ANIMAL HEAT. 



681 



the economy and the degree of the external 

 temperature, watery vapour tends to refrigerate 

 still more in winter, and to add to the heat in 

 summer. 



The state of the atmosphere in regard to 

 motion or rest modifies to a great extent the 

 effects of a given temperature upon the body. 

 Refrigeration by simple contact increases in 

 amount with the rate of motion of the air. The 

 same law holds good in regard to evaporation, 

 and indeed this process always complicates the 

 results proceeding from simple contact. The 

 cause of refrigeration in this case is consequently 

 double. It is easy, therefore, to imagine how 



rerful a cause of cooling a cold wind must 

 But observation can alone give any ade- 

 quate idea of the extent of its influence in this 

 respect. Mr. Fisher, one of the surgeons in 

 the expedition under the command of Sir 

 Edward Parry to the Polar Seas, has given us 

 an account of its extraordinary effects. In the 

 frozen regions around the arctic circle, the 

 hardy voyagers under Capt. Sir E. Parry found 

 that they could stand a cold adequate to freeze 

 mercury when the air was perfectly calm, much 

 more easily than a temperature nearly 50 F. 

 higher when it blew. The air in motion in 

 this case, therefore, produced a sensation of 

 cold that was equal to such a depression of 

 temperature as is indicated by a fall of 50 of 

 the scale of F. a most prodigious difference. 



Sudden transitions of temperature also exert 

 a great influence independently of any limits ; 

 in the first place, because the intenseness of 

 the sensation of cold or of heat is in propor- 

 tion to the suddenness of the abstraction, or of 

 the communication of heat ; and again, be- 

 cause the faculty of adaptation to different 

 degrees of external temperature is not acquired 

 all at once, but is only attained in a certain 

 lapse of time, and by gradual modifications 

 in the constitution. We therefore see that 

 those countries of which the temperature is 

 very high in the day, but very low in the night, 

 are subject to diseases that seem to belong 

 more peculiarly to cold and moist latitudes, 

 or to marshy lands where malaria prevails. 

 But the transition from hot to cold is not 

 limited to the suddenness of the thermal de- 

 pression ; it extends to the refrigeration by the 

 action of the wind. This is another among 

 the many reasons why in the latitudes of Eng- 

 land, France, &c. spring is a more dangerous 

 season than autumn. There are, however, cer- 

 tain cases of sudden transition that are useful 

 and salutary, as for instance,when the heat of the 

 body is excessive, and is doing mischief, whe- 

 ther it be induced by an elevated external 

 temperature, or proceeds from the violent and 

 involuntary action of our organs. Then re- 

 frigeration even of the most sudden kind, pro- 

 vided it be restrained within proper limits, 

 becomes beneficial. It is thus that the 

 affusion of cold water produces such excellent 

 effects in cases of extreme excitement, and 

 where the temperature is really above the 

 natural standard. This process is even to be 

 regarded as one of the most brilliant tri- 

 umphs of modern medicine. It is much to 



be regretted that recourse is not had to it more 

 frequently. It is evident that the proper time 

 for the use of this powerful means is that in 

 which congestion has not yet passed into ob- 

 stinate engorgement, that is to say, in the 

 beginning of the disease, in which by allaying 

 excitement congestion is diminished. The 

 favourable moment for using the cold affusion is 

 that in which the skin is hot and dry, which 

 is also the period of the highest excitation. 

 The experiments upon the effects of baths, 

 quoted above, tend also to show the propriety of 

 the practice ; in citing these, we mentioned that 

 the diminution of temperature produced in the 

 body lasted for hours, and that the reaction 

 consequent upon the use of the bath did not 

 carry the temperature higher than the pitch it 

 possessed at starting. It is obvious that the 

 effects of the cold affusion are to be derived 

 from the principles previously established; 

 since we have referred the production of heat 

 to two general conditions of the economy, 

 one of which is the state of the nervous sys- 

 tem. Now the affusion of cold water acts 

 directly upon this system. There is another 

 powerful method of tempering animal heat, 

 which flows from the other general condition, 

 upon which the production of heat depends, 

 viz. the state of the blood. We have seen 

 above that the respective proportions of the 

 serous mass of the blood and of its red glo- 

 bules exert an important influence; that in 

 the class of vertebrate animals which produce 

 smaller quantities of heat, the proportion of 

 the serum was in the inverse ratio of the 

 faculty of calorification. Whence it follows, 

 that in cases of excessive heat of body, to 

 reduce the quantity of red globules would 

 prove an effectual mode of reducing the tem- 

 perature. Now this is precisely what is done 

 by bloodletting. The effect, however, in this 

 way is not instantaneous. The first influence 

 of bloodletting is simply to lessen the quan- 

 tity of the blood, and this is the extent to 

 which ideas of the influence of the abstraction 

 of blood are generally confined. There is, 

 however, a consecutive influence, which is at 

 the least as important, and which proves much 

 more lasting. As the person who has been let 

 blood confines himself at the same time to 

 low diet, and principally to liquids, it is obvious 

 that the blood is recruited in its quantity 

 principally by additions of watery particles, 

 without any notable or even sensible addition 

 of globules. The blood is therefore altered 

 essentially in its constitution; the proportion 

 of its component fluid and solid elements is 

 changed, and this in direct proportion to the 

 extent and frequency of the venesections. The 

 consequence of this is a diminution of tempe- 

 rature, unless other causes oppose such an ef- 

 fect. 



Bloodletting, it must be observed, is not the 

 sole means of accomplishing such a change in 

 the constitution of the blood. We can pro- 

 duce a similar effect by exciting one or all of 

 the secretions which are thrown off by the 

 body. Secretion is performed at the cost of 

 the blood, which supplies both of its elements 



