The point selected to operate on is, in the horse, ox or dog, in front of the anterior border of the ninth rib, at its lower end or close to its union with the cartilage. The point of the trochar should be directed slightly upward and forward to avoid the possibility of injuring the diaphragm. The skin is first rendered aseptic by shaving, followed by a thorough soapy wash and a free use of mercuric chloride solution (1:500). It is then pricked with a lancet, then drawn aside that the wounds in the skin and muscles may not correspond after the cannula has been withdrawn. The trochar is then pushed steadily through the intercostal space till all obstruction has been overcome, when it may be concluded that the pleural sac has been reached. The trochar is now withdrawn and the fluid allowed to flow from the cannula until there is presumably some risk of the introduction of air, when the brass piece is to be applied and the remainder drawn off with the syringe or aspirator. As a substitute for the aspirator a caoutchouc tube, eighteen inches long, put on the cannula or needle and having its lower end plunged in a solution of boric acid will prevent the entrance of germs. A prob has often to be introduced to prevent plugging of the cannula by floating false membranes, and a new puncture in a different place may be necessary. In the case of excessive accumulation it is often advisable to draw it off at two operations, as recommended in large abscess of the pleura and for the same reasons. The need for such a precaution will be understood when it is stated that in bad cases the chest contains as much as six or seven ordinary stable bucketfuls of the liquid. If, however, it is limited in amount it may be all withdrawn at once. The most successful cases in the horse have been upon young, vigorous animals, from four to eight years old, during the first month of illness, and where the pleurisy has been confined to one side. Dr. Bowditch lays down the following rules for the adoption of paracentesis in man (_Clinical Medicine_, by Prof. W. T. Gairdner):— “I now never operate unless I find some distension or rounding out of the chest, and filling up of some of the intercostal spaces, so that the chest presents a uniform curve, and not alternate depressions and elevations as in the healthy chest. I operate under the following circumstances when I feel certain there is fluid: “1. When there is _severe permanent dyspnœa_—orthopnœa—however acute the disease if I find fluid filling the pleural cavity, or nearly filling it. “2. When there are occasional attacks of orthopnœa threatening death, even if there be not sufficient to fill more than half of the cavity. If the fluid seems to be the cause of the dyspnœa I operate, because occasionally I have lost a patient while waiting for more extensive physical signs. This rule I apply to acute and chronic cases.