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the proportion of one to forty: now the perpetual tendency of 

 thirty-nine parts in forty is to become cold; the blood therefore in 

 the lungs should be at least many degrees hotter than that which 

 has reached the extremities, and which has imparted heat in its 

 course to structures so much more considerable than the organs 

 by which the heat was produced: and the same is to be said on 

 the supposition of any other source of heat; whereas there is in 

 fact no regular and assignable difference in the temperature of 

 blood at different places.* This fact seems a sufficient refutation 

 of a local source of caloric, while, if the fact were otherwise, it 

 would prove only a variety of temperature at different places; but 

 not that the seat of the highest temperature was the source of 

 caloric to all the rest; unless it were first shewn that the increase 

 of temperature in this seat was adequate to such a purpose, and 

 unless there was no reason to suppose that the faculty of producing 

 heat was elsewhere possessed. 



6 That heat is not conferred upon the structures by any 

 medium, as from a source, seems also to follow from the irregulari- 

 ties of temperature of different surfaces, either spontaneous or 

 producible by artificial means: thus I have known the temperature 

 of a paralytic arm generally, I believe always, lower than that of 

 the sound arm in the same subject; thus, aho, a division of nerves 

 will reduce the temperature of a limb, perhaps permanently, or at 

 least until re-union of the nerves has taken place, or their function 

 is otherwise supplied. The same consequence has succeeded to 

 the ligature on the arterial trunk of an extremity. Both these 

 instances conjoined prove that heat has a diffused, and not a 

 precise or exclusive, source; and that the degree of it is liable to 

 be affected by an agency upon parts which are intimately con- 

 nected with the vitality of the structures. Conceding then that 

 heat is formed in no single organ, but is a function common to all 

 structures (or some of their components), it is next to be inquired, 

 by what process this evolution of heat takes place, or what rela- 

 tions are engaged in the process] 



7. In speaking of the fecundated ovum, we have seen that 

 the ovum of viviparous animals commences immediately after 

 fecundation, the processes which establish fetal existence; while 

 in oviparous animals the ovum may wait a considerable time after 

 fecundation, and the same processes are never commenced in it 

 until it has acquired a temperature which it afterwards maintains. 



8. It appears therefore that heat is essential to. life, that the 

 ovum of viviparous animals, suffering no interval between fecun- 

 dation and the acquirement of heat, immediately begins the 

 characteristic acts oj lijc; while the ovum of birds, &c. do not 

 begin these acts until their life is adapted for the purpose by ex- 



* It is said that the temperature of blood is raised two decrees by the 

 conversion of venous into arterial blood: this increase of temperature, how- 

 ever, if true, may be easily shewn to be inadequate. 



