EVOLUTION OP HEAT. 615 



ternal temperature is reduced below a certain point, there is an entire cessation 

 of all vital activity. 1 But there are certain tribes, especially Birds and Mam- 

 mals, which possess the power of generating Heat within themselves, to such a 

 degree as to render the rate of their vital processes almost entirely independent 

 of external influences j and there is probably no one species that can exercise 

 this power more effectually, and through a greater range of external conditions, 

 than Man is able to do. Of this we shall presently have evidence. The evolu- 

 tion of Light, again, is by no means an unusual phenomenon among the lower 

 tribes of Animals ; but where it does occur, it usually appears to have some 

 special purpose, as is obvious enough in the case of the glowworm and other 

 luminous Insects. But the luminosity which is occasionally exhibited in Man 

 must be regarded as an altogether abnormal phenomenon, whose physiological 

 interest arises out of the peculiarity of the circumstances under which it presents 

 itself. Of the degree in which Electricity is generated in the living body, we 

 know comparatively little. There is strong evidence that a disturbance of Elec- 

 tric polarity must take place in every action of Organic as well as of Inorganic 

 Chemistry ; and thus that every molecular change in the Animal as well as in 

 the Vegetable organism must involve an alteration in its electric condition. But 

 it would seem that in the Animal body generally, these alterations are made to 

 balance each other so exactly, that no considerable disturbance of the electric 

 equilibrium ordinarily takes place in the organism as a whole ; and it is only in 

 certain peculiar cases (as in the Electric Fishes) that a provision exists for the 

 generation of Electricity in considerable amount and intensity, with a view to 

 some special purpose. In the Human subject, however, an extraordinary pro- 

 duction of free Electricity, as of Light, occasionally presents itself; and this, 

 taken in connection with other evidence, would seem rather to indicate a depart- 

 ure from the usual balance between the opposite electrical changes continually 

 taking place, than to be due to the introduction of any extraordinary sources of 

 electric disturbance. 2 



2. Evolution of Heat. 



650. All the vital actions of the body of Man, as of that of "warm-blooded" 

 animals generally, require an elevated temperature as a condition of their per- 

 formance ; and the high degree of constancy and regularity which is observable 

 in these actions appears to depend in great degree upon the provision which 

 the organism contains within itself for the maintenance of that temperature at 

 a fixed standard. This constancy and regularity are most remarkably exhibited 

 in the various periodical changes to which the body is subject both in health 

 and disease; the uniformity of whose recurrence is due to a corresponding uni- 

 formity in the rate of vital action taking place in the interval. Thus, as will 

 be shown hereafter, the period of parturition is in great degree determined by 

 the maturation of the foetal structures ; and the uniformity of the time which 

 this requires (like the corresponding uniformity in the period of development in 

 the embryo bird) may be fairly attributed to the regularity of the supply of 

 Heat, which is the power that especially determines the formative operations. 

 For the periods of all similar phenomena in " cold-blooded" animals, which have 

 no power of maintaining an independent temperature, exhibit no such uniformity; 



1 See "Princ. of Phys., Gen. and Comp.," CHAP. in. Sect. 3, Am. Ed. 



2 Having recently had an opportunity of witnessing some of the experiments made by 

 M. Du Bois Reymond with a magneto-electrometer of extraordinary sensitiveness, the Au- 

 thor can bear his personal testimony to the fact, that the electricity even of the corre- 

 sponding fingers of the two hands is very seldom equally balanced, and that the existence 

 of even the slightest scratch or abrasion of surface upon one of them produces a very 

 marked disturbance. 



