. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 ^'^" Adequate diffusion occurs with slight differences in oxygen pressure, owing to the large surface area of the lungs. Calculations of pressure differences across the resp
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. 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 ^'^" 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 ^'' 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 h
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