1 88 THE SALAMANDER 



TYvt pulmonary vein (v.pul.) lies close against the dorsal wall of the 

 sinus, and anteriorly it actually lies within the wall. It enters the left 

 auricle at its extreme right dorsal angle. The opening is not valved, 

 so that regurgitation of the blood into the vein can only be prevented 

 by the contraction of the surrounding muscles of the auricular wall. 



The auricles (aur.) are separated from one another by a septum 

 composed of a muscular network covered on either side by the car- 

 diac endothelium. Thus the septum within the meshes of the net- 

 work is exceedingly thin, and frequently appears to consist merely 

 of the double layer of endothelial cells. It is probably due to this 

 construction that the older authors thought it was perforate. The 

 walls of the auricles are also formed of a muscular network, but it is 

 much denser, so that, although the thickness varies greatly, they are 

 never so extremely thin as the septum atriorum. The inner surfaces 

 of the walls of the auricles are not smooth, neither are the cavities 

 entirely simple, since the muscle bands form numerous ridges, and 

 even isolated trabeculae, which are detached from the wall for a 

 greater or shorter distance, thus forming a number of small pits and 

 cavities within the wall. This is particularly the case in the left 

 auricle and the upper part of the right. Owing to this peculiar 

 formation of the auricular walls certain bulges appear on the external 

 surfaces of the contracted auricles which have been named 'recesses' 

 by Rose. Thus the recessus sinister lies at the ventro-posterior portion 

 of the left auricle, the recessus dexter is the right ventro-posterior 

 part of the right auricle, the recessus intermedius is the main cavity, 

 and the recessus dorsalis the right antero-dorsal portion of the same 

 auricle. 



The auriculo-ventricular opening is a single oval aperture with the 

 long axis in the horizontal plane, and the free edge of the septum 

 atriorum stretching along the short axis in a dorso-ventral direction. 

 Thus the blood from both auricles has to pass through a single 

 opening into the ventricle. Regurgitation is prevented by a pair of 

 flap valves, one dorsal and one ventral. Nevertheless, in the speci- 

 men modelled, there is a hint of a double nature of the opening, 

 since at its left end there is a shorter pair of valves, partially 

 separated from the main large pair by a fold. It is perhaps not safe 

 to generalize from a single specimen, and it is not practicable to 

 detect this feature except from sections. In any case the valves 

 must be regarded as a single pair from a functional point of view. 

 Nevertheless it is interesting to note that Osawa describes two pairs 

 of valves in Cryptobranchus^ where the heart is much larger and the 

 valves easier to see, and they consist of one large and one small pair. 



