5f DR. C. F. SON^TtAG on the dOMPAilATIVE ANATOMY 



as stout ectocarotids. The former give off no branches in the 

 neck, and enter the carotid foramen in the basisphenoid. The 

 latter pass antero-laterally between the exoccipital process and 

 the mandible, and the former separates it from the external 

 jugular vein. In Trichosurus vulpecula the exoccipital process 

 does not separate these vessels. 



The Descending Thoracic Aorta lies first on the right half of 

 the body of the fifth dorsal vertebra, but passes more to the 

 right as it runs backwards through the thorax. It is connected 

 to the oesophagus by a triangular fold of pleura, whose base is 

 attached to the diaphragmatic pleura. It has the usual relations 

 and gives off the usual branches. 



The vertebral arteries come off from the subclavians, but in my 

 specimen of Trichosurus they arise from axes (text-fig. 61, E). 



The Abdominal Aorta differs from the common Marsupial 

 condition in that it is entirely to the left of the postcaval vein. 

 Hochstetter (20) examined several species, and found that the 

 postcaval vein concealed the lower end of the aorta in all but 

 Petaurus taguanoides, but he did not include Phascolarctos 

 cinereus in his list. Beddard (21) also found the postcaval vein 

 covering the aorta in all his species. My observations on Tricho- 

 surus vtdpecula and Dendrolagii,s ursinus confirm those of Hoch- 

 stetter and Beddard. 



Relations. — At the point where the abdominal aorta I'eaches 

 the level of the posterior surface of the liver, it is separated fi'om 

 the right suprarenal capsule by the right crus of the diaphragm. 

 These structures separate it from the postcaval vein. 



A little posterior to the capsule the solar plexus lies between 

 the aorta on the left, the postcaval vein on the right, and the 

 cceliaco-inesenteric artery ventrally. 



Posterior to the solar plexus the postcaval vein lies immediately 

 to the right side of the <aorta till the latter divides into the 

 internal iliac and mid-sacral arteries, and then the vein crosses 

 the surface of the aorta. The whole trunk of the aorta is 

 visible. 



The relations to other structures show nothing unusual. 



Branches. — With the exception of the four pairs of lumbar 

 arteries which arise at regular intervals from the sides, the 

 following is the order of the branches from before backwards : — 



Phrenics, cceliaco-mesenteric, suprarenals, renals, ovaiians, 

 external iliacs, internal iliacs, and middle sacral (cavidal). 



After giving off the external iliac arteries, the trunk of the 

 aorta diminishes in calibre to a little more than half of that of 

 either of the external iliac arteries. 



The Cceliaco-mesenteric Artery divides into cceliac axis and 

 anterior mesenteric artery, and a branch of the latter replaces 

 the posterior mesentei'ic branch of the abdominal aorta of the 

 higher Mammalia,. 



The cceliac axis divides into gastric, hepatic, and splenic 

 arteries, whose distribution has already been described. 



