402 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



stem to their insertions (see Fig. 103). These insertions are both into the free 

 margin of the valve and into the whole extent of that surface of it which 

 looks toward the wall of the ventricle, quite up to the ring. By means of this 

 arrangement of the cords each curtain is held taut from base to apex through- 

 out the systole of the ventricles, the opposed surfaces being kept in apposition, 

 and the parts of the curtains between these surfaces and the ring being kept 

 from bellying unduly toward the auricle. Each curtain is held sufficiently 

 taut from side to side as well, because the tendinous cords inserted into one 

 lateral half of the curtain spring from a widely different part of the wall of 

 the heart from those of the other lateral half of it (see Fig. 103). At all times, 

 therefore, even when the walls of the ventricle are most closely approximated 

 during systole, the cords may pull in slightly divergent directions upon the 

 two lateral halves of each curtain. This arrangement of the cords may also 

 cause them, when taut, to pull in slightly convergent directions upon the 

 contiguous lateral halves of two neighboring curtains and thus to favor the 

 pressing of them together (see Fig. 103). 



Papillary Muscles and their Uses. In the left ventricle the tendinous 

 cords arise in two groups, like bouquets, from two teat-like muscular projec- 

 tions which spring from opposite points of the wall of the heart, and which 

 are called the "papillary muscles" (see Fig. 103). One of these gives origin 

 to the cords for the right half of the anterior and for the right half of the 

 posterior curtain ; the other papillary muscle gives rise to the cords for the 

 left halves of the two curtains. Each papillary muscle is commonly more or 

 less subdivided (see Fig. 103). The same principles are carried out, but less 

 regularly, for the origins of the tendinous cords of the more complex tricuspid 

 valve. Various opinions have been held as to the use of the papillary muscles. 

 It seems probable that during the change of size and form wrought in the 

 ventricle by its systole, the origins of the tendinous cords and the auriculo- 

 ventricular ring tend to be approximated and the cords to be slackened in 

 consequence. Perhaps this is checked by a compensatory shortening of the 

 papillary muscles, due to their sharing in the systolic contraction of the mus- 

 cular mass of which they form a part. Observations have recently been made 

 which have been interpreted to mean that the papillary muscles begin their 

 contraction slightly later and end it slightly earlier than the mass of the 

 ventricle. 1 



Semilunar Valves. The anatomy and the working of the semilunar 

 valves are the same in the aorta as in the pulmonary artery, and one account 

 will answer for both valves. Each valve is composed of three entirely sepa- 

 rate segments, set end to end within and around the artery just at its origin 

 from the ventricle. The attachments of the segments occupy the entire cir- 

 cumference of the vessel (Fig. 103). Like the tricuspid and mitral valves, 

 each semilunar segment is composed of a sheet of tissue which is tough, thin, 

 supple, and slippery ; but the semilunar valves differ from the tricuspid and 



1 C. S. Roy and J . G. Adami : " Heart-beat and Pulse-wave," The Practitioner, 1890, i. 

 p. 88. 



