THE THORAX AND ITS CONTENTS 97 



Thymus gland: In young, and often in adult rabbits, the 

 thymus gland, a fatty-appearing pink mass, occupies much of 

 the sternal wall of the thoracic cavity craniad to the heart and 

 may extend onto the neck in young, and often in adult rabbits, 

 while in old adults it is usually small or quite indistinguishable. 

 Leave the thymus gland loosely attached by its right side until 

 actual dissection necessitates further dissection or its complete 

 removal. 



Mediastinum : Determine the position, extent, form, and 

 relations of the mediastinum, with as little cutting as possible. 

 It is essentially a chambered septum with translucent walls 

 composed primarily of two layers of serous membrane, the 

 pleura, which separates the thorax into right and left pulmonary 

 portions, and a central portion. What important structures 

 occur in each portion of the thorax? 



Pericardium : Note the attachments, structure and relations 

 of the pericardium and then slit this sac open on the ventral side 

 to expose the heart. 



B. HEART AND GREAT BLOOD VESSELS 



(Figs. 26, 27) 



Clear the fat, and, if necessary, the thymus gland and other 

 tissues away in order to expose the heart and great blood 

 vessels for study. 



Heart: Note the form and position of the heart, the thick- 

 walled ventricles and the collapsed thin-walled auricles, which as 

 previously stated, usually contain a quantity of blood clot. 

 The wall of the left ventricle is much firmer than that of the 

 right, due to the differences in actual thickness which in turn is 

 correlated with the functions of the two ventricles. 



7,7^. Pulmonary artery : Cranioventral end of right ventricle 

 to lungs. It lies ventrad to the aorta at its origin, but passes 

 to the left and bifurcating sends one branch over the dorsal sur- 

 face of the aorta to the right lung and the other over the dorsal 

 side of the left subclavian vein to the left lung. The pulmonary 



