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 eolumnce carnece, 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 FIG. iss.-Left auricle and ven. 

 triCUSpid ; (2) the pulmonary ; (3) the tricle, opened and part of their 



mitral ; and (4) the aortic. Thepulmo- ^ZttSS$?& 

 nary and aortic valves are sometimes short; i', cavity of left auricle; 

 spoken of as the semilunar valves. ' *> 3 'o' rt t ^ f w t a h 1 e ^m^th^n' 



The triCUSpid valve (Fig. 159) is larymuscle attache?; 6 5, the other 



situatrd at the right auriculoventric- papillary muscles; 6, 6', the seg- 



i -i , i. ments of the mitral valve ; 7, the 



alar orifice, and, as its name implies, %ure m aorta is placed V er the 



Consists of three CUSpS Or segments, semilunar valves; 8, pulmonary 



The bases of these cusps are attached to ^Sen 'ThomsorT) and lt branches 

 the opening, while the other edges are 



free, and to them are attached the chordce 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. 



