66 INCESSANT CHANGE IN EVERYTHING LIVING. 



in its course through the body, our attention will 

 naturally be drawn to the examination of those re- 

 maining processes viz., Respiration and Secretion, by 

 which great and most important changes are brought 

 about in the condition of the circulating fluid, various 

 substances being separated from it for ulterior uses, or 

 for complete removal from the body. Lastly, comes 

 Generation, the process by which the multiplication 

 of individuals is effected. 



101. Incessant change in everything living. Living 

 organisms, as well as every particle of living matter, 

 are incessantly undergoing change in every part, but 

 the rate of change varies marvellously in different 

 cases. Some materials, passing through the stages 

 of living or forming matter, formed matter and pro- 

 ducts of disintegration, in a few minutes, while others 

 last for many years as formed structure, and perform 

 an important office during the whole time. New 

 parts are constantly being formed, which grow, arrive 

 at maturity, pass through certain stages of existence, 

 and then, having performed their office, die, are cast 

 away, and succeeded by others. 



102. Of the minute elementary parts of an Organ or 

 Tissue. The changes in question affect every one of 

 the microscopic anatomical elements of which every 

 tissue and every organ is composed. The anatomical 

 unit, which performs the unit of work, seldom 

 measures more than the one- thousandth of an inch 

 in diameter, and, in some cases, the part possessing 

 structure and performing function, is far less than 

 this. 



In order, therefore, to form any correct idea of the 

 changes which go on in every part of the body of a 

 complex living being, it is necessary to study care- 

 fully the structure and the changes of one of these 

 numerous elementary units, of which every tissue 

 and organ may be regarded as a collection. Since 

 the time of Schwann and Schleiden, who wrote in 



