ORGANS OF RESPIRATION. 385 



affected^ and the dilatation of the vessels may be considerable ; 

 the quantity of blood also contained in such vessels will be 

 very great. In cases of trivial or partial congestion a slight 

 retardation in the progress of the blood only occurs ; in the 

 more severe and complete cases the blood accumulates in the 

 vessels to such an extent as that the circulation is totally 

 annihilated, death being the necessary result of such a con- 

 dition of things. 



In what way may the occurrence of this congestion be 

 explained, and what is the cause of the cessation of the cir- 

 culation ? The capillary vessels of the lungs are, of course, 

 incapable of conveying beyond a certain quantity of blood : 

 any causes, therefore, and there are several, which drive in 

 upon the lungs a greater amount of the vital fluid than its 

 vessels are capable of circulating would produce congestion ; 

 the first effect of which is an accumulation of blood in the 

 vessels ; the second, an enlargement of those vessels, the coats 

 of which are highly elastic, as a necessary consequence of the 

 first ; third, a total cessation of the circulation, arising from 

 the rapid aggregation of the blood corpuscles in the vessels. 

 The various degrees of congestion may be followed out with 

 the microscope in the most satisfactory manner in the tongue 

 of the frog, or in the web of the feet of that creature. Some 

 of the vessels will be but slightly dilated ; other capillaries 

 which, in their normal state are capable of conveying only a 

 single row of corpuscles, will now be observed to contain two 

 or three rows ; in others, again, the blood will have ceased to 

 move altogether. 



These several changes in the condition of the blood and 



O 



its vessels during congestion, are all unaccompanied by 

 structural alterations, by which character congestion may be 

 distinguished from inflammation. 



Pneumonia. The phenomena of congestion precede those 

 of pneumonia, of which indeed it may be considered as the 

 first, stage ; the second stage of pneumonia consists in the 

 rupture of some of the capillary vessels, and the escape of a 

 portion of their contents (whence proceeds the characteristic 

 rust-coloured expectoration), as well as the effusion without 



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