AND PHYSIOLOGY. 



251 



enter the lungs, they immediately divide and subdivide, until 

 they have diminished to a diameter about one fiftieth of an 

 inch, and some of them are within one eighth of an inch of 

 the outside of the lung. After this they are changed in their 

 structure, and become channels hollowed out in the cellular 

 tissue of the lung, and are called Intercellular Passages. 

 These terminate in minute 

 cells, called Csecal Air-Cells. 

 These cells have an average 

 diameter of rloth of an inch, 

 and accumulate around each 

 terminal bronchus to the num- 

 ber of 17,790, making a total 

 in the lungs of 600,000,000, 

 which, if spread out, would 

 make an area of cell surface 

 of 132 square feet; Dalton 

 says 1,400 square feet. 



443. Composition of the 

 Air-Tubes. Their Mucous 

 and Pleural Surfaces, — 

 These air-tubes just described 

 are essentially composed of 

 cartilage and fibro-cartilage, 

 and lined throughout with 

 mucous membrane. Hence 

 w r e see that this membrane, 

 though situated nearest the 

 center of the body, lines the 

 outside of the lungs, while 

 the Pleura, although nearer 

 the surface of the body, lines 



Longitudinal Section of the Termina- 

 tion of a Bronchus. 1, Bronchus. 2, C ce- 

 cal Air-Ceil. 3, Orifice of the same. 



Fig. 254. 



Small Bronchial Tube laid open. 



442. What are the bronchial tubes subdivided into? How large are the csecal air-cells'; 

 How many around each terminal bronchus ? What is their aggregate number ? Wh,at 

 is the area of cell surface in the lungs? 443. What is the composition of the air-tubes? 

 Which is really the inside and which the outside of the lungs? 



