550 THE RABBIT CHAP. 



the jugular and brachial veins. Note in the thoracic cavity thus laid 

 open : 



1. T\\Q pericarditim and heart, and the thymus (Fig. 135). 



2. The collapsed lungs : make a small aperture in the trachea and 

 inflate. 



3. The parietal and visceral layers of the pleura, and the media- 

 stinum (Fig. 138). To make out the relations of these and of the 

 mediastinal space more accurately, cut away a flap from the right wall 

 of the thorax, as you have already done on the left side, leaving the 

 sternum intact. 



4. The relations of the pericardium, to see which cut through the 

 posterior end of the sternum : separate the mediastinum from the dorsal 

 surface of the bone, turn the latter forwards and remove it : then cut 

 the pericardium open longitudinally, noting the pericardial fluid. 



II. Dissect away the pericardium, the thymus, and any fat about the 

 base of the heart which may obscure the vessels arising from it. Follow 

 out these vessels to the head by clearing away the connective-tissue, 

 fat, &c., by which they are surrounded, but taking care not to injure 

 any of the nervt s of the neck. Make out : 



1. a, The left and right ventricles and auricles, and the ramifications 

 of the coronary artery and vein ; b, the two bronchi. 



2. a, The ptdmonary artery and its division into left and right trunks : 

 b, the pulmonary veins (the course of which will be better seen at a 

 later stage) ; c, the two precaval veins, each formed by the union of a 

 subclavian, an external jugular, and a smaller internal jugular ; d, the 

 thoracic portion of the postcaval vein. 



3. a, The gullet (cervical and thoracic portions) ; b, the phrenic 

 nerves (p. 532), and the thoracic portions of the vagi (Fig. 135) ; 



j c, the arch of the aorta, continuous with the dorsal aorta, and the short 

 ligament (vestige of the foetal ductus arteriosus] connecting the former 

 with the pulmonary artery ; d, the thoracic portions of the sympathetic 

 nerves and their ganglia, lying on the heads of the ribs ; e, the azygos 

 vein (p. 525), best seen by turning the heart and lungs over to the left 

 side of the animal. The thoracic dtict (p. 528) cannot easily be made 

 out at this stage. 



4. a, The innominate artery, giving off the left and right common 

 carotids and the right subclavian ; b, the left subclavian artery ; c, the 

 division of each common carotid, at about the level of the anterior end 

 of the larynx, into an external and an internal carotid. 



