338 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



the mitral valve. The process of fibrous tissue extends from each 

 festoon towards the mouth of the artery, forming a loose semilunar 

 curtain, which is the basis of the valve. Each of these curtains 

 presents a convex attached border, and a concave free border, 

 interrupted at its middle, or bisected so as to form two concave 

 borders, by a slight thickening of the fibrous tissue, which forms 

 a small spherical body called the corpus Arantii ; it also has a 

 convex surface directed towards the ventricle, and a concave 

 surface directed towards the artery. The bundles of fibres which 

 constitute this fibrous basis of each valve are disposed in fes- 

 toons, some parallel to the fibrous festoon of the attached border ; 

 others, and the greatest number, parallel to the smaller con- 

 cave borders on each side of the Corpus Arantii. The fibres 

 nearest the attached border of the valve are considerably more 

 developed than those near its free margin. Indeed, for a 

 space of about three or four lines from the edge of the valve the 

 fibrous tissue is extremely thin and almost transparent. It is 

 at this part of their ventricular surface that the valves come in 



contact when they close the 

 arterial orifices, forming a mu- 

 IgEgr-- tnal support to each other, and 

 leaving the main stress of the 

 backward pressure of the co- 



Fibrous tissue of a semilunar valve beneath the lumn f bl d in th ^^^ 



endocardium. t o k e borne by that portion of 



each valve which intervenes between this and its attached border. 

 Hence the greater thickness of the fibrous basis in this situation. 

 It is worthy of notice that the tissue of the endocardium is nearly, 

 if not completely identical with the inner longitudinal fibrous tunic 

 of arteries ; a fact which explains the close similarity between the 

 diseased states of the arterial tissue and those of the endocardium 

 and the heart's valves. 



Mechanism of the Valves. The valves are closed by the mere 

 hydraulic pressure of the blood. When the blood accumulates in 

 the ventricles, it pushes up the auriculo-ventricular valves towards 

 the auricle until their several portions come in contact with each 

 other, and close the orifice. But the simple contact of the curtains 

 of these valves with each other would not prevent regurgitation ; for 

 the same hydraulic pressure which brings them in contact would push 

 them into the auricle, were it not that their margins are connected 

 with the walls of the ventricle and the musculi papillares by means 

 of the chordae tendinese. When the ventricle contracts, it draws 



