8o4 THE POPULAR SCIENCE MONTHLY. 



unnaturally abundant discharge ceases. Not so, however, when the 

 superabundance is due to the effects of cold ; for, in the latter case, a 

 diseased condition is set up, which will only disappear when the effects 

 of the exposure upon the nervous system have passed away. 



Having demonstrated that cold is not produced by the action of 

 cold air playing upon the part affected, but that, on the contrary, it is 

 an effect of cold acting upon a distant part of the body, it will be 

 necessary to explain how this is brought about. If a person sits in a 

 draught of cold air, and this draught is directed upon the back of his 

 head, the chances are that a catarrh of the nasal passages will result, 

 and this is produced by what is called reflex action of the nerves. 

 Here it. will be necessary to diverge a little, and explain what reflex 

 action is. It must be understood, then, that there are numerous ner- 

 vous centers connected with the spinal cord. These nervous centers 

 send filaments of their nerves to various portions of the body. For 

 example, a nerve-center may be placed alongside the spine in the neck, 

 and from this point nerves may be distributed to the back of the head 

 and the mucous membrane of the nose. One important function of 

 these little bodies is to control the supply of blood to different surfaces 

 and tissues and organs. This is done by a system of minute nerves 

 which are distributed on the arteries, by which the vessels are kept in 

 a state of contraction. Now, if these nerves are severed from the 

 main trunk, the blood-vessels immediately expand to the full extent of 

 their caliber, and congestion is the result ; or, if these nerves are para- 

 lyzed, the same effect is produced. Sometimes a very slight shock 

 produces a temporary paralysis of these minute nerves when a rush of 

 blood takes place into the arteries, of which blushing is a good exam- 

 ple ; but the nerves soon recover their control over the blood-supply, 

 and the blush passes away. Then, again, the shock may produce quite 

 the opposite effect : this may be so severe as to cause such extreme 

 contraction of the blood-vessels that a deadly pallor pervades the face, 

 as, for instance, in severe shock from fear. This, however, is caused 

 more by the effect of shock acting upon the nerve-centers which sup- 

 ply the heart with motor power. 



But let us suppose that one extremity of a nerve arising from a 

 particular nerve-center is irritated ; this is communicated to that cen- 

 ter, Avhich is affected thereby, it may be slightly or more severely. 

 The irritation may be so great as to prostrate for the time being the 

 nerve-center, and, in consequence, all the nerves arising from it are 

 thrown into a state of inaction. This is called the reflex action of that 

 nerve-center, because the effects of the irritant applied to one part of 

 the body are thereby reflected to other parts. Instances of reflex 

 action may be seen frequently in every-day life. Take, for example, 

 the action of the eyelid when an object threatens to enter the eye. 

 The retina perceives the object advancing ; this is telegraj^hed to the 

 nervous center supplying the muscles which open and shut the eyelids, 



