360 THE HEART. 



through the auricular orifice ; the papillary muscles, shortening as the cavity of the 

 ventricle itself shortens, prevent the valve from yielding too much towards the 

 auricle. 



According- to Kiirschner, 1 there are three kinds of cords to each segment ; a, the first set, 

 generally two to four in number and proceeding from two different sets of papillse, or from 

 one of these and the wall of the ventricle, run to the attached margin of the segment, and 

 are there connected also with the tendinous ring round the auriculo-ventricular opening ; 

 5, the second set, more numerous, and smaller than the first, proceed also from two adjacent 

 papillary muscular groups, and are attached at intervals to the back or ventricular surface 

 of each segment along two or more lines extending from the points of attachment of the 

 tendons of the first order at the base of the valve to near its free extremity ; c, the third set, 

 which are still more numerous and much finer, branch off from the preceding ones, and are 

 attached to the back and edges of the thinner marginal portions of the valves. A few 

 muscular fibres prolonged from the neighbouring walls of the auricles sometimes penetrate 

 into the segments of the auriculo-ventricular valves ; blood-vessels accompany these, but in 

 all other parts the valves are non- vascular. 



Stretching obliquely across the cavity of the right ventricle there is usually a 

 more isolated fleshy trabecula known as the moderator band, which extends from the 

 base of the anterior papillary muscle upwards and inwards to the septum. It varies 

 much in size, and is occasionally fibrous ; or it may be divided into two or more 

 parts. This band is more largely developed in the heart of the ox and some other 

 mammals. 



The valve at the orifice of the pulmonary artery consists of three flaps, a right 

 and left, and a posterior, named from their shape semilunar or sigmoid (fig. 309 ; 

 fig. 314, I) : they are constructed similarly to those on the left side at the root of 

 the aorta ; and as the characters of the last named are better marked, the more 

 complete description will be reserved until these are treated of. 



The left auricle occupies the left and posterior part of the base of the heart. 

 The atrium presents from behind, where it is best seen, an oblong appearance, with 

 its longer diameter directed transversely. In front it is in contact with the aorta 

 and pulmonary artery ; behind, it receives two pulmonary veins on each side, those 

 from the left lung entering very close together ; on the right, it is in contact with 

 the other auricle. The auricular appendage (fig. 306, 6) is the only part of the left 

 auricle seen from the front : it extends forwards from the left side of the atrium, 

 and curves towards the right side, resting on the pulmonary artery. It is more 

 curved as well as longer and narrower than that of the right auricle, and its margins 

 are more deeply indented. 



The interior of the appendix has musculi pectinati somewhat similar to 

 those on the right side of the heart, but the walls of the atrium are alto- 

 gether smooth and even, and are also thicker than those of the right auricle. 

 Posteriorly the openings of the pulmonary veins are seen, usually two on each side, 

 and without valves (fig. 312, 1). The two veins of one or both sides sometimes 

 unite into one before entering the auricle ; while in other cases there is found an 

 additional opening, more frequently on the right side. In the lower and fore part 

 of the auricle is situated the left auriculo-ventricular orifice. It is of an oval form, 

 and is rather smaller than the corresponding opening between the right auricle and 

 ventricle. On the septum between the auricles is seen a vestige of the foramen 

 ovale in the form of a lunated depression (!'), comparable to the mark made by a 

 finger-nail on a soft surface, and placed on a higher level than the fossa ovalis of the 

 right auricle. The depression is limited below by a slight crescentic ridge, having 

 the concavity turned upwards ; this is the border of the now adherent membranous 

 valve, which during foetal life is applied to the left side of the then open foramen 



1 Wagner's Handworterbuch, art. " Herzthatigkeit. " 



