DISSECTION OP THE DOG 47 



speaking generally, the pulmonary veins are most ventral. (2) Pulmonary 

 artery distinguished from the veins by the greater thickness of its wall, and 

 by a somewhat more dorsal position. (3) Bronchus, a large air-tube produced 

 by the division of the trachea. The bronchus occupies the dorsal part of 

 the lung root. (4) Bronchial vessels of small size. (5) Nerves in the form of 

 the pulmonary plexus, which is produced by the intercommunicating branches 

 from the vagus and sympathetic nerves, and is connected with the cardiac 

 plexus. (6) Lymphatic vessels. 



Each root has certain structures closely related to it. The vena azygos 

 curves round the root of the right lung ; and the aorta is similarly disposed 

 on the left side. Both roots are crossed by the vagus nerve ; and the phrenic 

 nerve, though not in contact with, is ventral to the root. Both roots, moreover, 

 have a pulmonary ligament continued towards the diaphragm. 



N. phrenicits. — The origin of the phrenic nerve by three roots from the 

 fifth, sixth, and seventh cervical nerves has already been noted. Both right 

 and left nerves enter the thorax ventral to a subclavian artery and pass 

 through the precardial mediastinum. They then cross the pericardium ventral 

 to the roots of the lungs. The left nerve continues through the post-cardial 

 mediastinum to the diaphragm, but the right nerve courses along the lateral 

 aspect of the caudal vena cava and is therefore not contained within the medi- 

 astinal septum, but in the special fold of pleura provided for the caval vein. 

 There is a further difference in the relations of the two nerves. The left 

 crosses the commencement of the aortic arch and the pulmonary artery, the 

 pericardium intervening ; whereas the right lies along the lateral face of the 

 cranial vena cava. 



Pericardium.— The pericardium is a fibro-serous sac somewhat loosely 

 enveloping the heart. It is conical in form with the base of the cone looking 

 towards the vertebrae and the entrance to the chest. The apex points towards 

 the sternum and the diaphragm. Between the pericardium and the sternum 

 there is no attachment ; but the apex of the sac is connected with the fleshy 

 part of the diaphragm by a strong, flattened ligament. The outer face of the 

 pericardium is covered by pleura (pleura pericardiaca), and, as has just been 

 noticed, is crossed by the phrenic nerves. 



The outer layer of the pericardium is composed of fibrous tissue, and is 

 continued, as tubular investments, onto the large vessels associated with the 

 base of the heart. 



Dissection. — Make a crucial incision through the pericardium and examine 

 its interior. 



Within the fibrous pericardium is a serous membrane disposed after the 

 customary manner of its kind : that is to say, the parietal part of the 

 membrane lines the fibrous pericardium, and is reflected along the great 

 vessels onto the heart itself as the mceral portion or epicardium. It will be 



