40 PHYSIOLOGY. 



increased that they cannot bear the smallest degree of light or 

 noise."" 4. The state of the same medullary substance, as 

 affected by heat or cold. " A certain degree of heat is neces- 

 sary to the subsistence of the body in the living state ; but if 

 life consists especially in a certain condition of the nervous 

 system, it will follow that heat has indeed the power of giving 

 that condition to the nervous system upon which the functions of 

 life, and perhaps sensation, necessarily depend. Hence, accord- 

 ing to the different degrees of heat applied to our nerves, they 

 are more or less exquisitely sensible; and I have no doubt 

 that this is the just solution of the ordinary fact of the greater 

 sensibility of men inhabiting warmer climates, than that of men 

 inhabiting cold climates. But if there is any doubt with regard 

 to the effects of heat, there is none with regard to the effect of 

 cold. We have no fair instance of the effect of cold on the 

 retina and auditory nerve ; but with regard to the nerves 

 more exposed to surrounding bodies, we know most certainly, 

 that a certain degree of cold destroys the feeling altogether, 

 and considerably affects the sensibility of the gustatory and 

 olfactory nerves. I say, therefore, the sensibility is according 

 to the state of the medullary substance as affected by heat and 

 cold. But however heat may be produced, it is probable that 

 it is connected particularly with the blood, and is distributed to 

 the different parts by the blood ; for we find that upon subtracting 

 the circulation, the heat ceases as soon as it would do in a body 

 of the same bulk which has acquired it. The action of the 

 arteries, therefore, which are joined to the nervous extremities, 

 has the effect of giving not only the tension, but also the heat 

 necessary to sensation." 5. The state of it, as produced by 

 former impressions (XLIII. LIV.) " We see this particu- 

 larly in its power of diminishing the power of those impres- 

 sions which are the cause of fear ; and the courage of the sailor 

 and soldier, for example, are to be explained in no other way 

 than by their having the impressions so often repeated as to lose 

 their force ; and we have instances of the same in other parts 

 of human life. Let us consider what may give a general char- 

 acter of more timidity and intrepidity ; we observe it in the 

 difference between the inhabitants of a rude and uncultivated 

 climate, and those of a civilized and polished nation. The in- 



