THE ORIGIN OF NERVES. 275 



but one kind of impulse or impression, the impression in 

 the present case resulting from that disturbance of the ether 

 and light-rays which gives origin to the sense of sight, 

 just as disturbance of another kind, producing sonorous 

 vibrations, results in the production of sound, and in its 

 appreciation by sense-organs, the ears, specially adapted 

 to receive such an impression. Received by the organ of 

 sight, the impression is conveyed to the nearest " nerve- 

 centre," represented in this case by a part of the brain. 

 Only when the impression has reached the brain do we 

 " see," in the true sense of the term. For the sense of 

 sight, involving a knowledge and appreciation of what is 

 seen, is resident not in the eye, but in the brain, as repre- 

 senting that part of the nervous system where the act of 

 ''knowing" is performed. Thus we note that an impulse 

 is conveyed inwards to the brain ; and we may call this 

 a "sensory" impression, since it has been received by a 

 " sense "-organ, and has moreover given rise to a "sensa- 

 tion," that of sight. 



But the actions which follow the impression made upon 

 the organ of seeing do not end thus. Active exertion the 

 withdrawal of the head and the closure of the eyelids 

 follows the sensation. How, then, is this action related to 

 the appreciation by eye and brain of the threatened danger ? 

 Because, we may reply, the brain transmits another and a 

 different impulse or command to the muscles of the head 

 and neck and to those of the eyelids, sets these muscles in 

 action, and thus produces movements destined to save the 

 body from the act of our assailant. There is thus illustrated 

 the great principle of reflex action, with the discovery and 

 enunciation of which the name of Marshall Hall is so 

 worthily associated. We note that an impression which we 

 have named " sensory " passed inwards (Fig. 46) through a 

 "gateway of knowledge," the eye (E), to the brain (B) ; and, 

 conversely, we note that a second impulse is sent outwards 

 from the brain to the muscles of the neck and eyelids (s), 

 directing the movement of the former and the closure of the 



