HEART OF BIMANA. 



527 



ventricle : but the walls are relatively thicker to those of the 

 left ventricle than in Birds. 



The left auricle, figs. 408 and 409, LA, lies to the left and 

 back part of the base of the heart, is transversely oblong and 

 subquadrate behind ; its auricular appendage comes forward into 

 view curving to the right, upon the base of the pulmonary artery. 

 The walls of the ' sinus venosus ' are thicker than those in the 

 right auricle : the terminal orifices of the pulmonary veins, 

 usually one on each side, sometimes two on the right and one on 

 the left, are undefended by valves : on the septum, the foetal 

 foramen is feebly indicated by a crescentic depression. The 

 opening into the left ventricle is smaller than the right auriculo- 

 ventricular one : it is defended by the pair of triangular folds of 

 endocardium, called the ' bi- 

 cuspid ' or ' mitral ' valve. 

 Of these the largest, fig. 

 407, a, hangs between the 

 auricular and aortic orifices, 

 and is in part reflected from 

 the sclerous ring of the lat- 

 ter : a small fold commonly 

 also projects at each angle 

 of junction of the larger 

 folds. The chief conical 

 ' columnae ' are two in num- 

 ber, and larger than those 

 of the right ventricle ; their 

 apices are shown at fig. 407, 

 p, p, each contributing tendinous cords to the portion of the mitral 

 valve, «. The distribution of the chordae tendineae, from each 

 column to contiguous borders of the two parts of the mitral, ob- 

 viously illustrates the adaptation to bring those margins together 

 in the contraction of the ventricle. The semilunar valves at the 

 aortic orifice, ib. d, are thicker than those of the pulmonary 

 artery, the ( Valsalval sinuses,' e, are deeper, and the ' corpora 

 arantii ' larger : the muscular walls of the left ventricle are about 

 three times thicker than those of the right: some of the inner 

 longitudinal fibres, ib. b, are attached to that part of the aortic 

 ring, not preoccupied by the larger mitral fold, a. The left 

 ventricle is longer and narrower than the right and alone forms 

 the apex : the two large mammillary columns occupy the lower 

 three-fourths of the cavity, rising in its axis : the fibres radiate 

 from their base and wind round the axis, being progressively 



Semilunar valves and portion of mitral valve, left 

 ventricle, clxxxvii". 



