4 HUMAN PHYSIOLOGY 



structure, e.g. a muscle-fibre, can spread itself throughout 

 the entire structure by conduction. 

 The stimuli are classified as : 



(1) Chemical, those which, by means of chemical action 

 upon the irritable substance, cause dissimilation. Electrical 

 stimuli act as chemical stimuli because the current produces, 

 by polarization at the places of entrance and exit, chemical 

 changes which are able to stimulate. 



(2) Physical; this includes mechanical (hitting, pulling) ; 

 thermal (heating) and photical (light which, for example, 

 stimulates the retina). The action of these physical stimuli 

 can be reduced to a common principle, that of concussion, 

 by which chemical changes producing stimulation are called 

 forth. 



Substances which can be decomposed by concussion are the 

 explosive bodies; they are compounds in which the union of the 

 atoms is unstable and which, by decomposition, form more stable 

 compounds. The irritable substance of the living being is, per- 

 haps, such an unstable compound. 



Influences which decrease the amount of metabolism are 

 called inhibitory agents and their effect is called inhibition. 

 The condition of inhibition in which the vital phenomena 

 entirely cease without the power of life being destroyed is 

 called latent life or biostition. In biostition there are many 

 living beings at the lowest temperatures which do not 

 destroy life, also desiccated seeds of plants, spores of 

 Bacteria, etc. 



Strong stimulation can also cause inhibition, which is then 

 called fatigue or exhaustion. This fatigue is due to the excessive 

 consumption of irritable material and partly to the harmful effect 

 of the dissimilation products (fatigue-substance) retained in the 

 irritable structure. A fatigued structure at rest recovers by means 

 of the reconstruction of the irritable substance and the removal of 

 the fatigue-substance. 



