154 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



be injured, and the normal surface l>c compared with the dying or dead cross 

 section, the second phase is absent. If the nerve be frequently excited, each 

 excitation awakens a separate currenl of action. 



Although nerves are excited most readily by electric currents, negative 

 variations of the demarcation current maybe called out by various chemicals 

 — e.g., salt or glycerin ' — and by mechanical excitation, such as a sharp cut 

 with the shears. 2 It is a physiological phenomenon, for a negative change 

 may be observed to accompany a uerve impulse which has been caused by 

 the spread of excitation from central neurones along their peripheral axones. 



Du Bois-Reymond observed with the galvanometer a lessening (''negative 

 variation ") of the demarcation current (" current of rest ") when in strychnia- 

 poisoning the spinal motor neurones were sending vigorous impulses along 

 their axones and causing cramp-like tetanic muscular contractions. Gotch 

 and I Iorslev ; applied electrodes connected with a capillary electrometer to 

 peripheral nerves, spinal nerve-roots, and tracts of motor fibres within the 

 spinal cord, and discovered that if the cortical brain-cells in the motor zones 

 were excited, the nerves showed currents of action corresponding in rate to 

 the discharge of motor impulses from these brain-cells — e.g., if the epilepti- 

 form convulsions were occurring at the time, the capillary electrometer 

 revealed changes of potential of like rate in the nerves. 



Macdonald and Rwd ' observed currents of action in the phrenic nerve 

 of mammals which corresponded in time with the respiratory movements. 

 These were due to the normal discharge of nerve impulses from the respira- 

 tory centers. When a condition of apnoea was established and the respiratory 

 movements ceased, the electrical change failed to appear; when the respiratory 

 movements were quickened in dyspnoea, the rhythmic movements of the gal- 

 vanometer were quickened to correspond ; when during asphyxia the respi- 

 rations were of the Cheyne-Stokes type, the currents of action showed a like 

 rhythm. 



Even physiological sensory nerve-impulses have been found to produce 

 negative variation currents. Stcinach 5 observed currents of action to be 

 caused by mechanical pressure on the skin of the frog. If the pressure was 

 continued, the negative change gradually decreased, and a new negative 

 variation was seen if the pressure was suddenly removed. 



Light falling on the retina of the eye of a frog has been seen to cause a 

 negative variation of the current of rest of the optic nerve. 



The electrical change which we call the current of action can be thought 

 to sweep over the nerve as a wave, having in the inedullated nerves of the 

 frog a length of IS mm., and travelling at the rate of 28 meters per second. 

 The duration of the negative variation is different in different kinds of nerves 



1 Kiihne un<l Steiner : I r ntersuchungen mix der physiologischen Laboratorium i* Heidelberg, 1880, 

 Bd. iii. S. 1 19. 



2 Hering: Lotos, Neue Folge, 1888, Bd. 9, S. 3-",. 



3 Physiological Transactions, 1891, vol. 182, pp. 267-526. 



* Macdonaltl and Keed : Journal of Physiology, 1898, vol. xxiii. p. 100. 

 5 Steinach: Pftuger , a Archiv, 1896, Bd. lxiii. 8. 495. 



