^6 THE DATA OF BIOLOGY. 



the probable interpretation of tlie visual process; though 

 whether it is the accepted one I do not know. 



But now, carrying with us the conception made clear by 

 the first eases and suggested by the last, we shall appreciate 

 the extent to which this general physiological method, as we 

 may call it, is employed. The convulsive action caused by 

 tickling shows it conspicuously. An extremely small amount 

 of molecular change in the nerve-endings produces an im- 

 mense amount of molecular change, and resulting molar 

 motion, in the muscles. Especially is this seen in one whose 

 spinal cord has been so injured that it no longer conveys 

 sensations from the lower limbs to the brain; and in whom, 

 nevertheless, tickling of the feet produces convulsive actions 

 of the legs more violent even than result when sensation 

 exists: clearly proving that since the minute molecular 

 change produced by the tickling in the nerve-terminals can- 

 not be equivalent in quantity to the amount implied by 

 the muscular contraction, there must be a multiplication of 

 it in those parts of the spinal cord whence issue the reflex 

 stimuli to the muscles. 



Returning now to the question of metabolism, we may see 

 that the processes of multiplication above supposed to take 

 place in muscle, are analogous in their general nature to 

 various other physiological processes. Carrying somewhat 

 further the simile used in § 15 and going back to the days 

 when detonators, though used for small arms, were not used 

 for artillery, we may compare the metabolic process in 

 muscle to that which would take place if a pistol were fired 

 against the touch-hole of a loaded cannon : the cap exploding 

 the pistol and the pistol the cannon. For in the case of the 

 muscle, the implication is that a nervous discharge works in 

 certain unstable proteids through which the nerve-endings 

 are distributed, a small amount of molecular change; that 

 the shock of this causes a much larger amount of molecular 

 change in the inter-diffused carbo-hydrate, with accompan3'ing 

 oxidation of its carbon ; and that the heat liberated sets up a 

 transformation, probably isomeric, in the contractile substance 



