ATROPHY. 



285 



when there is no longer an adequate call for their functional activ- 

 ities. 



This form of atrophy is probably attributable in some measure 

 to a diminished flow of blood to the part, for in health, when the 

 functional activity of an organ is called into play, there is an in- 

 creased volume of blood conveyed to that organ. But this element 

 in the innutrition does not account for the whole process. The 

 intracellular metabolism also falls below the normal level, and this 

 appears to reduce the state of nutrition of the cellular constituents. 



2. Pressure-atrophy (Figs. 257 and 258). When a part is sub- 

 jected to moderate but constant, or oft-repeated pressure, it under- 

 goes atrophy through a disturbance in its nutrition. This may be 



FIG. 257. 



Section from an emphysematous lung. (Ribbert.) The pulmonary alveoli are enlarged ; their 

 walls are stretched and thinned ; atrophied because of repeated excessive air-pressure 

 within the alveoli. In more extreme cases of emphysema the atrophy of the alveolar 

 walls may lead to their total destruction in places, so that the cavities of neighboring 

 alveoli communicate. (Compare with Fig. 150.) 



partly due to a direct influence exerted by the pressure upon the 

 processes carried on in the cells of the tissue, but it is probable that 

 interference with the circulation, including the lymph-currents, has 

 a greater influence in bringing about the lack of nourishment. Ex- 

 amples of this form of atrophy are furnished by cases in which a 

 contracting cicatricial tissue is formed between the parenchymatous 

 cells of an organ, as the result of a chronic interstitial inflammation. 

 Those cells then undergo atrophy and may eventually disappear (Fig. 



