 best means that we have of artificially exciting the nerves. Using electricity, we can 

 regulate with exquisite nicety the degree of stimulation ; we can excite the nerves long 

 after they have ceased to respond to mechanical or chemical irritation ; the effects of 

 different currents can be noted ; and, finally, this mode of stimulation produces a peculiar 

 and interesting condition of the parts of the nerve not included between the poles of the 

 battery. For these reasons, it seems proper to devote some consideration, in this con- 

 nection, to the effects of the application of this agent to the nerves. 



So long as the nerves retain their irritability, they will respond to an electrical stimu- 

 lus. Experiments may be made upon the exposed nerves in living animals or in ani- 

 mals just killed; and, of all classes, the cold-blooded animals present the most favorable 

 conditions, on account of the persistence of nervous and muscular irritability for a con- 

 siderable time after death. Experimenters most commonly use frogs, on account of the 



