SYSTEMATIC SYNTHESIS OF TISSUES INTO ORGANS. 337 

 FlO. 221. 



Fig. 221 represents a cross section of the chest and contents, just above 

 the level of 10, semilunar valves of the heart. 1, Body of vertebra ; 2, sec- 

 tion of ribs; 3, breast-bone ; 4, 5, lungs; 6, heart; 7, P. artery; 8, left, 9, 

 right branch; 11, part of left, 12, part of right auricle; 13, vena cava; 

 14, left, 15, right bronchus; 16, oesophagus; 17, aorta; 18, cavity between 

 the pleura, owing to the contraction or collapse of lungs, when the cavity 

 is opened and air admitted; 19, pleura (5, Fig. 220), applied to walls of 

 chest ; 20, where 19 is continuous with 22 ; 23, the pleura pulmonalis ; 21, 

 space called posterior mediastinum ; 24 corresponds to 20. From 24 to 25 

 the pleura and heart-case adhere ; 26, a space unduly large, called anterior 

 mediastinum. Notice also the two layers of muscles between the ribs. 



989. THE PLEURA COSTALIS AND PULMONALIS are, in 

 health, constantly moistened with serous fluid, so that 

 they move against each other without any friction, and, 

 from the elastic structure of the lung, only touch in 

 the gentlest manner imaginable (except the lung is 

 closed) ; and, as the former is attached to the inner sur- 

 face of the ribs, forming, indeed, a v lining to the chest, 



Describe Fig. 221. Are the lungs represented as full or empty ? In the sound 

 chest how are they ? 989. What are ? 

 15 



