WHAT IS A COLD f 805 



and immediately a message is sent back to the eyelids to shut, and ex- 

 clude the particle of matter that threatens to enter the eye. All this 

 is done so quickly that it is hardly possible to realize that there is time 

 for reflex nervous action being brought into play. 



Another instance of reflex action, but this time influencing the 

 secretions, may be cited. Who is not familiar with the effect of a 

 savory smell, or the sight of some luxiu-y, upon the salivary secretion, 

 so that, to use a common expression, " the mouth waters " ? In the 

 first, the olfactory nerve is the means by which the impression is con- 

 veyed to the nerve-center ; in the other it is the optic nerve which is 

 the transmitting agent ; but in each case the impression is reflected to 

 that nerve controlling the salivary secretion, with the effect of produc- 

 ing an increased flow of saliva. We thus see that the secretions can 

 be influenced by one nerve conveying its impression to another whose 

 filaments take origin in a common center. 



Now, to come to the subject more directly under consideration in 

 this paper, we must comprehend how cold acting on one part of the 

 body produces catarrh of the nasal mucous membrane. Exposure to 

 the most intense cold for a lengthened period will not produce this 

 effect. Indeed, we find it invariably the case that severe frost in win- 

 ter is, so far as catarrh is concerned, the healthiest weather we can 

 have. During the prevalence of frost, as a rule, colds are at a mini- 

 mum. The system here shows its power of accommodating itself to 

 the circumstances surrounding it, and actually benefits by the prevail- 

 ing low temperature. Let us, however, suppose a person to be sit- 

 ting in a room the temperature of which is, say, 70 Fahrenheit, 

 and that a current of cold air is rushing in at an open door or win- 

 dow, and playing upon the back of his head, or it may be on his 

 legs or feet, and the probability is that he will "catch cold," and in 

 nine cases out of ten this cold will be a catarrh in the head, and, what 

 may appear more remarkable still, only one nostril will at first be 

 affected. Now, if the catarrh was due to the inhalation of cold air, 

 both nostrils would suffer ; but it is not so ; for, as each side of the 

 body is supplied by its distinct set of nerves, so only that side is af- 

 fected through which the reflex disturbance has been transmitted. The 

 modus operayidi is the following : The draught of cold air, acting, we 

 will suppose, on the back of the head, conveys through the sympathetic 

 nerve, which ramifies on the scalp, a shock to the nervous center from 

 which these nerve-fibers proceed ; but we must understand that this 

 nerve-center sends its filaments to other jjortions of the body, and so 

 the shock which this center receives by one set of nerves is reflected 

 by another set to some surface quite remote from that primarily acted 

 upon, and in this way a temporary paralysis of the nerves supplying 

 the blood-vessels of the mucous membrane of the nose is brought about. 

 In consequence, these vessels become dilated and engorged, and the 

 shock which has brought about this congestion continuing, disturbs the 



