552 THE POPULAR SCIENCE MONTHLY. 



our nervous power is produced. You might liken it roughly to the 

 galvanic battery by which the electric telegraph acts, the white or 

 fibrous portion of the brain and nerves being like the conducting wires 

 of the telegraph. Just as the fibres of the nerves establish a commu- 

 nication between the organs of sensation and the ganglionic centres, 

 and again between the ganglionic centres and the muscles, so do the 

 white fibres, which form a great part of the brain, establish a commu- 

 nication between the gray matter of the convoluted or folded surface 

 of the Cerebrum and the Sensory Ganglia at its base. Now, I believe 

 that this sensory tract which lies at the base of the skull is the real 

 Sensorium, that is, the centre of sensation ; that the brain at large, the 

 cerebrum, the great mass of which I have been speakiug, is not in itself 

 the centre of sensation ; that, in fact, the changes which take place in 

 this gray matter only rise to our consciousness only call forth our 

 conscious mental activity when the effect of those changes is trans- 

 mitted downward to this Sensorium. Now, this Sensorium receives the 

 nerves from the organs of sense. Here, for instance, is the nerve from 

 the organ of smell, here from the eye, and here from the body generally 

 (the nerves of touch), and here the nerves of hearing every one of 

 these has its own particular function. Now, these Sensory ganglia 

 have in like matter reflex actions. I will give you a very curious 

 illustration of one of these reflex actions : You all know the start we 

 make at a loud sound or a flash of light ; the stimulus conveyed through 

 our eyes from the optic nerve to the central ganglion causing it to send 

 through the motor nerves a mandate that calls our muscles into action. 

 Now, this may act sometimes in a very important manner for our pro- 

 tection, or for the protection of some of our delicate organs. A very 

 eminent chemist a few years ago was making an experiment upon some 

 extremely explosive compound which he had discovered. He had a 

 small quantity of this compound in a bottle, and was holding it up to 

 the light, looking at it intently ; and whether it was a shake of the 

 bottle or the warmth of his hand, I do not know, but it exploded in his 

 hand, the bottle was shivered into a million of minute fragments, and 

 those fragments were driven in every direction. His first impression 

 was, that they had penetrated his eyes, but to his intense relief he found 

 presently that they had only penetrated the outside of his eyelids. 

 You may conceive how infinitesimally short the interval was between 

 the explosion of the bottle and the particles reaching his eyes ; and yet 

 in that interval the impression had been made upon his sight, the man- 

 date of the reflex action, so to speak, had gone forth, the muscles of his 

 eyelids had been called into action, and he had closed his eyelids before 

 the particles reached them, and in this manner his eyes were saved. 

 You see what a wonderful proof this is of the way in which the auto- 

 matic action of our nervous apparatus enters into the sustenance of 

 our lives, and the protection of our most important organs from injury. 

 Now I have to speak of the way in which this Automatic action of 



