THE LUNGS. 147 



thoracic duct, and those of the right into the large lymphatic 

 trunk coming from the right upper extremity. 



The nerves come principally from the par vagum. Some of 

 them are distributed with the bronchus, and may be traced 

 easily far along its branches, forming beautiful anastomoses 

 around them : their texture there resembles much tliat of the 

 sympathetic: they are thought to be, finally, spent upon the mu- 

 cous membrane: others seem to be more specifically appro- 

 priated to the vessels. 



It wil'l now be understood that the root of each lung is formed 

 by the pulmonary artery, the two pulmonary veins, and the bron- 

 chus, covered by the pleura, where the latter extends from the 

 lung to the pericardium. The relative situation is such, that 

 the pulmonary artery is above, the bronchus in the centre and 

 behind, and the pulmonary veins below. 



The texture of the lung is so light and spongy after an ani- 

 mal has once breathed, that its weight is very inconsiderable 

 when compared with its volume. Its cells are left much dis- 

 tended, even when the animal is dead; and, notwithstanding 

 from its unusual elasticity, it expels a great quantity of air 

 when the thorax is opened, and is thereby reduced to a third of 

 its size during life; yet it retains enough air to make it float in 

 water, or even in spirits of wine. The quantity of air which 

 the lungs contain differs very considerably in different individu- 

 als, depending entirely on the capaciousness of the thorax. Its 

 medium amount is computed at one hundred and forty-five cu- 

 bic inches, thirty cubic inches of which are changed at every 

 act of respiration. 



Of the Pleura. 



Each lung has a perfect covering, called Pleura, to which it 

 is indebted for its shining surface. This membrane is also re- 

 flected from the internal surface of the lung to the adjacent side 

 of the pericardium, and is then spread over the interior peri- 

 phery of that half of the thorax to which it belongs, by lining 

 the ribs and intercostal muscles, and covering the convex face 

 of the diaphragm. There are, therefore, two pleurae, each of 



