THE THORAX IN THE HORSE. 
681 
The substance of the lung in the horse, in its normal 
condition, is dry, less rich in blood, than the pulmonary 
substance in man. The division into lobules is less marked, 
the air-cells are contracted, very small and delicate, insomuch 
that under the microscope they appear much finer (wrought) 
than in man. This is the reason why, under pneumonitis, 
the substance of the lung appears so finely granular that it 
seems as though it were homogeneous : though, for all this, 
the bronchial tubes preserve, throughout, their finest ramifica- 
tions and their cartilaginous ring-like structure. Respiration, 
normal and unagitated, is in general carried on so softly that 
no vesicular murmur is heard : that is only to be detected 
when the breathing becomes accelerated. As a general rule, 
vesicular respiration denotes a morbid change already com- 
menced, an incipient pneumonia, or else an acute catarrh. 
Often, likewise, is this to be heard in the sound parts of the 
lungs, when the other parts of the organ are impermeable to 
air. Through the ramifications of the trachea likewise, formed 
of cartilaginous rings, penetrating close upon the surface of the 
lung, whenever serous exudations take place in the chest, 
and compression of the air cells by the liquid occur, the respi- 
ratory murmur may be heard, although in another part the 
lung is impermeable. 
In the horse there exists, behind the heart and between 
the two compartments of the chest, underneath the oesopha- 
gus, a longitudinal space of square and ample dimensions, 
being four inches in breadth, and above ten inches in length ; 
within which interval is lodged a lobe of the lungs of a tri- 
angular form, commonly regarded as an appendix to the 
right lung. This lobe, which is found only in the horse, 
causes the greatest difficulties to diagnosis in the diseases of 
the lungs. The pulmonary serous membrane also inclines 
from the same causes to particular modifications. This third 
pulmonary lobe, said to be an appendix to the right lobe, is 
naturally enveloped within a proper serous membrane, con- 
tinuous with the same on the right side by a duplicature 
reflected from underneath by the posterior vena cava. Within 
the space formed by this duplicature, which cannot be re- 
garded as belonging to the mediastinal interstice, there is de- 
posited, in horses well kept, a notable quantity of adipose 
tissue. Through disease or emaciation this tissue grows 
more fluid, so that at times this collection gives rise to a 
belief of morbid exudation. 
With horses exempt from any disease of chest, the serous 
duplicatures which form a partition between the lobes of the 
lungs, exhibit constantly perforations of the size of peas, 
xxvi. 68 
