THE MOTION OF THE BLOOD. 169 



then, passing into the aorta, is distributed through the arterial 

 system of the body in general and the coronary vessels of the 

 heart itself, i 



" Having proceeded from the extreme twigs of the general 

 arterial system into the radicles of the veins, and from the 

 coronary arteries into the coronary veins, it finally is poured into 

 the two venae cavae, and then again pursues the same circular 

 course. 



" The regularity of this circular and the successive motion 

 through the cavities of the heart is secured, and any retrograde 

 motion prevented, by valves, which are placed at the principal 

 openings, viz. at the openings of the auricles into the ventricles, 

 and of the ventricles into the pulmonary artery and aorta." 



" Thus the ring, or venous tendon, which forms the limit , of 

 the anterior auricle and ventricle, descending into the latter 

 cavity, becomes these tendinous valves. r These were formerly 

 said to have three apices, and were, therefore, called triglochine 

 or tricuspid: they adhere to the fleshy pillars, or, in common 

 language, the papillary muscles. 



" In a similar manner, the limits of the posterior auricle and 

 ventricle are defined by a ring of the same kind, forming two 

 valves, which, from their form, have obtained the appellation of 

 mitral." 8 



They are duplicatures of the lining membrane, with the addition 

 of intervening fibrous membrane. 



"At the opening of the pulmonary artery 1 and aorta u are 

 found the triple semilunar or sigmoid valves x , fleshy and elegant, 

 but of less circumference than the mitral." 



These are merely duplicatures of the lining membrane. 



" It is obvious how these differently formed valves must pre- 

 vent the retrocession of the blood into the cavities which it has 

 left. They readily permit the blood to pass on, but are expanded, 



q " Consult Achil. Mieg, Specimen . ObservationumBotanicarum, &c. Basil, 

 1776. 4to. p. 12. sq." 



r " Eustachius, tab. viii. fig. 6. tab. xvi. fig. 3. Santorini. Tab. Posth. ix. 

 fig. 1." 



* " Eustachius, tab. xvi. fig. 6." 



* " Eustachius, tab. xvi. fig. 4." 



u " Eustachius, tab. xvi. fig. 5. Morgagni, .Adversar. Anat. i. tab. iv. fig. 3. 

 Santorini, 1. c." 



x " Consult Hunter, -who treats very minutely of the mechanism of these valves 

 in his work On the JBlood, p. 159." 



