THE HEART. 



307 



two left pulmonary veins, the former coming from the right and 

 the latter from the left lung. The left veins sometimes join, and 

 have but a single opening, in which case there would, of course, 

 be but three openings instead of four. At these openings there 

 are no valves. 



The left ventricle (Fig. 158) is by far the most powerful of the 

 four subdivisions of the heart. Its walls are three times as thick 

 as those of the right ventricle. The 

 capacity of its cavity is the same as 

 that of the right. The left auricle and 

 ventricle communicate by the left 

 auriculoventricular orifice, at which is 

 situated the mitral valve. Connected 

 with this ventricle is the aorta, the 

 opening of communication being the 

 aortic orifice, at which is situated the 

 aortic valve. 



On the inner surface of the ventri- 

 cles the muscular tissue projects, and 

 forms the eolunmce camece, or fleshy 

 columns ; some of these are ridges 

 only, while others are attached at both 

 ends, but are unattached in the middle, 

 while still others project into the 

 cavity and are attached at one ex- 

 tremity only ; the latter are the mus- 

 culi papillares, or papillary muscles. 



Cardiac Valves. There are four 

 sets of valves in the heart : (1) The 

 tricuspid ; (2) the pulmonary ; (3) the 

 mitral ; and (4) the aortic. The pulmo- 

 nary and aortic valves are sometimes 

 spoken of as the semilunar valves. 



The tricuspid valve (Fig. 159) is 

 situatrd at the right auriculoventric- 

 ular orifice, and, as its name implies, 

 consists of three cusps or segments. 

 The bases of these cusps are attached to 

 the opening, while the other edges are 

 free, and to them are attached the chordae tendinece, or tendinous cords, 

 the other ends being connected with the free extremities of the 

 musculi papillares to which reference has been made. This valve, 

 when shut, closes the right auriculoventricular orifice ; when 

 open the segments are in the cavity of the right ventricle. The 

 tendinous cords prevent these segments from passing into the cav- 

 ity of the auricle at the time of the valve's closure, while the papil- 

 lary muscles by their shortening keep the cords taut at the time of 

 the ventricle's contraction, as will be seen later. 



FIG. 15S. Left auricle and ven- 

 tricle, opened and part of their 

 walls removed to show their cavi- 

 ties ; 1, right pulmonary vein cut 

 short; 1', cavity of left auricle; 



3, 3", thick wall of left ventricle ; 



4, portion of the same with papil- 

 lary muscle attached ; 5, the other 

 papillary muscles; 6, 6', the seg- 

 ments of the mitral valve ; 7, the 

 figure in aorta is placed over the 

 semilunar valves; 8, pulmonary 

 artery ; 10, aorta and its branches 

 (Allen Thomson). 



