There is in this operation no danger of piercing the lung. The trocar must be driven upward and onward, very far and very forcibly, to induce such an effect. The lung is protected from all lawful violence by the water, on the top of which it floats. There is, however, a dispute concerning how much of the fluid should be extracted. It is a good rule to take all you can get, or all the condition of the horse will permit to be abstracted. Do not commence the operation with any determinate quantity in your mind. Take all, if the horse will suffer so much to be withdrawn but if the animal, after the loss of a quart, shows signs of approaching faintness, withdraw the trocar, let the skin fly back, and wait a more favorable opportunity for the next attempt. In an hour or two the trial can be repeated. Make a new opening (for never risk exciting irritation in the original wound, by again thrusting the trocar through it.) There are but few precautions to be observed 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 longer, 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.