2 Descriptions of Preparations. 



relatively to the craniospinal canal superiorly, as also to the digestive 

 tract next inferiorly, and the heart most inferiorly, is held by the 

 aorta in all vertebrata. The singleness of the aortic trunk in the 

 adult state is characteristic of all warm-blooded animals ; but mam- 

 mals, as is seen here, differ from birds in having* the single trunk 

 arching' from the heart over the left and not over the right lung's 

 root. Behind and to the right of this black bristle from before 

 backwards are to be seen, firstly, the fourth lobe of the right lung 

 in its pleural cavity resting on the diaphragm below, and in relation 

 above with the heart, and on the left with the phrenic nerve ; 

 secondly, the oesophagus, a lowly vascular tube the small calibre 

 of which is correlated with the working of the dental apparatus in 

 these creatures ; thirdly, the third lobe of the right lung placed 

 far back and to the right, and, like the lungs of all mammals, freely 

 suspended in its pleural cavity and bearing no impressions on its 

 exterior from the different bony constituents of the thoracic cavity ; 

 fourthly, the vena azygos ^ of the left side between the aorta and 

 the vertebral column, passing up to arch over the root of the left 

 lung, and join the vena cava descendens of that side; and fifthly, 

 the spinal cord. The complete diaphragm, forming a dome-shaped 

 floor, with the heart and lungs in relation with its convex, and the 

 liver, stomach, spleen, and kidney in relation with its concave 

 surface, and receiving a large nerve, the phrenic, from the cervical 

 region, is eminently characteristic of Mammalia. The upper part 

 of the pericardial sac has been removed, and the two ventricles 

 (less distinctly separated from each other than in many mam- 

 mals) and the left auricle are brought into view. The anterior 

 surface of the heart is more equally shared in by the two ven- 

 tricles than is the case in many mammals, in which the right 

 ventricle forms nearly the entire anterior aspect of the organ. 

 The left vena cava descendens, a trunk which is found in most 

 Rodents, except the Guinea Pig and Agouti, is seen to pass in 

 front of the root of the left lung in company with the phrenic 

 nerve round to the back of the heart to end in the right auricle. 

 The vena azygos of the left side is seen to join it just above the 

 root of the left lung, and at a point some way above this, the vein 

 from the fore-leg, which is in connection with the nerves going to 



'' For the various arrangements observable in the system of the vena azygos, see 

 Milne-Edwards, 'Lefons sur la Physiologie,' vol. iii. p. 598, ibique citata. 



