THORACIC CAVITY 137 



bronchial lymph glands, associated with the intra-thoracic part of the 

 trachea and the extra-pulmonary parts of the bronchi and the visceral 

 pleura. (7) Broncho- pulmonary lymph glands, which lie in the hila of the 

 lungs. (8) Pulmonary lymph glands, which are situated in the angles of 

 division of the bronchi in the substance of the lung. The lymph from the 

 lungs and the visceral pleura passes through the pulmonary, broncho- 

 pulmonary, tracheo-bronchial, and intertracheo-bronchial glands, on the 

 way towards the terminal lymph vessels. As it traverses the glands the 

 carbon particles, which have passed from the air in the pulmonary alveoli 

 and then through the walls of the alveoli and the walls of the lymph 

 capillaries into the lymph in the lymph capillaries, are removed from the 

 lymph by the cells of the lymph glands, and are deposited in their 

 substance and in the stroma of the glands which, as a consequence, 

 gradually become blacker and blacker as life continues. 



Dissection. Cut through the descending aorta immediately 

 above the diaphragm. Detach its upper end from the left vagus 

 and the left recurrent nerve, which were previously fastened to 

 it ; then draw it forwards and divide the intercostal and subcostal 

 arteries, which arise from its posterior surface, close to their 

 origins, and remove the aorta. The lateral parts of the aortic 

 intercostal arteries have already been displayed (p. 40). Now 

 clean their most medial parts and the transverse parts of the hemi- 

 azygos and accessory hemiazygos veins. Then proceed to the 

 study of the aortic intercostal arteries and the revision of the 

 intercostal veins and nerves. 



Arterise Intercostales. There are eleven pairs of inter- 

 costal arteries. The upper two pairs are derived indirectly 

 from the subclavian arteries ; the remaining nine' pairs are 

 branches of the thoracic part of the descending aorta. 



The Aortic Intercostal Arteries. The nine pairs of aortic 

 intercostal arteries spring from the posterior surface .of the 

 descending aorta, either separately or by a series of common 

 trunks, one for each pair. The right arteries are longer than 

 the left because the aorta lies to the left of the median plane ; 

 and, since the descending aorta commences only at the level 

 of the lower border of the fourth thoracic vertebra, the four 

 or five highest pairs have to ascend to gain the level of 

 the spaces to which they are distributed (Figs. 13 and 14). 



The right aortic intercostal arteries run across the anterior 

 aspects of the bodies of the vertebrae, lying posterior to the 

 thoracic duct and the vena azygos; then they turn back- 

 wards, between the sides of the bodies of the vertebrae and the 

 parietal pleura ; and, finally, immediately before they enter 

 the intercostal spaces, they pass between the sides of the 

 bodies of the vertebrae medially and the sympathetic trunk 

 laterally. 



The shorter left aortic intercostal arteries run backwards, first 



