ELECTRICAL CUERENTS DURING ELECTROTONUS. 



5 6l 



Rectinal and Eye Currents. If a freshly-excised eyeball be placed on the non-polarisable 

 electrodes connected with a galvanometer, and if light fall upon the eye, then the normal eye- 

 current from the cornea ( + ) to the transverse section of the optic nerve ( - ) is at first increased. 

 Yellow light is most powerful, and less so the other colours {Holmgren, M'Kendrick and 

 Deioar). The inner surface of the passive retina is positive to the posterior. When the retina 

 is illuminated there is a double variation, a negative variation with a preliminary positive 

 increase ; while, when the light ceases, there is a simple positive variation. Retinae, in which 

 the visual purple has disappeared owing to the action of light, show smaller variations {Kiihne 

 and Steiner). 



Stimulation of the secretory nerves of the glandular membranes, besides causing secretion, 

 affects the current of rest {Roeber). This secretion-current passes in the same direction in the 

 skin of the frog and warm-blooded animals as the current of rest, although in the frog it is 

 occasionally in the opposite direction {Hermann). If the current be conducted uniformly from 

 both hind feet of a cat, on stimulating the sciatic nerve of one side, not only is there a secretion 

 of sweat ( 288), but a secretion- current is developed {Luchsinger and Hermann). If two 

 symmetrical parts of the skin in the leg or arm of a man be similarly tested, and the muscle of 

 one side be contracted, a similar current is developed. Destruction or atrophy of the glands 

 abolishes both the power of secretion and the secretion-current. There is no secretion-current 

 from skin covered with hairs, but devoid of glands {Bubnoff). [The secretion-current from the 

 submaxillary gland is referred to in 145 {Bayliss and Bradford).] 



333. ELECTROTONIC CURRENTS IN NERVE AND MUSCLE. [When a 

 constant current called the ; ' polarising current " is passed though a stretch of 

 nerve, the nerve is thrown into a peculiar condition, called the " electrotonic con- 

 dition," or briefly electrotonus. In this condition, the vital properties of the 

 nerve are modified, i.e., 



(1) Its electromotivity ( 333). 



(2) Its excitability ( 335). 



The former is considered in this section, and the latter in a subsequent section.] 



1. Positive Phase of Electrotonus. If a nerve be so arranged upon the 

 electrodes (fig. 407, I) that its transverse section 

 lies on one, and its longitudinal on the other elec- 

 trode, then the galvanometer indicates a strong 

 current. If now a constant current be transmitted 

 through the end of the nerve projecting beyond the 

 electrodes (the so-called "polarising" end of the 

 nerve), and if the direction of this current coincide 

 with that in the nerve, then the magnetic needle 

 gives a greater deflection, indicating an increase of 

 the nerve-current " the positive phase of elec- 

 trotonus." The increase is greater the longer 

 the stretch of nerve traversed by the current, the 

 stronger the galvanic current, and the less the dis- 

 tance between the part of the nerve traversed by 

 the constant current and that on the electrodes. 



2. Negative Phase of Electrotonus. If in the 

 same length of nerve, the constant current passes 

 in the opposite direction to the nerve-current (fig. 

 407, II), there is a diminution of the electro- Fig. 407. 

 motive force of the latter "negative phase of Nerve-current in electrotonus. a, 

 electrotonus." galvanometer; b, electrodes; E, 



3. Equator. If two points of the nerve equi- constan * current. 



distant from the equator be placed on the electrodes (III), there is no deflection of 

 the galvanometer needle (p. 555, 4). If a constant current be passed through one 

 free projecting end of the nerve, then the galvanometer indicates an electro-motive 

 effect in the same direction as the constant current. 



Electrotonus. These experiments show that a constant current causes a change 

 of the electro-motive force of the part of the nerve directly traversed by the constant 



2n 



