408 THE POPULAR SCIENCE MONTHLY. 



thought and feeling can be estimated in heat-units; he even asserts 

 that there is no common measure between intelligence and heat ; but 

 less timid physiologists are not wanting who reduce every kind of 

 vital manifestation to the strict laws of thermo-dynamics. A few suc- 

 cinct remarks may perhaps shoAV that such physiologists 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 differ- 

 ence that the former 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, lias 

 in itself an ever-present, never-exhausted power of action, of which the 

 energy is its peculiar property. 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 limited 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, re- 

 mains 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 thoughts, desires, pleasures, and pains, make up a world withdrawn 

 from the common conditions of determination. That superior force 

 which, ruling all the highest animal activities, decides, suspends, 

 checks, and governs the very transformation of heat into movement ; 

 which, asserting its independence within us, call it by what oldest 

 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. 



II. 



Independently of the slight and usual variations that heat may 

 present in the same species, and those it exhibits in passing from one 

 zoological group to another, we may consider the changes it undergoes 

 in the same individual, influenced by the various disturbances of the 

 system. Although it remains almost insensible to modifications of the 

 surrounding temperature, it is not the same when the complete equi- 

 librium of the organs is affected. The concord between the different 

 parts of the organism and the functions they discharge is so perfect 

 that the least trouble is reflected among them, and sends disorder 

 everywhere. The nervous system, charged with keeping up harmoni- 

 ous communication between all points of the living being, first takes 

 note of the change befalling, and transmits its abnormal impression 

 into all quarters. It is not the generator, but it is the regulator, of 



