280 



RESPIRATORY APPARATUS OF MAMMALS. 



this cavity, by their summit or apex ; and are covered by a 

 serous membrane termed the pleura, which also lines the 

 thorax, being reflected from one surface to the other precisely 

 in the manner of the pericardium ( 43). Thus the pleura of the 

 outer surface of the lung is continually in contact with that 

 which forms the inner wall of the thorax ; they are both kept 

 moist by fluid secreted from them ; and they are so smooth, 

 as to glide over one another with the least possible friction. 



The lungs themselves are 

 very minutely subdivided ; 

 and thus expose a vast ex- 

 tent of surface in proportion 

 to their size. The air-cells of 

 the human lung, into which 

 the air is conveyed by the 

 branches of the wind-pipe, and 

 on the walls of which the 

 blood is distributed, do not 

 average above the 1-1 00th of 

 an inch in diameter. In the 

 accompanying figure is repre- 

 sented, on one side, the lung, 

 d, presenting its natural ap- 

 pearance ; and on the other, 

 the ramifications of the air- 

 passages or bronchial tubes, 

 c, e, by which air is con- 

 veyed into every part of the 



Fig. ^.-AIR-TUBES AND LUNG OF MAN. lungs. The trachea^ or wind- 

 pipe, 6, opens into the 



pharynx or back of the mouth, by the larynx, a. The con- 

 struction of this is especially destined to produce the voice, 

 and will be explained under that head (Chap, xin.); but 

 it may be here mentioned that the entrance from the 

 pharynx into the larynx consists of a narrow slit, capable of 

 being enlarged or closed by the separation or approximation 

 of its lips, which form what is called the glottis. The aper- 

 ture of the glottis is regulated by the muscular apparatus of 

 the larynx ; the actions of which are not under the direct 

 control of the will, but are automatic, like those concerned in 

 swallowing ( 194) ; and the purpose of this provision is to 



