404 ANATOMICAL AND PATHOLOGICAL OBSERVATIONS. 



in the organism to which it belongs, be called on to perform. 

 This variableness in the average age of each species of cell, is 

 dependent on those circumstances which have been named 

 " nervous agency," " peculiarity of constitution," " irritability 

 of the parts," " morbid action," but may be studied independ- 

 ently of these agencies. The variableness in the term of ex- 

 istence of cells can no more be explained at present, than the 

 variety in the duration of the lives of species of animals and 

 vegetables : but the fact being known, its laws ascertained 

 will afford a clue to the explanation of many organic pheno- 

 mena and processes. 



In the study of absorption, nutrition, and secretion, 

 attention has been directed to the vessels, as the active agents 

 in the performance of these processes. It is only a short 

 time since we have been willing to admit that the new 

 matter which is constantly replacing the old materials of the 

 frame, is selected and laid down, not by the ultimate vessels, 

 but by the non-vascular portions of the textures. It is only 

 now that we are beginning to know that secretion differs 

 from nutrition in its anatomical relations, and not in its 

 intimate nature. We still, however, retain in full force the 

 old belief in the active absorbent powers of the vessels, and 

 in the agency of the capillary and lymphatic vessels in re- 

 moving parts and modelling the forms. 



It is not my intention to question entirely the active 

 agency of the veins and lymphatics in absorption and 

 ulceration, but merely to direct attention to the subject ; and 

 to point out, in some of the following chapters, a few organic 

 processes in which these actions appear to be functions in- 

 dependent of the vessels, the latter to be passive agents, mere 

 ducts for conveying away the products of action. 



A rapidly-extending ulcerated surface appears as if the 

 textures were scooped out by a sharp instrument. The 

 textures are separated from the external medium by a thin 



