DISSECTION OF THE BACK AND THORAX. 121 



of the first pair of ribs. Tt passes backwards through the anterior 

 mediastinum, being there related to the great arteries, beneath and 

 to the right of which it is placed. 1 It enters the roof of the right 

 auricle. Besides the vessels already seen entering it on the left 

 side (page 1 4), it receives : — 



1. The Internal Thoracic Vein. 



2. The Vertebral Vein. 



3. A trunk formed by the union of the dorsal and superior cervical 

 veins. (These veins may enter independently.) 



4. The Great Vena Azygos (sometimes). This large vein begins 

 behind the hiatus aorticus, where it receives the first pair of lumbar 

 veins. Passing through the hiatus, it extends along the dorsal portion 

 of the spine to the right of the posterior aorta, the thoracic duct being 

 usually between the two vessels. At the 6th or. 7th dorsal vertebra 

 it leaves the spine and curves downwards to the right of the aortic 

 arch, the oesophagus, and the trachea ; terminating either in the 

 anterior vena cava, or in the auricle immediately behind the opening 

 of that vein. Besides the first pair of lumbar veins, it receives the 

 last thirteen intercostal veins of the right side and the last five or six of 

 the left. 



The Posterior Vena Cava. This great vein enters the thorax through 

 the foramen dextrum of the diaphragm. It passes directly forwards to 

 terminate in the right auricle, being included between the main mass of 

 the right lung and its small internal lobe, and placed at the upper edge 

 of a double serous fold belonging to the right pleural membrane. The 

 right phrenic nerve is in company with it. 



The Right Lymphatic Duct. This is a short lymphatic vessel (not 

 more than two inches) which empties itself into the initial part of the 

 anterior vena cava, at the angle of junction of the jugular and axillary 

 veins of the right side. Its opening is provided with a valve. It dis- 

 charges into the venous system the lymph collected in the right anterior 

 half of the animal, viz., the right fore limb, and the right side of the 

 head, neck, chest-wall, and diaphragm. 



Directions. — Cut out the trachea and lungs with the heart and great 

 vessels. Sever the lungs from the heart by cutting the great vessels at 

 the root, and set both lungs and heart aside in some preservative 

 solution to serve in the examination of the structure of these organs. 

 Or, since they are likely to be much decomposed, it will be better 

 to discard them if fresh organs can be obtained. In the mean- 

 time separate the sternum and costal cartilages as shown in Fig. 7, 

 and dissect the triangularis sterni muscle and .the internal thoracic 

 vessels. Portions of the longus colli and psoas muscles which are 



] The vein is in the natural position in Plate 25. In Plates 24 and 20 it is represented 

 as smaller than natural, and slightly lowered in position, in order to expose the arteries. 



