PATHOLOGY OF PULMONARY DISEASES OF THE HORSE. 
27 
the bronchitis. Tympanitis and ascites are further dangerous in 
that they favor the genesis of atelectasis. An indiopatliic 
bronchial catarrh develops no atelectasis by old animals, even 
when the bronchioli are complicated, and a viscid mucous is 
secreted, the normal performance of the inspiration muscles 
hinders the development of bronchial obstruction. But atelec¬ 
tasis is, however, developed, if tympanitis or ascites are present, 
or become present during the prevalence of the inflammatory 
processes in the lungs. The latter cause a shortening of the 
longitudinal diameter of the thorax, they extend the diaphragma, 
and consequently exert a disadvantage influence upon the processes 
of inspiration; they render the alveolis spaces smaller, compress 
the bronchi, and hinder the entrance of air into the same. 
Atelectatic parts always take up less room than those parts of the 
lungs filled with air. In a lobulus atelectatic it will be found to 
lie below the level of the remaining air containing parts of the 
lungs. The pleura covering an atelectatic piece of lung shows 
no changes—it is smooth and transparent. Small sections of 
atelectatic pulmonary tissues sink if thrown into water. Atelec¬ 
tatic tissue does not crepitate on cutting, it is dry, and its cut 
surface smooth ; such parts are dark or blue red in color, hyper- 
semic. Pulmonary tissue filled with air is of a delicate pink 
color, on account of the latter distribution of the blood through 
the distended tissue, and the opportunity given for its more perfect 
oxydation. An atelectatic portion of a lung must therefore have 
a dark red color, even though it contains no great quantity of 
blood; the part is retracted and the blood limited to a smaller 
space. The blood in such parts is at the same time more 
veinous because no oxygen gains access to the same. The capil¬ 
laries of such parts are distended on account of the cessation of 
the intra-alveolic atmospheric pressure. 
When a part of a lung becomes atelectatic, it is alone a conse¬ 
quence of retraction. If we open the thorax of a cadaver, the 
lungs at once fall together the moment the external air enters 
the thorax. The lungs are in a full condition of retraction, which 
can only take place, however, when the dispersion of the atmos¬ 
pheric pressure upon the inner (respiratory) and external surfaces 
