208 ANIMAL BIOLOGY. [Part I. 



Fig. 63, are strengthened with a network of muscular bands, the 

 musculi pectinati. In the septum between the auricles there is 

 an oval area which is thinner than the rest. This is the fossa 

 ovalis, and represents a communicating aperture, the foramen 

 ovale, of the fostus. The apertures of the pulmonary veins have no 

 valves. That of the postcaval has a fold (Eustacliian valve) from its 

 posterior margin towards the septum. That of the right precaval 

 has posteriorly a semi-lunar fold. That of the left precaval has 

 no valve. Just within it is the small aperture of the coronary 

 vein, bringing blood back from the tissues of the heart itself. 



Between the right auricle and ventricle there is a tricuspid 

 valve, consisting of three membranous flaps, of which the irre- 

 gular posterior edges are connected by tendinous threads, the 

 chordce tendinece, to little muscular cones on the ventricle, the 

 papillary muscles. These prevent the flaps being forced upwards 

 into the auricle. Between the left auricle and ventricle there is 

 a bicuspid or mitral valve, consisting of two such flaps, also pro- 

 vided with chordae and papillary muscles. The septum between 

 the ventricles is muscular and convex towards the right ventricle. 



Both the pulmonary artery and the aorta are guarded near 

 their point of origin from the ventricle by a set of three semi- 

 lunar valves, arranged like watch-pockets. If blood attempts to 

 flow towards the ventricles the watch-pockets fill, and their flaps, 

 meeting in the middle of the channel, completely block the 

 passage. 



The Pigeon (Fig. 65). The heart has two ventricles and two 

 auricles. The pulmonary artery bifurcates at once (pvl. ar.), its 

 two branches being the right and left pulmonary arches. The 

 pulmonary veins fall into the left auricle in the V-shaped space 

 between the two precavals. The aorta gives off two large 

 branches, almost immediately at its origin. These are the 

 innominate arteries (in. a.). The continuation of the aortic arch 

 is relatively smaller than these two branches. It crosses over 

 the right bronchus, and represents the right trunk in the frog. 

 Each innominate gives off a common carotid, and becomes the 

 subclavian artery (s. cl. a.), which branches into a smaller brachial 



