STRUCTURE OF THE MEDIASTINUM IN THE HORSE. 483 
the pleura. It is thin, diaphonous, tense, and composed of ex- 
ceedingly slender fibres, crossing in every direction, and forming a 
transparent tissue, having considerable resemblance to the con- 
struction and web of lace. The minute spaces which this .mem- 
brane present, crowded together, and scarcely to be distinguish- 
ed in the human subject, enlarge, in course of time, by the sole act 
of respiration, until at length numerous rounded and irregular 
openings manifest themselves here and there, and establish a di- 
rect communication between the two pleural cavities. This was 
easily to be seen in some specimens which M. Bouley produced 
before the Academy. 
From this natural construction of the membrane, it clearly 
results that pleurisy can never be a purely simple disease in the 
horse ; that both pleural sacs, if not equally, are actually affected 
at the same time, and, consequently, that this must always be a 
more serious and dangerous disease in the horse than in man, 
in whom inflammation of the membrane of both cavities is 
rarely observed. 
To these anatomical facts M. Bouley added, that whenever 
hydrothorax existed in the horse, auscultation indicated that the 
fluid stood at the same height in both cavities, and that the 
operation of paracentesis on one side of the thorax would effec- 
tually empty both sacs of their contents. He thought that, in 
this last case, the natural openings of the posterior mediastinum 
must be considerably enlarged, and that sometimes the mem- 
brane must have been completely torn. He did not state this 
as a fact that he had witnessed ; he only offered it as a con- 
jecture. He proposed to make some researches on the point. 
Professor Delafond observes on this, that he is completely of 
the same opinion with M. Bouley, as to the cause of the serious 
character of the double pleurisy of the horse ; namely, the 
passage of a portion of the effused fluid from one pleural sac to 
the other, across the posterior mediastinum. He has, indeed, 
recorded this structure of the mediastinum, and the accidental 
ruptures of it caused by the effused fluid, and thus producing a 
double pleurisy. 
M. Bouley inquires, whether the natural openings in the 
mediastinum are simply enlarged, or whether this frail mem- 
brane was torn by the pressure of the fluid. He could assure 
this distinguished practitioner that, in these cases, lesions of 
the mediastinum are frequently to be found. He has records 
of ten cases in which it has occurred in horses dying either from 
chronic or acute pleurisy. In chronic pleurisy, however, these 
openings are not always to be found, although there may be 
effusion in both pleural sacs. He has inserted one case in the 
