CHAP. XXVIII.] MUSCLE OF THE HEAIIT. 339 



firmly by means of the chordae tendineae upon the valves, and not 

 only keeps them closed, but causes them to exercise a considerable 

 pressure on the blood, which promotes its onward flow into the 

 artery. 



Thus, by the attachment of the chordae tendineae to the several 

 curtains of the valves, not only is regurgitation of the blood opposed, 

 but every part of the surface of each curtain is made to press upon 

 the blood with a force equal to that of the contraction of the ven- 

 tricle, and to aid in propelling it through the artery. Should, how- 

 ever, any imperfection of the valve exist, as by the imperfect 

 apposition of its several portions, a chink remains between their 

 margins, and regurgitation takes place to a degree proportionate 

 to the size of the chink. 



The semilunar or arterial valves are closed in their turn by the 

 pressure of the blood from the artery backwards to the heart. 

 The blood forcibly driven back by the elastic reaction of the arte- 

 rial walls, slips between the wall of the artery and the valves, at the 

 sinuses of Yalsalva, and throws the lattsr inwards, causing them to 

 meet and close the arterial aperture, thereby preventing regurgitation 

 into the ventricle. Thus the force by which the arterial valves are 

 closed, being the elastic reaction of the arterial walls, excited by the 

 expulsive force of the ventricle, bears a constant ratio to the con- 

 tractile power of the wall of the heart; and, therefore, the degree of 

 tension of the semilunar valves, and the sound which it developes 

 (the second sound of the heart), enables us to form an estimate of 

 the expulsive force of the ventricle, which is often of great value 

 in practice. 



Of the Muscular Tissue of the Heart. The heart is composed of 

 muscular fibres of very various sizes. In all essential points of struc- 

 ture, these fibres resemble very closely the striped fibres of the 

 external muscles, differing from them, however, in the extreme 

 tenuity of the sarcolemma. They interlace with each other in an 

 intricate manner, and assume opposite directions on different planes, 

 thus forming a complicated interlacement of fibres, which adds 

 greatly to the power of resistance possessed by the organ. By this 

 interlacement, the fibres of the heart adhere to each other, for 

 between them there is little or none of that areolar tissue which 

 exists in considerable quantity in the external muscles, and unites 

 their fibres and fasicles together. This interlacement takes place 

 irrespective of any subdivision of the fibres. Nevertheless, it has 

 recently been noticed by Kolliker and other good observers, that 

 a true anastomosis does take place between adjacent fibres by the 



