140 



THE VOYAGE OF H.M.S. CHALLENGER. 



papillary muscle supplies additional tendinous cords to the left cusp of the valve only. 

 The cusps lie one on either side of the auriculo-ventricular orifice, which, on the left 

 side of the heart does not exceed one-third of the diameter of the corresponding orifice 

 of the right side. The aortic orifice is provided as in other birds with three semi-lunar 

 valves. 



Variations. — Except in Spheniscus demersus, in which I found that the two valves, 

 which in other species meet both in front of and behind the orifices of the right superior 

 and inferior venae cavse, are only continuous with one another in front of, and not behind 

 these orifices, I have observed no variations in the hearts of the various Penguins 

 examined. The following table gives the dimensions of the heart of difi'erent species in 

 inches. 



AKTEEIAL SYSTEM. 



The Pulmonary Artery. 



The pulmonary artery (PL XL fig. 1), after leaving the right ventricle, divides into 

 two main trunks, one for each lung. Each of these trunks as it passes outwards lies above 

 the corresponding superior vena cava, and below the bronchus. Each divides into two 

 subordinate branches, which enter the lung substance separately. The pulmonary artery 

 is situated ventrad of the other constituents of the root of the lung. 



