48 DISSECTION OP THE DOG 



observed that the aorta and puhnonary artery are enclosed within a coramon 

 tubular sheath of the serous pericardium. Dorsal to these vessels is a passage, 

 the transverse sinus of the pericardium (sinus transversus pericardii). 



The serous membrane endows the fibrous pericardium and the surface 

 of the heart with a smooth glistening appearance, and, like all other serous 

 membranes, is always moist. 



Vena cava ceanialis. — The cranial vena cava is a large vessel formed, 

 on about the level of the middle of the first sternal segment, by the union of 

 the right and left innominate veins. Each innominate vein (v. anonyma) 

 in its turn results from the junction of the jugular and subclavian veins of its 

 own side. As collateral tributaries, each innominate vein has the internal 

 mammary vein and a common trunk formed by the vertebral and costo-cervioal 

 veins flowing into it. Any of these, however, may join the vena cava itself. 



The cranial vena cava lies in the precardial mediastinum ventral to the 

 trachea, and pierces the pericardium to the right of the aortic arch. The 

 vein terminates by opening into the right atrium of the heart. 



During the process of cleaning the vena cava, several mediastinal lymph- 

 glands (lympho-glandulse mediastinales) will be revealed. Some of these are 

 ventral to the vein : others are between the vein and the trachea. 



Vena azygos. — The azygos vein begins in the abdomen and drains a con- 

 siderable proportion of the wall of the chest. At the present moment only 

 its terminal part can be examined. The vein joins the cranial vena cava just 

 as this is entering the heart, or it may open into the right atrium itself. 



Vena cava catjdaijs. — The caudal vena cava begins on a level -ivith the last 

 lumbar vertebra, and enters the thorax by the foramen venae cavse of the 

 diaphragm. The thoracic part of the vein, which is all that should be examined 

 now, lies in a notch in the intermediate lobe of the right lung, enclosed in a fold 

 of pleura. The vein passes through the pericardium and opens into the right 

 atrium of the heart. 



The right phrenic nerve, as has already been said, lies lateral and ventral 

 to the vein. 



Dissection. — In order to give more room for the dissection of the heart, 

 etc., the lungs may now be removed. 



CoE. — The heart is a hollow muscular organ in the form of an irregular 

 cone placed so obliquely in the chest that its base (basis cordis) looks slightly 

 towards the vertebral column, but mainly towards the entrance to the thorax. 

 The blunt apex (apex cordis), on the other hand, is directed mainly towards 

 the diaphragm, but also towards the ventral aspect of the body and slightly 

 towards the left. 



The heart of the dog is flattened in an oblique dorso-ventral dii-ection, so 

 that it presents convex sterno-costal and diaphragmatic surfaces and thick 

 right and left borders. 



