THE VASCULAR SYSTEM 193 



a mechanism whereby the brain is assured of a supply of pure 

 oxygenated blood from the left auricle, and the body and respiratory 

 systems are supplied with a mixed blood, of which that going to the 

 former contains rather more oxygen than that to the latter system. 



This hypothesis, by discounting the importance of the septum 

 bulbi of Salamandra in effecting such separation of the blood, does 

 not explain why the septum should be absent from those Amphibia 

 which do not possess lungs, nor indeed why it should be present in 

 those which do. On the other hand it must be remembered that 

 the septum bulbi is but an exaggeration of the tendency to become 

 prolonged into ridges exhibited by all the distal valves. It seems 

 therefore not improbable that the inception of the septum bulbi may 

 be referred to similar mechanical stresses to those which, in the other 

 valves of the distal series, occasion the development of small rods of 

 hyaline cartilage (cf. p. 189). Whether this is so or not, a careful 

 observation of the contracting heart and bulbus cordis makes it cer- 

 tain that the presence of the septum enables the bulbus to empty 

 itsQ^ completely of blood, and thus ensures that the last drops of pure 

 oxygenated blood shall be forced into the vessels. In the case of an 

 apulmonate species, where the blood in the ventricle is homogene- 

 ously constituted with regard to oxygen, this complete emptying of 

 the bulbus would be of much less importance. IntheAnura,e.g.Rana, 

 the septum bulbi has developed more extensively, and the septum 

 principale of the truncus arteriosus approaches close to its anterior 

 end, so that the 'spiral valve' is able to execute the additional function 

 of ensuring more effectively than pressure alone can do that the 

 blood carrying least oxygen shall be delivered to the pulmonary artery. 



4. The Pericardium. 



The heart is surrounded by a coelomic space, th.Q pericardial cavity, 

 which is bounded externally by a tough, fibrous, transparent mem- 

 brane — th& pericardium. The cavity is completely separate from the 

 pleuro-peritoneal coelom, there being no trace of any pericardio- 

 peritoneal canal. 



The pericardium is attached to the heart at the anterior end of the 

 truncus arteriosus, at the ductus Cuvieri close to their formation 

 by the fusion of the venous trunks, and also to the dorsal wall of 

 the sinus venosus and to a part of the dorsal wall of the ventricle. 

 This attachment of the ventricle to the pericardium is an annular one, 

 involving a relatively large area (see Fig. 59). It is also present in the 

 Dipnoi and in many Reptiles^, but only as a ligamentous strand. In 

 ' Cf. also Hoffmann in Bronn's Thierreich, Band VI, Reptilia. 

 4038 o 



