336 DETAILED SYNTHESIS. 



expressions "arterial" and "venous" Blood are not proper, for the 

 character of the vessel has nothing to do with its contents ; but if they 

 are received with great force the vessel must be an artery ; if with light 

 force, a vein is sufficient. The vessel receiving blood from a heart is, 

 therefore, always an artery. 



985. THE INTERSPACES OP THE BRONCHII, BLOOD- 

 TUBES AND CELLS OF THE LUNGS are filled with the sinewy 

 tissue necessary in working them, and upon the elasticity 

 of which the expulsion of the air from the lungs is 

 chiefly dependent. 



986. The sinewy tissue near the surface of the lungs 

 is wrought into the form of a membrane, and covered 

 with basement membrane, that is covered with serous 

 cells, thus producing a serous coat, called pleura, and in 

 this position PULMONARY PLEURA. 



987. THE LUNGS thus constructed MUST BE PLACED 

 in an air-tight box, the dimensions of which can be en- 

 larged and diminished. How this is done is shown by 

 Fig. 220 and Fig. 221. 



Fio. 220. 



Fig. 220. 1, section of a lung; 2, the root, compos- 

 ed of branches and blood-tubes ; 6, the external cover- 

 ing or pleura, at the root turning up, extending out, 

 passing down at 5, to turn up over 4 (diaphragm), and 

 again up to 2, where it becomes continuous with 6. At 

 6 it is called pleura costalis, because against the ribs. 

 The two pleurae are shown to be merely one continuous 

 serous membrane. The lung is also shown to be at- 

 tached only at its root, and elsewhere has a free sur- 

 face ; not hanging from its root, but, filled by air, is 

 sustained by the action of the external walls of the 

 chest. 



Remark. There is no space between the box in which the lung 

 is placed and its surface. THE REPRESENTATION AT 7 is NOT CORRECT, 

 therefore, nor is that at 18, if the' chest is entire; but as soon as it is 

 open, the external air acts on the surface of the lung, balancing the 

 internal pressure, and the elasticity of the lungs causes them to con- 

 tract. The upper part of the lung will then fall down, and it will hang 

 by its root 



985. What said of ? 986. Describe . 98T. Where must ? Describe 

 Fig. 220. 983.1s? 



