14 ABSORPTION, ULCERATION, AND THE 



species may, by peculiar circumstances in the organism to which 

 it belongs, be called on to perform. This variableness in the ave- 

 rage age of each species of cell, is dependent on those circum- 

 stances which have been named " nervous agency," " peculiarity 

 of constitution," " irritability of the parts," " morbid action," 

 but may be studied independently of these agencies. The vari- 

 ableness in the term of existence 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 

 phenomena and processes. 



In the study of absorption, nutrition, and secretion, attention 

 has been directed to the vessels, as the active agents in the per- 

 formance of these processes. It is only a short time since we 

 have been willing to admit that the new matter which is con- 

 stantly 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 rela- 

 tions, and not in its intimate nature. We still, however, retain 

 in full force the old belief in the active obsorbent powers of the 

 vessels, and in the agency of the capillary and lymphatic vessels 

 in removing 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 independent 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 tex- 

 tures were scooped out by a sharp instrument. The textures are 

 separated from the external medium by a thin film. This film is 

 cellular in its constitution, and so far it is analogous to the 

 epidermis or epithelium. It is a peculiarly endowed cellular 

 layer, which takes up progressively the place of the subjacent tex- 

 tures, these being prepared for dissolution, either by the state of 

 the system, the condition of the part, or by some influence in* 



