of the koala and vulpine rhalanger. 569 



The Organs of Circulation. 



Phascolarctos cinereus. 



The Heart (text-fig. 61, A). — The fibrous pericardium is firmly 

 bound to the dorsal surface of the sternum and left ribs by 

 connective tissue. It narrows out and is lost on the great vessels. 

 These conditions are also present in Trichosurus vulpecida. 

 The area of adhesion is greater than that of Dendrolagus ursimis. 

 The long axis of the heart is antero- posterior and parallel to 

 the left side of the sternum, and the apex is level with the 

 posterior border of the fourth left costal cartilage. Its apex is 

 more obtuse than that of Trichosurus vul^jecula * . 

 Measurements : — 



Phascolarctos. Trichosurtos. 



Length 4-6 cm. 3 cm. 



Greatest width 2-4 „ 2'2 „ 



Greatest thickness ... 1'9 ,, 1"5 )? 



The characters agree with those already described for the 

 Marsupial heart. 



The Ascending Aorta (text-fig. 61 A) is embraced by the two 

 parts of the right auricular appendix, and is parallel to the 

 right precaval vein. It quickly gives way to the aortic arch., whose 

 most anterior point is level w^ith the anterior end of the first 

 intercostal space. At the level of the fourth dorsal vertebra it 

 gives way to the descending aorta ; this is the commonest level 

 for the transition in the Mammalia. The relations of the arch 

 are : — 



Ventral. — Left lung, left precaval vein, left azygos vein, left 

 vagus nei've, left phrenic nerve. 



Dorsal. — Trachea, oesophagus. 



Posterior. — Root of left lung. 



Anterior. — Great vessels of head, neck, and fore limbs. 

 Branches: — The right subclavian («), right common carotid (5), 

 and left common carotid (c) arise directly, and close to one 

 another, from the aortic arch, so there is no innominate artery. 

 Alter a short interval the left subclavian artei'y is given o^ {d), 

 Martin (8) and Owen (11) describe an arrangement similar to 

 that of Man, but Forbes (4) states that the innominate artery 

 gives off three vessels close together from a common trunk. In 

 Trichosurus vidpecula the aortic arch gives ofi" the innominate 

 and left subclavian arteries, and the former, after giving off the 

 left common carotid, divides into right common carotid and right 

 subclavian arteries ; the left common carotid arises almost at the 

 beginning of the innominate artery. 



The Common Carotid Arteries give oft" fine entocarotids at the 

 level of the anterior border of the hyoid bone, and are continued 



* Some specimens of Tricliosiirus vuljpecula have a more obtuse cardiac apex than 

 the above. 



