PATHOLOGY OF PULMONARY DISEASES OF THE HORSE. 
25 
direction, that is, from forward, backward. In general, however, 
the costal extension of the thorax is very small in comparison to 
the influence exerted upon the dimensions of the same by the dia- 
phragma. The thorax will be extended the least, on such part, 
when it has the least elasticity and flexibility. Accordingly, the 
anterior and middle part of the same will suffer the least exten¬ 
sion by inspiration; this is partly due to the arrangement of the 
ribs and partly to the hindrance to the same offered by the 
attachments and influence of the anterior extremities. If, there¬ 
fore, the development of a catarrhalic secrete—mucous and pus— 
in the bronchi has taken place, the same will self-evidently accu¬ 
mulate in those parts of the lungs which are least agitated and 
extended by inspiration—the anterior and middle parts. In the 
inferior middle section of the lungs, or, according to Leisering, 
“ where the inferior edges extend furthest downwards,” an obstruc¬ 
tion of the bronchi will most easily take place; because the 
secrete follows the laws of specific gravity, and must, therefore, 
sink to this, the lowest part. Aside from the form of the thorax, 
and the situation of the single sections of the lungs, the character 
of the inspiration muscles is of importance. This has been 
already considered. In proportion as the activity of these mus. 
cles is interfered with, so much the less will be the extension of 
the thorax, and this will be more apparent in regions where the 
normal excursions are the least. Therefore, atelectasis will natu¬ 
rally take place after a bronchitis in those parts of the lungs where 
expectoration is mostly interfered with , that is, where obstruction 
of the bronchi can most easily take place. We thus see that an 
acquired atelectatic condition may have a variable extension. A 
lobulus, or a part of the same, may be complicated; and, indeed, 
lobular atelectasis follows bronchiolitic catarrhalis, while partial 
lobular atelectasis follows a broncho-pneumonia. By the last the 
complication of the bronchi is the protopathic, while the compli¬ 
cation of the lungs is the deuteropatliic process; the process 
creeps from the bronchi to the alveolae of the lungs. By bron¬ 
cho-pneumonia, the alveolae around the bronchi are diseased, the 
central part of the lobulus suffers, while the peripheral remains 
intact. If the broncho-pneumonia processes lead to a long con- 
