I. The Bleeding of Guinea-Pigs. In serological work where guinea-pig's blood is essential, investigators are often seriously handicapped by not being able to obtain this fluid at all, or in sufficient quantities. This handicap may be due to a scarcity of these animals or to the expense resulting from their use in large numbers.
Several methods may be employed for drawing the blood from the animal. If a very small quantity is desired, it is possible to bleed the animal from its marginal ear vein and collect the blood in a Wright tube. This is, however, a slow process and can be carried out successfully only on an adult animal. In a few laboratories blood is drawn directly from the animal's heart with a sterile syringe. By this method a considerable quantity of blood may be taken readily, without any serious injury to the animal; the technic involved is, however, so difficult that few investigators make use of it.