XIII PHYLUM OHORDATA 463 



heart consists of two layers, a parietal, forming the waill of the 

 pericai'dial cavity, and a visceral, immediately iavesting the heart. 

 Between the two is a narrow cavity containing a little fluid — the 

 pericardial fluid. In general shape the heart resembles the heart 

 of the Pigeon, with the apex directed backwards and slightly to the 

 left, and the base forwards. Like that of the Pigeon, it contains 

 right and left auricles and right and left ventricles, the right and 

 left sides of the heart having their cavities completely separated 

 off from one another by inter-auricular and inter-ventricular 

 partitions. 



Into the right auricle open three large veins — the right and left 

 precaval veins and the single postcaval — the first into the anterior 

 part, the second into the left-hand side of the posterior portion, 

 and the third into the dorsal surface. Projecting forwards from it 

 is an ear-like auricular appendix, the inner surface of which is 

 raised up into numerous cords of muscular fibres, the rmcsculi 

 peciinati. A membranous fold , the remnant of the foetal Eustachian 

 valve, extends from the opening of the postcaval forwards towards 

 the auricular septum. The opening of the left precaval is bounded 

 behind by a crescentic fold, the valve of Thebesius. On the septum 

 is an oval area where the partition is thinner than elsewhere ; this 

 is the fossa ovalis (f, ov.) ; it marks the position of an aperture, 

 the foramen ovale, in the foetus. The crescentic upper rim of the 

 aperture is known as the annulus ovalis. The cavity of the right 

 auricle communicates with that of the right ventricle by the wide 

 right auriculo-ventricular opening. This is guarded by a valve, 

 the tricuspid (tri. v.), composed of three membranous lobes or cusps, 

 so arranged and attached that, while they flap back against the 

 walls of the ventricle to allow the passage of blood from the 

 auricle to the ventricle, they meet together across the aperture so 

 as to close the passage when the ventricle contracts. The lobes 

 of the valve are attached to muscular processes of the wall of the 

 ventricle, the musculi papillares (m. pap^, by means of tendinous 

 threads called the chordce tendinece. The right ventricle, much 

 thicker than the auricle, forms the right side of the conical apical 

 portion, but does not extend quite to the apex. Its walls are 

 raised up into muscular ridges called columnce carnece. It gives 

 off in front, at its left anterior angle, the pulmonary artery, the 

 entrance to which is guarded by three pouch-like semilunar 

 valves (sem. v.). 



The left auricle, like the right, is provided with an auricular 

 appendix. Into its cavity on its dorsal aspect open together the 

 right and left pulmonary veins. A large left auriculo-ventricular 

 opening leads from the cavity of the left auricle into that of the 

 left ventricle : this is guarded by a valve, the mitral, consisting of 

 two membranous lobes or cusps with chords tendinese and musculi 

 papillares. In the walls of the ventricle are columnae carnae, rather 



