AND PHYSICAL FORCES 13 



performs its manifold muscular movements. That this is so, 

 however, we know ; and we know, moreover, that as a mere 

 machine for the conversion of heat into mechanical energy, the 

 muscle far excels the best steam-engine which has ever been 

 constructed. 



3. Turning now to the third mode of vital activity — to that which 

 manifests itself in the display of nervous phenomena — we find that 

 these manifestations are also closely dependent upon the integrity 

 of certain material structures, and that their appearance coincides 

 with an increase in the quantity of heat appreciable in, or in the 

 neighbourhood of, these structures. 



It has been ascertained very definitely by the experiments of 

 Helmholtz and of Schiff, that the transmission of a stimulus 

 through a nerve is marked by a rise of temperature therein ; 

 while the extremely interesting experiments of Lombard seem 

 to show that a similar though minute rise of temperature takes 

 place in the brain itself when it is in a state of activity. 



The molecular motion or energy, set free in the nervous system, 

 subserves very different purposes. Upon evidence which cannot 

 now be gone into, it could be shown that the nervous system plays 

 an important part (a) in regulating the various secretions and in 

 influencing the nutrition of the body generally. It is nerve-force 

 again (6) which initiates or calls into play the activity of the various 

 muscles by which the countless movements within the bodies 

 of animals are produced, and also those by which locomotion and 

 external visible movements generally are effected. But nerve- 

 changes also (c) give rise to other manifestations — manifestations 

 altogether peculiar in kind and peculiar to the individual in whom 

 they occur. Feeling is the basis of ' Consciousness,' and of 

 ' Intellectual Actions ' and these are phenomena which are com- 

 monly believed to be called into existence by the activity and the 

 occurrence of molecular changes within certain parts of the 

 nervous system and of the brain generally. 



From a consideration of these different modes of activity of the 

 nervous system it may easily be imagined how hopeless the attempt 

 would be to establish anything like a quantitive estimate of the 

 amount of force answering to the above-mentioned different results 

 of nervous action. In considering the question of muscular activity 

 and its correlation with physical force, we have to do with a 

 measurable effect under the form of mechanical energy. But the 

 manifestations of the activity of the nervous system are much more 



