232 



THF- INTI.RNAL FNVIRONMENT OF TUT. BODY 



Part III 



esophagus 



pharyngeal 

 gill slits 



tongue 



Fig. 13.7. View into the open mouth of a fish (barracuda) showing the gill 

 slits and arches in the walls of the pharynx. (Courtesy, Weichert: Anatomy of 

 Chordates. New York, McGraw-Hill Book Co., Inc., 1951.) 



lung and the extension of its inner layer by partitions. In frogs the partitions 

 form alcoves, in toads open rooms, and in mammals the respiratory space is 

 completely divided up into minute cavities, the alveoli (Figs. 13.9, 13.10). In 

 each of these successive arrangements more area for blood capillaries is se- 

 cured. Increase in the numbers of blood capillaries parallels greater diffusion 

 of gases. Nerves, connective tissue, and lymphatic capillaries are also present 

 in highly developed lungs. 



The ventilation mechanism differs in various classes of vertebrates. With 

 tightly closed mouths frogs take air through the nostrils and into the mouth, 

 and by contracting the throat, press it into the open glottis, actually swallow- 

 ing air into their lungs. Reptiles enlarge the body cavity by pulling the par- 

 tially folded ribs forward. Air is then drawn through the nostrils, windpipe, 

 and into the lungs because of the reduction of pressure in the body cavity 

 around them. In birds the mechanism is complicated and, for the full action 

 of the lungs, depends largely upon the movements produced particularly 

 while flying. The main body of the bird's lungs is small but the extensions 

 of the lungs in air sacs are relatively large (Chap. 36). The upper surface of 

 the lungs adheres to the ribs; indentations of the latter show clearly when the 

 lungs are pulled away. A special membrane ventral to the lungs is also at- 

 tached to the ribs. In quiet breathing intercostal and abdominal muscles 



