142 ; NYDROTHORAX. 
ing the trocar through it.) There are but few precautions to be ob- 
served during the performance of tapping the chest. It is usual to 
teach, that the posterior border of the ribs is to be avoided, because this 
portion of the bones is grooved for the reception of the artery. Anatomy, 
however, shows that such vessels are amply protected by the grooves in 
which they travel. 
There is also some selection to be made in the trocar which shall be 
employed. If the tube be of too great a size and permits the fluid to 
gush quickly out, nature may sink under the sudden change induced: the 
water, consequently, ought to be very gradually abstracted. For this 
purpose, the instrument cannot well be too small. The most diminutive 
of those made for human practice will be quite large enough, so that the 
bulk of liquid within the chest may be insensibly removed, and the horse 
be scarcely aware of the change. Those trocars, however, which are 
made for the human practitioner will not be long enough; therefore one 
must be procured louger, but of the like bore. 
Sometimes, after the trocar is properly inserted, no fluid will pour 
forth: the operation is then all but hopeless. It must have been so 
long delayed that various substances have been secreted. These cover 
the interior of the chest. They obstruct the mouth of the cannula and 
prevent the liquid issuing by the tube. 
It is customary, in these cases, to employ a whalebone probe. This 
is inserted up the trocar, and then moved about in different directions. 
The intention is to break down the layer of pus or lymph lining the 
thorax, and to allow the water to leave the cavity. But this is almost, 
needless, as the author does not recollect a single case of this description 
which ultimately survived. 
It is also advisable to draw off the fluid from both sides at the same 
time, so there may be no pressure upon the delicate divisions of the 
chest, and upon the important vessels within them. But happily the 
fluid is, in the first instance, generally confined to one side only. 
Always pull a piece of skin either backward or forward, before the 
incision is made through the integument. The reason for doing this is, 
because, when the trocar is removed, the skin may resume its proper 
place, and act as a valve, keeping out the atmosphere from the cavity; 
for external air, getting into the interior of the chest, is proved to be 
most injurious to life. 
There is to be tendered but one last admonition; even this has been 
in a great measure anticipated by the previous observations. The 
animal must not be left during the operation. Whatever time may be 
consumed by the withdrawal of the liquid, the operator must remain a 
patient spectator of the slow abstraction; for if the horse should be 
