666 
MR. YOUATTS VETERINARY LECTURES. 
even by the slightest filament. The mediastinum is the continua¬ 
tion of the investing tunic of the lungs,—the pericardium of that 
of the heart. They are in contact with each other, but they are as 
distinct and unconnected as if they were placed as distant from 
each other as the frame would admit. Is there no meaning in 
all this? Have we been sufficiently aware of the important object 
thus effected ? It is to preserve the perfect independence of 
organs equally important, yet altogether different in structure and 
function;—to oppose an insuperable barrier to hurtful sympathy 
between them, and especially to cut off the communication of 
disease. 
The Effect of Purging in Pleurisy and Pneumonia .—I must 
not too much anticipate, but perhaps a little light begins to be 
thrown on a circumstance of which, I hope, you will not have too 
frequent and painful experience. I mean the fact, that while 
we may administer physic, or mild aperients at least, in pleurisy, 
not only with little danger, but with manifest advantage, we may 
just as well give a dose of poison, a& a physic-ball to a horse 
labouring under pneumonia. The pleura is connected with the 
lungs, and with the lungs alone. There is not a connecting fila¬ 
ment with any other viseus; and although connected with the 
lungs, the organization is so different, that there is very little 
sympathy between them. A physic-ball may therefore act as a 
counter-irritant, or as giving a new determination to the vital 
current, without the propagation of sympathetic irritation; but 
the lungs or the bronchial tubes that ramify through them are 
continuous with,—it is a somewhat roundabout way,—but they 
are continuous with the mucous membranes of the digestive as 
well as all the respiratory passages; and on account of the conti¬ 
nuity and similarity of organization, there is much sympathy be¬ 
tween them. The irritation caused in the one is easily propagated 
to the other; or if there be irritation excited at the same time in . 
two different portions of the same membrane, it is probable that, 
instead of being shared between them, the one will be transferred 
to the other—will increase and double the other, and act with 
fearful and fatal violence. Do not you, gentlemen, require too 
many lessons of this kind to teach you caution. 
The Mode of Investment .—A few words, and these in the 
simplest and perhaps superficial way, must now be said as to the 
manner in which the thorax and lungs are covered by the pleura. 
I take away a portion of the wall of the thorax, and by careful 
dissection I arrive at this membrane, forming the inner lining of 
that portion of the ribs which I am removing. I cut through 
this membrane, and introduce my hand into the chest, and I feel 
the smoothly polished pleura covering the bony wall of the tho- 
