[L. vacca, a cow.] Jenner in 1796 noticed that persons who had much to do with cows, and who had caught from these animals a mild disease known as cow-pox, did not afterwards catch small-pox. The lymph by which the cow-pox is transmitted is a clear fluid obtained from a cow or a calf, or from the swelling on the arm of a vaccinated person. This lymph is introduced into the body by scratching the arm and then rubbing into the blood the lymph from little glass tubes.