x889.] Etiological Classification of Diseases. 957 
plicate the interstitial also. This condition of things is far more 
common in regard to the extension of interstitial disturbances 
to the parenchyma, than the contrary, especially if parenchyma- 
tous disturbances are of short duration. Again, it is far more 
common in some organs than others, For instance, a parenchy- 
matous pneumonia may exist to an excessive degree without any 
proliferation to speak of occurring in the stroma of the lungs ; 
or a chronic bronchitis, which is partially parenchymatous and 
partly interstitial as to the bronchial tubes, may be present with- 
out any very serious disturbance of the alveola-epithelium, but a 
well-diffused and active interstitial pneumonia cannot occur with- 
out the most severe parenchymatous complications resulting in 
the complicated parts, due principally to the encroachment of the 
mterstitial processes upon the walls of the blood-vessels circula- 
ting through the interlobular spaces, thus leading to severe circu- 
lation disturbances, and, eventually, stasis in many parts of the 
lungs. The condition seen in contagious pleuro-pneumonia 
in cattle is a striking example of this kind of extension. On the 
contrary, it is very seldom that we have severe and prolonged 
disturbance of the active substance in the dense glandular organs, 
such as the liver or kidneys, without more less extension of the 
processes from the parenchymatous to the interstitial tissue, 
especially in the vulnerable spots around the larger blood-vessels, 
or where bifurcation of vessels is present. Hence it is better to 
look upon this classification as having reference to the tissues in 
which the disease began, or where the disturbances have been 
initially the most severe, rather than that they have been, or are, 
limited to a certain tissue, as the names selected appear to indi- 
cate. 
Again, we classify diseases according to the product, the 
nomenclature being self-explanatory, such as, caseous, fibrinous, 
or catarrhal pneumonias; a fibrinous or, a serous effusion; a 
hemorrhagic, or purulent, process, and so on. Such other 
means of differentiation as acute or chronic, or the various critical 
sub-divisions, need not be here mentioned. 
With these few and imperfect remarks as an introduction, let us 
turn our attention to the etiological classification. 
