A final source of information is the instruments themselves. Some are engraved with the name of the manufacturer, and a few are even engraved with the date of manufacture. Some have been taken apart to study the spring mechanisms and others examined in the Conservation Analytical Laboratory of the Smithsonian Institution to determine their material content. The documentation accompanying the instruments, while sometimes in error, may serve to identify the individual artifact by name, place and date of manufacture, and to augment our knowledge of the historical setting in which these instruments were used. Bleeding: The History The history of bloodletting has been marked by controversy. The extensive literature on bloodletting contains numerous polemical treatises that both extol and condemn the practice. Bloodletting was no sooner criticized as ineffective and dangerous than it was rescued from complete abandonment by a new group of zealous supporters. From the time of Hippocrates (5th century B.C.)--and probably before, although no written record is available--bloodletting had its vocal advocates and heated opponents. In the 5th century B.C. Aegimious of Eris (470 B.C.), author of the first treatise on the pulse, opposed venesection, while Diogenes of Appolonia (430 B.C.), who described the vena cava with its main branches, was a proponent of the practice. Hippocrates, to whom no specific text on bloodletting is attributed, both approved and recommended venesection.[3] The anatomist and physician Erasistratus (300-260 B.C.), was one of the earliest physicians to leave a record of why he opposed venesection, the letting of blood from a vein. Erasistratus, who practiced at the court of the King of Syria and later at Alexandria, a celebrated center of ancient medicine, recognized that the difficulty in estimating the amount of blood to be withdrawn and the possibility of mistakenly cutting an artery, tendon, or nerve might cause permanent damage or even death. Since Erasistratus believed that only the veins carried blood while the arteries contained air, he also feared the possibility of transferring air from the arteries into the veins as a result of venesection. Erasistratus was led to question how excessive venesection differed from committing murder.[4]