152 BIOLOGY: GENERAL AND MEDICAL 



an enormous number of nerve fibres connect with the 

 brain, while to utilize the impressions a second group of 

 fibres must connect the receiving cells with many other 

 parts of the brain, and a third group of fibres leaves the 

 brain in efferent course to apply the information in some 

 such form as muscular action, for example, to the general 

 good of the body as a whole. 



It is difficult for one not acquainted with the details 

 of nervous structure to conceive of the complexity of 

 nervous activity arising in the course of a single and 

 apparently trifling act. A few moments ago, having clip- 

 ped some papers, you carelessly laid the sharp pointed 

 scissors on the desk where a little later they were covered 

 with some papers and a blotter. Moving your hand to 

 brush the accumulation aside, you felt a sharp prick, 

 found your hand involuntarily drawn away, and recog- 

 nized that you had unexpectedly injured yourself. The 

 point of the scissors touching the skin stimulated a 

 peripheral nerve ending in so violent fashion that a 

 double excitation followed, almost simultaneously regis- 

 tering pain in the receptive centres of the brain and 

 stimulating a motor centre in the spinal cord by which 

 an impulse was sent out to the muscles of the arm 

 which was quickly drawn away by their contraction. 

 In the meantime, the metal impression is being rapidly 

 passed about from cell group to cell group until it arrives 

 at a group of cells formerly stimulated in the same 

 manner which now feebly revive the sensation, as one 

 produced by a sharp object, and then to another group 

 of cells which recall the scissors, and from these to others 

 by which you become reminded of all that was done a 

 short time before and that you had left the scissors on 

 the table. The revived memories in these nerve cells 

 thus define themselves as thoughts, appearing at first 

 with such rapidity that they were very indistinct, but 

 becoming more and more clear as time is allowed for each 

 to arise, and as attention is directed toward it. Indeed, 

 if no means of interrupting the course of nervous dis- 



