7 30 CHEMISTR Y OF RESPIRA TION. 



some of them play in respiration. The oxygen in the air swallowed is 

 almost entirely absorbed in the stomach ; the carbon dioxide is gener- 

 ally 20 to 90 per cent, of the gas present in the intestines, and will 

 therefore have a partial pressure greater than that of the carbon dioxide 

 in the blood and tissues, and will diffuse from the intestines into the 

 blood, to be ultimately discharged in the lungs. As regards the 

 nitrogen, the quantity present in the alimentary canal is considerable, 

 but its partial pressure is generally below that of the atmosphere, and 

 of the tissues, and under these conditions there will be a diffusion of 

 nitrogen from the blood and tissues into the intestinal tract. It is 

 important to remember the presence of nitrogen and marsh-gas in the 

 alimentary canal, for thus it is possib^ to explain those cases in which 

 an absorption or discharge of nitrogen has been observed during 

 determinations of the respiratory exchange. When carbon dioxide or 

 hydrogen-sulphide is injected into the rectum, a portion of the gas is 

 absorbed and excreted by the lungs. 1 



Paul Bert 2 observed that a kitten with ligatured trachea lived twenty-one 

 minutes when a current of air was passed through the alimentary canal, 

 whereas a kitten of similar age died in thirteen minutes, when the only 

 operation performed was ligature of the trachea. A similar absorption of 

 oxygen from the alimentary canal probably takes place in man under special 

 circumstances ; for swimmers who can remain under water for an exceptional 

 length of time, state that they swallow air in addition to taking a deep inspira- 

 tion before a dive. 



In warm-blooded animals the alimentary canal plays an unimportant part 

 in respiration, but this is not the case in some fish, for all the members of the 

 loach family respire partly by the alimentary canal. The air discharged 

 under normal conditions from the rectum of Colitis fossilis has the following 

 composition: 87*18 per cent, nitrogen, 12'03 per cent, oxygen, and 0*79 per 

 cent, carbon dioxide ; but if the fish be prevented from swallowing air for 

 several hours, the percentage composition is 91*33 nitrogen, 7 '9 4 oxygen, and 

 0*73 carbon dioxide. 8 Erman 4 opened the abdomen of one of these 

 fish, and noticed that when air was swallowed the intestinal veins and the 

 liver became bright red, but with hydrogen or nitrogen the colour was, very 

 dark purple. The mucous membrane of the intestine of Colitis fossilis is, 

 according to Leydig, 5 composed almost entirely of capillary blood vessels, and 

 a little connective tissue. In the Calliclttliys asper, a fish found in Brazil, the 

 respiration by the alimentary canal is essential for life, for if the fish be 

 prevented from coming to the surface of the water to swallow air, it dies 

 within two hours. The air discharged by the rectum contains 1*5-3*8 per 

 cent, of carbon dioxide. 6 



The respiration of the foetus. The respiration of the foetus was first 

 understood and described in 1674 by Mayow, 7 who in his treatise, "De 

 Respiratione Foetus in Utero," maintains that the placenta is to be looked 

 upon as a lung, from which the umbilical vessels take up the nitre-aerial gas 



1 Bernard. " Le9ons sur les effets des substances toxiques et medicamen tenses," Paris, 

 1857, p. 59; Bergeon, Compt. rend. Acad. d. sc., Paris, tome civ. p. 1812; Hanriot and 

 Richet, Compt. rend. Soc. de biol., Paris, 1887, p. 307 ; Flint, Med. News. Phila., 1887, 

 vol. li. p. 670. 



2 "Physiol. corap. de la respiration," Paris, 1870, p. 173. 



3 Baumert, '*Chem. Untersuch. ii. d. Respir. d. Schlammpeitzgers," Breslau, 1855, 

 S. 24. 



4 Ann. d. Phijs. u. Chem., Leipzig, 1808, Bd. xxx. S. 113. 



5 Arch. f. Anat., Physiol. u. wissensch. Med., 1853, S. 3. 



6 Jobert, Ann. d. sc. nat., Paris, 1877, Zool. (6), tome v., Art. No. 8. 



7 "Traetatus Tertius, de Respiratione Foetus in Utero et Ovo," Oxon., 1674. 



