An oblique cut is now made in the vein, as shown in the illustration, the cannula is introduced, and the upper ligature is tied. [Illustration: Fig. 12.--INSERTION OF THE CANNULA IN A VEIN] The barrel of the air-lock, with its contained saline solution, is then fixed firmly on to the rubber bung, so that the nozzle of the delivery tube projects into the saline solution. Meanwhile, an assistant has fixed a rubber bellows on to the side tube of the flask; a short piece of glass tubing loosely packed with cotton-wool should be interposed between the bellows and the flask to prevent any particles of dust being blown over into the flask from the bellows, which is not sterilized. The clip near the cannula is released, and some positive pressure is produced inside the flask by means of the bellows. The citrated blood then rises in the delivery tube, and a corresponding quantity of saline solution is displaced from the air-lock into the patient’s circulation. The blood then flows from the nozzle of the delivery tube into the air-lock, and the remainder of the saline solution is driven on into the patient. Finally the blood flows steadily through the cannula, and the rate at which it is flowing can be observed in the air-lock. The presence of this air-lock facilitates, as has been seen, the introduction of the cannula, into the vein, since there is no leakage of blood to obscure the operation. In addition, the operator can see at a glance whether the blood is flowing in properly, and can regulate the rate of flow to a nicety by varying the pressure in the flask by means of the bellows. If a very slow injection is required, the blood can even be made to run drop by drop. If the patient is suffering from acute anæmia, the blood can be pumped in rapidly, 750 cc. of blood being given in the course of twenty minutes. If, on the other hand, the patient has a plethora of fluids, such as is seen in some cases of secondary anæmia, the blood must be given very much more slowly than this, since it is dangerous rapidly to increase the blood volume. A half to three-quarters of an hour must be occupied in giving 500 cc., and even then the patient may complain of a sensation of tightness in the chest and of dyspnœa, due to embarrassment of the right heart during the transfusion. This complaint, however, is usually transient, and will disappear quickly if the injection be stopped for a few minutes.