164 HISTOLOGY 



part of it is observed to be full of air." He added that " these blood vessels 

 have their origins in the heart, for in whatever direction they happen to 

 run, they traverse the other viscera without in any way losing their dis- 

 tinctive characteristics as blood vessels; whereas the heart is, as it were, 

 a part of them" (Historia Animalium, Book 3, trans, by Thompson). 

 Subsequently the term artery was applied to the aorta and its branches, 

 which were found partly empty of blood after death, and were believed 

 to convey air; the windpipe was called the arteria aspera. 



Vesalius described an artery as "a vessel similar to a vein, membranous, round, 

 and hollow like a pipe, by means of which vital spirit and warm blood, rushing impetu- 

 ously, are distributed throughout the entire body; by the aid of these, and thus 

 through the motion of the artery itself (which is by dilatation and contraction) the 

 vital spirit and the natural warmth of the several parts are renewed" (De corporis 

 humani fabrica, 1543, 4th ed., 1604). Vesalius described the arteries and veins as 

 composed of coats (tunica;) in which he found loose tissue and layers of fibers- 

 circular, oblique, and longitudinal. 



The valves of the veins, consisting of thin membranes projecting into their lumens, 

 were first described and clearly figured by Fabricius, under whom Harvey studied 

 at Padua (De venarum ostiolis, 1603). Fabricius observed that the ostiola are 

 found chiefly in the veins of the limbs and are "open toward the roots of the veins but 

 closed below." He considered that "to a certain extent they hold back the blood, 

 lest, like a stream, it should all flow together either at the feet, or in the hands and 

 fingers." He stated that the veins can be easily dilated and distended, since they are 

 composed of a simple and thin membranous substance; and concluded that the veins 

 have valves to prevent over-distention, but the arteries, because of the thickness 

 and strength of their walls, do not require them. 



In demonstrating the circulation of the blood (in 1628) Harvey contributed 

 little to the knowledge of the structure of the vessels. He could not find the micro- 

 scopic connections between the arteries and veins, but they were discovered not many 

 years later by Malpighi (De pulmonibus, Ep. II, 1661). In the membranous lungs 

 of frogs and turtles, Malpighi found a rete or network of vessels connecting the artery 

 and vein, so that the blood was not poured out into spaces, but was driven through 

 tubules. He concluded that if in one case the ends of the vessels are brought together 

 in a rete, similar conditions exist elsewhere, and he observed the circulation taking 

 place in the diaphanous anastomosing vessels of the distended bladder of frogs. 

 Leeuwenhoek (1698) clearly figured the minute vessels which pass from the arteries 

 to the veins in the caudal fin of eels, and noted that the line of separation between the 

 artery and vein is arbitrary. 



The vessels which connect the arteries with the veins, because of their 

 hair-like minuteness, were later called capillaries. Physiologically they 

 form the most important part of the vascular system, and anatomically 

 they are the most fundamental. They consist merely of endothelial 

 tubes. All larger vessels, not only the arteries and veins, but also the 

 heart, are derived from endothelial tubes and retain their endothelial lining. 

 The endothelium, however, becomes surrounded by layers of smooth 

 muscle fibers and connective tissue, which form the substance of the 



