PHYSICAL MECHANISM OF MAN. 23 



the temperature declining, the body becomes of a marble coldness charac- 

 teristic of that terrible disease. 



The animal mechanism is thus the focus of intense chemical changes, 

 and great quantities of material are required in- very brief Necessity of re- 

 spaces of time for its support. We have seen what is the {^^ft* 1 **" 

 use of the combustible matter employed as food, what of the wastes. 

 water, what of the air, how, these reacting on one another, a high but reg- 

 ulated temperature is kept up. 



Much of what has been thus far said has had reference only to the de- 

 struction of tissues. This waste of matter arises for a double reason, 

 partly to give origin to the heat which animals require, and partly as a 

 consequence of intellectual activity and muscular motion ; for no move- 

 ment can be made without a destruction of muscular fibre, and all mental 

 and nervous actions imply the waste of a certain quantity of vesicular 

 substance. For this reason, after an animal has undergone violent mus- 

 cular exercise, the quantity of urea and sulphuric acid in the urine is in- 

 creased, this being the channel through which those results of the de- 

 struction of muscular, fibre are removed ; or, after severe mental or intel- 

 lectual duty, there is more phosphoric acid than usual in the urine, be- 

 cause of the greater oxidation of phosphorus which has taken place in 

 the brain. 



But of course this destruction of tissue must be compensated by a re- 

 pair if a normal condition and health are preserved. The action of the air 

 is not directly upon the food, for intermediately and temporarily the food 

 is converted into the living mechanism. The dead material is awakened 

 into life, and for a time, though only for a time, becomes a portion of the 

 living and feeling mass. 



The functions and actions we have been considering imply the provi- 

 sion of many complicated mechanisms. There must be means various mech- 

 for effecting the introduction of the air ; these, in man, depend anisms wanted 



iv - , ,. .. A { A i f r removal of 



on calling into operation its pressure. A system of tubes is waste and for 

 necessary for its distribution to the points at which it is re- repair, 

 quired, and in like manner a system is required for carrying away the 

 wasted products of decay. The new material which is destined to re- 

 place the parts which are thus disappearing, and to keep the economy in 

 repair, must be submitted to such processes of mechanical and chemical 

 preparation that it may be dissolved in the blood, and carried wherever it 

 is wanted. It must therefore be cut and crushed by teeth driven by pow- 

 erful muscles, dissolved by acid and alkaline juices in digestive cavities 

 set apart for that purpose. From these it must be taken by arrange- 

 ments which can absorb it and carry it into the torrent of the circulation. 

 Physical means must be resorted to, not only for the impulsion of these 

 newly-absorbed nutritive juices, but likewise to drive the blood in its 



