162 



ANIMAL BIOLOGY 



problems of protecting and keeping moist the greatly increased 

 respiratory surface which their more active metabolism — pro- 

 portionally greater energy requirements — demands. Accordingly 

 the gill slits persist merely as transient embryonic reminders of 

 evolutionary history ; the function of the gill pouches being taken 

 over by a huge outpocketing of the ventral wall of the pharynx 

 into the anterior portion of the body cavity, which constitutes the 



VtH 



Fig. 116. — Diagram of the 

 respiratory (tracheal) system 

 of an Insect. The other in- 

 ternal organs are omitted. 



Fig. 117. — Diagram of a 

 vertical section through the 

 head region of Fish (above) 

 and Reptile or Bird (below) 

 to show the paths of the re- 

 spiratory currents (a) and 

 food (6). See Fig. 109. 



lungs. Thus, even in Man, the respiratory membrane which lines 

 the lungs is, from the standpoint of development, a specialized 

 part of the epithelium of the alimentary canal. Furthermore, the 

 establishment of lungs entails, in turn, a complex respiratory 

 mechanism so that the air within them may be changed at frequent 

 intervals. (Fig. 235.) 



A. Lungs 



In the human respiratory process, air after entering the nostrils, 

 anterior nares, passes along the nasal passages and out the 

 posterior nares into the lower part of the pharynx. Air may 

 also enter through the mouth. From the pharynx it passes over 

 the epiglottis, and through the slit-like glottis into the larynx, 

 or so-called Adam's apple, and then down the windpipe, or 

 trachea. Within the larynx are the vocal cords which vibrate in 

 response to air currents; the amplitude of the vibrations and the 

 tension of the cords being responsible for the voice. (Fig. 109.) 



