166 THE MOTION OF THE BLOOD. 



a, lungs. d, pulmonary veins. g, left auricle. 



b, trachea. e, aorta. A, right ventricle. 



c, pulmonary artery. f, right auricle. i, left ventricle. 



same nature as the dura mater, though thinner ; the other a true 

 serous membrane, lining the inside of this, closely enveloping the 

 substance of the heart, and " very firm, accommodated to the 

 figure of the heart, and moistened internally by an exhalation.'* 



It lies between the two pleurae, and behind the anterior, and 

 before the posterior, mediastinum. " Its importance is evinced by 

 its existence being, in red-blooded animals, as general as that of 

 the heart ; and by our having but two instances on record of its 

 absence in the human subject. m 



" The heart alternately receives and propels the blood. Receiv- 

 ing it from the whole body by means of the superior and inferior 

 vena cava, and from its own substance through the common orifice 

 of the coronary veins, that is supplied with a peculiar valve n , it 

 conveys that fluid into the anterior sinus and auricle, and thence 

 into the corresponding ventricle, which, as well as the auricle, 

 communicates with both orders of the heart's own vessels by the 

 openings of Thebesius. 



171 " Consult, v. c. Littre, Hist de C Academic des Sc. de Paris. 1712. p. 37. 



Baillie, Transactions of a Society for the Improvement of Medical and Chirurgical 

 Knowledge, t. i. p. 91." 



n " Gasp. Fr. Wolff on the origin of the large coronary vein, Act. Acad. Scient. 

 Petropol. 1777. P. i. 



Petr. Tabarrani on the same subject, Atti di Siena, vol. vi." 



" Respecting these openings, consult, among others, J. Abernethy, Philos. 

 Trans. 1798. p. 103." 



