ON VITAL PRODUCTION 77 



energy is transformed into mechanical work done by the muscles 

 of the body and limbs, and the rest is transformed into heat. 

 That is the energy transformation. 



Assume an adult animal in perfect health and of stationary 

 weight; then the working substance enters the tissues in the 

 form of proteid, fat, and carbohydrate, and leaves them in the 

 form of water, carbonic acid, and urea. That is the chemical 

 transformation. Looked at both from the point of view of 

 available energy and that of chemical structure there is degrada- 

 tion. First, in that a high intensity of chemical energy becomes 

 a low intensity; and, second, inasmuch as complex chemical 

 compounds become simple ones. 



The working substance of life is now water, carbonic acid, and 

 urea. Let us trace its further history. The water and carbonic 

 acid undergo no further change (just yet, at all events). But the 

 urea (and the other proteid degradation products) do. 



The Fate o! the Proteid Residues. — The urea (and other 

 nitrogenous substances) contained in the urine find their way, 

 through drains and by other means, into the water of rivers and 

 of the sea, or on to the land, and then it is at once attacked by 

 micro-organisms. Now a few words about the latter. 



Micro-organisms are (in the present connection) bacteria, 

 moulds, yeasts, and infusoria. The bacteria are exceedingly 

 minute, single-celled organisms which cannot be said to be either 

 animals or plants, since their modes of generation are altogether 

 special. Many of them (but still a small minority) are called 

 pathogenic organisms, and are the causes of certain infectious 

 and epidemic diseases, such as cholera, enteric fever, pneumonia, 

 " influenza," diphtheria, catarrh, septicaemia, etc. Being exces- 

 sively minute, capable of living in water, and even of being 

 partially dried, they may be distributed in liquids, in dust, on 

 infected clothing, etc. When they enter a suitable " soil " — that 

 is, a liquid containing certain organic substances in solution — 

 they multiply at an incredibly high rate. The human body 

 possesses certain defences against these pathogenic bacteria — 

 that is, the phagocytes of the blood can ingest and destroy them, 

 or the plasma can form certain substances which can neutralise 

 their poisonous activity. If the infected animal fails to set up 

 such adequate defences, the invading pathogenic bacteria multiply 

 with great rapidity and form toxins which are injurious to one 

 or more tissues, and so instigate a condition of disease. 



