618 Professor Jagadis Chunder Bose [May 10, 



tion, whereas the falling portions exhibit recovery (see curve in 



Fig- I)- 



If, instead of a single stimulus, a succession of 



Incomplete stimuli be superposed, the frequency of individual 



Tetanus. contractions also increases ; the muscle has not 



time to recover; we get a jagged curve (a 1 , Fig. 11) . 



But when the frequency is sufficiently increased, 



Complete the intermittent effects are fused, and we get an 



Tetanus. almost unbroken curve. When the muscle attains 



its maximum contraction (corresponding to the 



frequency and strength of stimulus), it appears to be held rigid, and 



recovers only on the cessation of stimulus (b 1 , Fig. 11). 



When the muscle is continuously excited it grows 

 Fatigue. fatigued. The height of the curve grows con- 



tinuously less. This is seen in a series of single 

 twitches (Fig. 4). It is also seen in tetanus, where there is a decline 

 of the upper portion of the curve. 



Drugs may act as stimulants, or produce depres- 

 Influence sion, according to their nature. As extreme cases 



of Drugs. of such depressing agents we may instance poi- 



sons, which kill the response of living tissue. All 

 signs of sensibility then disappear. 



This mechanical method of studying the response 



Other Modes of living substances is, however, very limited in 



of Expression its application. For example, when a piece of 



of Living nerve is stimulated, there is no visible change of 



Response. form. When light falls on the retina there is no 



change of form, but it responds by transmitting 



to the brain a visual impulse. What, then, is this visual impulse 



which is sent along the optic nerve, causing the sensation of light? 



Thanks to the work of Homgren, Dewar, McKendrick and others, 

 it is possible to answer this question. If we excise an eye, say of a 

 frog, and substitute a galvanometer in the visual circuit in the place 

 of the perceiving brain, it is found that each time a flash of light 

 falls on the eye there is produced an electric twitch — that is to say, 

 there is a sudden ju'oduction of current, which ceases on the cessation 

 of light-stimulus. Stronger light produces stronger electric twitch 

 in the galvanometer, just as it produces stronger sensation in the 

 brain. The visual impulse thus appears to be the concomitant of an 

 electric impulse. This conclusion is supported by the fact that a 

 luminous sensation is occasioned (without the action of light), by 

 simply sending an electric current to the brain through the optic nerve. 

 The visual circuit is therefore like an electric circuit. The retina 

 is a potential voltaic element. The nerve is the conductor. The brain 

 is the detector of current, or a very highly sensitive galvanometer. 

 Unless these three elements are in good order, no light-message can 

 be perceived. We must have the current-generator or retina, and 



