220 Comparative Animal Physiology 



dae, Pedipalpi, Araneidae, Porcellionidae, Janellidae). Book lungs of the 

 scorpion, Opisthoplithahwus capensis, for instance, with no ventilating appara- 

 tus, provide for practically all the respiratory exchange of the organism. Re- 

 moval of the book lungs reduces the gas exchange to approximately zero.-^'^" 

 Adequate diffusion occurs with slight differences in oxygen pressure, owing 

 to the large surface area of the lungs. Calculations of pressure differences 

 across the respiratory epithelium indicate that values as low as 1 mm. Hg in 

 the chilopod, Sciitigera,-^^' and 2 mm. Ilg in the air-breathing pulmonate, 

 Arion,^-'' are sufficient for gas exchange. 



Ventilation Lungs. The respiratory efficiency of \'entilation lungs is im- 

 proved as a result of the higher ax'crage oxygen gradient maintained across the 

 luno surfaces. Two kinds of ventilating mechanisms are found among \'erte- 

 brates, a positive-pressure type in which air is forced into the lungs by swallow- 

 ing (as in the frog), and a negative-pressure system in which air is drawn 

 into the respiratory cavities by increasing the space about the lungs (as in 

 man). During eupnea (normal breathing) in mammals, inspiration is active 

 and expiration largely passive, but in some animals both movements may be 

 effected by active muscular contraction, as in birds-'^"* and turtles.-^'' 



Increase in metabolic requirements of higher animals is correlated with 



.^r-'i^M-s^^i^'*^^'' 



Fig. 41. X-radiograph of physoclyst perch, Perca, showing the swim bladder, an 

 emergency oxygen store as well as a hydrostatic organ. From Hall.''*' 



greater complexity of the lungs and with less dependence on cutaneous and 

 gill respiration. The lungs are characterized by (1) increased surface area 

 and partitioning into alveoli, (2) greater vascularization, and (3) more ade- 

 quate ventilation. 



Although the lungs ol some mammals have been described as being partially 

 devoid of "respiratory epithelium," thereby exposing mesodermal endothelium 

 of the capillaries to the alveolar air, the recent re\iew of Guieyssc-Pellissier''''' 

 concludes that the evidence is based on inadequate technique and that the 

 respiratory epithelium, although occasionally enucleate, is complete. Muscle 

 Hbers can be demonstrated in the walls of reptilian lungs'"'' and down to the 

 alveolar sacs in those of the mammal. Active contractions of the respiratory 

 musculature correspond to the \'entilation rhythm. 



Gas exchange across the moist respiratory membranes seems adequately pro- 



