Spencer—TZhe Llood Vessels of Ceratodus. We 
“diaphragm ” between the pericardium and the body cavity behind, distension of the 
canal would simply mean the obliteration of the lumen of the blood vessels, unless 
these were protected as they both are by stout walls. 
In this same part also the walls of the lung are very thick and muscular. 
C.—Pulmonary vein (figs. 17 and 18, pwd. v.). 
On the ventral aspect of each half of the lung there can be clearly seen a large 
vessel running parallel to and at some distance laterally from the branch of the right 
artery of its own side. Traced forwards, these two veins, which are richly supplied 
on both sides with branches, converge towards the median ventral line until slightly 
in front of the point of divergence of the two arteries they unite to form a single 
thick-walled pulmonary vein. The right branch passes under the corresponding 
artery before joining its fellow so that the vein lies to the left side of the main right 
artery. The two pass close together round the cesophagus (figs. 16 and 18) and then 
the vein runs forwards and towards the left side dorsad of the inferior vena cava 
(to be described below) and the sinus venosus to enter the auricular chamber on the 
left side. 
A curious communication by means of a well-marked transverse vessel on each 
side may here be noted, which exists between the two branches of the right artery 
and of the vein (fig. 18, 2.). 
TV.—Tue VEIns. 
These have been very briefly noted by Dr. Giinther whose short account refers 
almost entirely to the vena cava posterior and its main branches including the renal 
portal system. 
The study of the veins is more difficult than that of the arteries but the venous 
system shows, as might be expected, affinities to both that of a fish on the one hand 
and that of an amphibian on the other. It may be said that in its anterior portion 
Ceratodus is typically piscine, whilst in its posterior, or trunk and caudal regions it 
presents a mixture of piscine and higher types such as is probably not found outside 
the group Dipnoi. 
Cc 
