186 ANATOMY OF VERTEBEATES. 



precaval, ib. b, winds round the posterior part of the left auricle to 

 open into the lower part of the sinus ; just before its termination 

 it receives the coronary vein, so that this does not open separately 

 into the auricle as in most Mammals. The postcaval vein, ib. c, 

 terminates in the sinus just above the orifice of the left precaval, 

 and a semilunar valvular fold, ib. h, analogous to that of the 

 coronary vein in man, is extended forward between these orifices 

 so as to separate them, and afford a protection to the mouth of 

 the left precaval, in addition to that which it derives in common 

 with the other veins from the larger valves at the mouth of the 

 sinus. 



The disposition of the valves between the sinus and auricle 

 seems more especially destined to prevent regurgitation into the 

 sinus, when the pulmonary circulation may be impeded. A 

 strong oblique semilunar muscular fold, ib. g, commences in the 

 Emeu by a band of muscular fibres running along the upper part 

 of the auricle, and expanding into a valvular form extends along 

 the posterior and left side of the sinus, terminating at the lower 

 part of the fossa ovalis, ib. i. A second semilunar muscular 

 valve, ib./, of equal size, extends parallel with the preceding 

 alonar the anterior border of the orifice of the sinus, its lower 

 extremity being fixed to the smooth floor of the auricle, its upper 

 extremity being continued into a strong muscular column running 

 parallel to the one first mentioned across the upper and anterior 

 part of the auricle, and giving off from its sides the greater part of 

 the musculi iiectinati. From this structure it results that the more 

 powerfully the musculi pectinati act in overcoming the obstacle 

 to the passage of the blood from the auricle to the ventricle, the 

 closer will the valves be drawn together, and the stronger will be 

 the resistance made to them by the regurgitation of the blood 

 from the auricle into the sinus. The valves f and g are homo- 

 logous with the pair dividing the auricle, 6>, from the sinus, 5, in the 

 Crocodile's heart (vol. i. fig. 339). The parietes of the auricle 

 in the interspaces of the muscular fasciculi are thin and trans- 

 parent, consisting in many parts only of the lining membrane of 

 the cavity and the reflected layer of the pericardium blended 

 together. T^q fossa ovalis, fig. 90, i, is a deep depression situated 

 behind the posterior semilunar valve, which, we may observe, bears 

 nearly the same relation to the fossa as the annulus ovalis in the 

 human heart. The membranous septum closing the foramen 

 ovale is complete and strong, but thin and semitransparent. The 

 appendix auriculod, ib. x, is the most muscular part of the cavity ; 



