find the first glimmering of the truth with regard to the nature of the circulating medium and also with regard to the course which it pursues in one part of its circuit—that part, namely, where the two kinds of vessels become capillary in character. His substitution of air for blood in the arteries is plainly the principal error in his scheme.
(b) The Teaching of Galen and of Caesalpinus with Regard to the Nature of the Blood and Its Mode of Distribution.—Galen, in the second century of the present era, disputed the correctness of the doctrine taught by Erasistratus. His objections are thus stated: "Inasmuch as blood flows from an artery when it is wounded, one of two things must be the truth. Either blood was already contained in the vessel before it was wounded, or it must have found its way in from the outside. But, if the blood comes from the outside into a vessel which contains only air, then air must necessarily escape from that vessel (when wounded) before blood does—which is contrary to the fact, as blood alone flows out. Therefore arteries contain only blood." As a further proof of the correctness of his statement Galen carried out the following experiment: In a living animal he placed two ligatures around an artery at points situated not far apart, and then made an opening in the vessel between the two ligatures. The intervening section of the artery, it was thus found, contained only blood. This experiment, it might reasonably be supposed, would have definitely settled the question; but such was not the case. The followers of Erasistratus immediately raised this objection: If the arteries contain blood, how may the air which is drawn into the lungs find its way to all parts of the body? Galen replied that the inhaled air does not pass through the lungs, but is rejected by them after it has cooled the blood. This refrigerating process, he claimed, constitutes the sole purpose of the respiratory act.
Although Galen's idea regarding the true function of respiration is not in harmony with the doctrine taught by modern physiologists, it nevertheless represents a marked advance over the belief previously maintained. Even as
