16 Macueay Memoriat VounvuMe. 
(a) Right pulmonary artery (figs. 17 and 18, ~ Azd. art). 
From its point of origin this passes backwards and shghtly inwards till it comes 
to lie close to the right side of the vertebral column. It passes beneath the cceliac 
and subclavian arteries, and entering the body cavity passes directly to the dorsal 
surface of the right half of the lung, along the length of which it runs in close 
contact with the dorsal aorta. It therefore only gives off branches on the right side. 
On the left side there is no artery in a corresponding position. Its course and 
branches can easily be determined by injecting from the pulmonary artery on the 
right side close to the epibranchial vessel. A little way beyond the latter it passes 
through stiff fascize in which its course is difficult to make out unless the vessel has 
been previously injected. 
(b) Left pulmonary artery (figs. 17 and 18, 2. pu. art.). 
The course of this is very different from the simple straightforward one of the 
right artery. From the fourth epibranchial vessel of the left side it passes backwards 
until it reaches the level of the subclavian artery : at this point it lies close to the 
left side of the cesophagus. It now bends suddenly forwards and at the same time 
downwards towards the ventral side of the cesophagus, then it once more turns 
backwards and crosses under the cesophagus just in front of the opening of the lung 
until it reaches the right side. Here it runs backwards close by the pulmonary vein 
and where the lung comes to be dorsad of the alimentary canal divides into two 
branches, one running down either side of the mid ventral stripe and giving off 
branches to the half lung of its own side. Till it comes in contact with the 
alimentary canal its walls are fairly but not noticeably thick. From the level of the 
glottis to the dorsal surface where it passes on to the ventral aspect of the lung— 
that is in the region where it is firmly attached to the alimentary canal—its walls 
are extremely thick and strong and the same is true of the pulmonary vein. 
The two modifications, (a) the curious loop-like course and (b) the strong walls 
just described, are probably both to be regarded as adaptations to prevent any 
interference with the flow of blood when the animal is feeding. 
Judging by the contents of their intestine which is always crammed full of 
vegetable matter—grass, waterweeds, gum-tree fruit, &c.—the animals must be 
somewhat voracious feeders. The food passing down the cesophagus distends the 
latter, around which, loosely attached to its walls, the artery passes : the loop in this 
position allows of its distension. When the cesophagus passes through the stout 
