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 for investigating a question of this nature. Capillary blood-vessels are invisible to the naked eye, and may be studied only with the aid of a microscope; but this instrument was not available until long after the time (1605-1616) when Harvey was engaged in carrying out his investigations into the circulation of the blood.

Other Discoveries Relating to the Vascular System.—To Vesalius is due the credit of having discovered the fact that anastomoses exist between the carotids and the vertebral arteries, thus explaining how a man may continue to live even after both carotids have been severed or ligated. His great rival, Fallopius, described these anastomoses in the most detailed manner, and he noted the further fact that an anastomosis with the basilar artery exists.

By the end of the sixteenth century a certain amount of progress had been made toward a correct knowledge of the lymphatics. Bartholomaeus Eustachius, for example, discovered the existence (in horses) of the thoracic duct, but he supposed it to be a vein. His description of this vessel reads as follows:—

In these animals there is a large vessel which extends downward from the inner aspect of the clavicular vein (= left subclavian vein). At the point where it joins the vein it is closed by means of a semicircular valve. This vessel is of a whitish color and it contains a scanty watery fluid. Not far from its starting-point it divides into two branches which very soon, however, join together again, and then, as a single trunk from which no further branches are given off, it passes down along the left side of the spinal column, penetrates the diaphragm, spreads itself out over the aorta, and ends in a manner unknown to me.

About one hundred years later (1647), Jean Pecquet of Dieppe, France, professor in the Medical School of Montpellier, rediscovered (in a dog) this same duct, with its tributary chyle ducts and also its point of entrance into the left subclavian vein; and, as he had rightly interpreted its nature, anatomists by common agreement accorded him the rights of discoverer.