170 THE MOTION OF THE BLOOD. 



like a sail, against it, by any attempt at retrograde movement, 

 and thus close the openings." The mere attempt at retrocession 

 by the blood closes the semilunar valves : but the contraction of 

 the muscular bands attached by tendons to the edges of the 

 tricuspid and mitral valves during the systole of the ventricles 

 will assist in closing the auriculo-ventricular openings. 



" The texture of the heart is peculiar : fleshy, indeed, but very 

 dense and compact, far different from common muscularity. y 



" It is composed of fasciculi of fibres, more or less oblique, here 

 and there singularly branching out, variously and curiously con- 

 torted and vorticose in their direction, lying upon each other 

 in strata, closely interwoven between the cavities, and bound by 

 four cartilaginous bands at the basis of the ventricles, which 

 thus are, as it were, supported, and are distinguished from the 

 fibres of the auricles."* 



The heart was shown by Dr. Alexander Stewart a , about the 

 beginning of the last century, to be resolvable by boiling water 

 into a semicircular muscle, with all its fibres running parallel to 

 the base. Being rolled round in a funnel form, the left ventricle 

 is produced with the apex, which thus belongs entirely to it ; and 

 the second turn produces the fight ventricle, by the space between 

 it and the first layer. The walls of the left ventricle, except the 

 septum, are strengthened by another turn, which the right ven- 

 tricle has not ; so that the left ventricle is thicker than the right. 

 The auricles are distinct, and by boiling drop off from the ven- 

 tricles. They are very thin. 



The interior of the heart is lined by the same membrane which 

 forms the inner coat of the arteries and veins, being firmer and 

 more opake in the left or arterial cavities, which are continuous 

 with the arteries, than in the right or venous cavities, which are 

 continuous with the venae cavae. 



M. Gerdy has arranged the fibres of the ventricles into three 

 orders the one running from the heart's apex towards its 

 base, and ending in tendons which are attached to the tricuspid 



* " Leop. M.A. Caldani, Memorie lette neW Acad. di Padova. 1814. 4to. 

 p. 67." 



z Casp. Fr. Wolff, Act. Acad. Sdentiar. Petropol. for the year 1780. sq., 

 especially for 1781. P. i. p. 211. sq., on the cartilaginous structure of the heart, 

 or on the cartilagineo-osseous bands, and their distribution at the base of the 

 heart." 



* Phil. Trans, vol. ix. abridg. 



