243 THE HEART. 



them back through tlie auriculai; orifice : the papillary mitscles, shortening- as 

 the cavity of the ventricle itself shortens, are sui^posed thus to prevent the valve 

 from yielding too much towards the auricle. In the angles between each pair of 

 the principal segments of the auriculo-ventricular valves there may be found, 

 but not constantly, as many small mtermediate lobes. 



According to Kiirschner (Wagner's Handworterbuch. art. '• Herzthatigkeit"), 

 three kinds of cords belong to each segment ; a, the first set, generally two to 

 four in number and proceeding from two different sets of papillae, or from one of 

 these and the wall of the ventricle, rmi to the base or attached margin of the 

 segment, and are there connected also with the tendinous ring round the auriculo- 

 ventricular opening ; h. 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 Arentricular 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 fi'ee 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 poi-tions of the 

 valves. A few muscular fibres prolonged from the neighboming walls penetrate 

 into the segments of the auriculo-ventricular valves. 



A fibrous band, sometimes muscular, is often found stretching across 

 the cavity of the right ventricle from the base of the anterior papillary 

 -muscle to the septum. It represents the strong "moderator"' band 

 found in the heart of the ox and of some other animals. 



The valve at the orifice of the pulmonary artery consists of three 

 flaps, a right and left anterior and a posterior, named from their shape 

 semilunar or sigmoid (figs. 167, G ; 172 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 

 "svill be reserved until these are treated of. 



The left auricle (fig. 170, l') occupies the left and posterior part 

 of the l)ase of the heart. The atrium presents from behind, where 

 it is best seen, a quadrilateral ' appearance. In front it is in con- 

 tact with the aorta and pulmonary artery ; liehind, it receives two 

 pulmonary veins on each side, those fi-om the left lung entering 

 very close together ; on the right, it is in contact with the other 

 auricle. The auricular appendage (fig. 165, 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 pulmo- 

 nary 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 presents musculi pectinati somewhat 

 similar to those in the right side of the heart, but the walls of the sinus 

 Tenosus are altogether 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. 170, 1). The 

 two veins of one or both sides sometimes unite into one before enter- 

 ing the auricle, whilst in other cases there is found an additional open- 

 ing, most 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, a 



* Sibson proposes to term the whole apparatus guarding the arterial orifices the 

 aortic or pulmonary valre, reserring the terms semilunar and sigmoid to denote the 

 icdividual fla2)s or segments. 



