6 BIOTIC STKUCTURE AND BIOTIC ENERGY 



colloidal combinations intrinsically present in the heart protoplasm, 

 and the osmotic pressure of the products of chemical reaction occur- 

 ring in each discharge. Just as tightening or slackening the hair- 

 spring will quicken or slow the beat of the watch, so increasing or 

 diminishing carbon- dioxide pressure will quicken or slow the heart- 

 beat. Again, more permanent chemical combinations such as 

 those of products of fatigue, anaesthetics, or cardiac drugs with 

 heart proteins, will alter heart rhythm in a way more comparable 

 to change in weight of the balance wheel. In both cases it is to be 

 specially observed that the mechanism is only capable of bearing 

 interference from outside within well-defined limits. Thus, pursu- 

 ing our mechanical analogy a little farther, it may be pointed out 

 that if we pull the hair- spring a little too tight, instead of causing 

 the watch to tick faster and faster we only succeed in stopping it 

 altogether, and contrariwise, by giving greater and greater length of 

 slacker spring, we again simply cause stoppage. So also in the 

 case of the heart, stimulants or sedatives carried beyond cer- 

 tain limits interrupt the most delicately balanced oscillating 

 equipoise. 



This law of a working interval with a minimum and maximum 

 not very widely apart runs through the whole realm of organic 

 nature, both animal and vegetable, and always it is found that 

 excess is as poisonous and deadly as defect of any essential con- 

 stituent in the interaction. 



Great wealth and great poverty are as dangerous to the living 

 cell as to the body politic of a civilised community, and the cell, 

 and the animal composed of cells, lives best and healthiest when it 

 is possessed of neither poverty nor riches, but endowed with a 

 comfortable competence for which it yields its due meed of labour. 



This great law is seen in operation in the case of the process of 

 oxidation to choose the most general biochemical example seen in 

 all living protoplasm from the unicellular organism up to man. 

 Absence of oxygen, or oxygen starvation, kills ; but excess is no less 

 fatal : breathing is only possible between well-marked limits, and 

 health and well-being between much narrower limits still. Excess 

 of oxygen as truly asphyxiates as does its defect. Also, in the great 

 converse process of organic nature proceeding upon such a gigantic 

 scale all over the green earth, in the building up from inorganic 

 carbon dioxide by means of the sun's energy of the total food of the 

 world, this law is seen in operation. That same carbon dioxide, 



