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. Thenerve-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 heat. 
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ures, and pains, make up a world withdrawn from the 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 tomechanism. Such is the 
conviction, moreover, of Claude Bernard and of Helmholtz. 
