HEAT AND LIFE. 141 



physiologists are not wanting who reduce every kind of 

 vital manifestation to the strict laws of thermo-dynamics. 

 A few succinct remarks may perhaps show that such physi- 

 ologists err. 



A comparison between the muscular and the nervous 

 systems from the point of view of their connection with 

 heat is a bold one for many reasons. Between nerve and 

 muscle there exists this enormous difference that the for- 

 mer is endowed with a spontaneity denied to the latter. 

 Muscular fibre never contracts of its own accord ; it needs 

 a stimulus its energy is borrowed. The nerve-cell, on the 

 contrary, has in itself an ever-present, never-exhausted 

 power of action, the energy of which is its peculiar prop- 

 erty. Both evidently derive the principle of the activity 

 that marks them from the same external and internal 

 media ; but, while the muscle, a mechanical organ, is lim- 

 ited to the obedient transformation of the force assigned to 

 it, under the form of heat, into a measurable amount of 

 work, the nerve, a vital organ, remains impenetrable and 

 inaccessible to our calculations, and exerts its characteristic 

 and sovereign powers in its own way, through a series of 

 operations that escape all estimates of their force and he at. 

 On the part of the muscular system, every thing can be 

 measured; on the part of the nervous system, nothing. 

 Impressions, sensations, affections, thoughts, desires, pleas- 

 ures, and pains, make up a world withdrawn from the 

 common conditions of determination. That superior force 

 which, ruling all the highest animal activities, decides, sus- 

 pends, checks, and governs the very transformation of heat 

 into movement ; which, asserting its independence within 

 us, call it by whatever name we may soul, will, or freedom 

 remains the most undeniable, though the most mysterious, 

 certainty of our consciousness this force protests against 

 the degradation of cerebral life to mechanism. Such is the 

 conviction, moreover, of Claude Bernard and of Helmholtz. 



