7 so 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 possible 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- 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; fur 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 < 'obitis fossil is has the following 
composition: 87T8 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, 7'94 oxygen, and 
0"73 carbon dioxide. 3 Errnan 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 Cobitis fossilis is, 
according to Leydig, 5 composed almost entirely of capillary blood vessels, and 
a little connective tissue. In the Calliehthys asper, a fish found in Brazil, the 
respiration by the alimentary canal is essential for life, for if the fish he 
prevented from coming to the surface of the water to swallow air, it dies 
within two hours. The air discharged by the rectum contains l'S-S^S per 
cent, of carbon dioxide. 6 
The respiration of the fcetus. — The respiration of the foetus was first 
understood and described in 1674 by Mayow," who in his treatise, " De 
Bespiratione Fcetus in Utero," maintains that the placenta is to be looked 
upon as a lung,' from which the umbilical vessels take up the nitro-aerial gas 
1 Bernard. " Lecons sur les etfets des substances toxiques et medicamentenses," Paris, 
1857, p. 59; Bergeon, Compt. rend. Acad. d. sc, Paris, tome civ. p. 1812; Hatrriot and 
Richet, Compt. rend. Soc. de hiol., Paris, 1887, p. 307 ; Flint, Med. News, Phila., 1887, 
vol. li. p. 670. 
2 "Physiol, comp. de la respiration," Paris, 1870, p. 173. 
3 Bamnert, '"Chem. Untersuch. ti. d. Respir. d. Schlammpeitzgers," Breslau, 1855, 
S. 24. 
4 Ann. d. Phys. u. Chem., Leipzig, 1808, Bd. xxx. S. 113. 
5 Arch. f. Anat., Physiol, v. wissensch. Med., 1S53, S. 3. 
B Jobert, Ann. d. sc. not., Paris, 1877, Zool. (6), tome v., Art. Xo. 8. 
7 "Tractatus Tertius, de Respiratione Fcetus in Utero et Ovo," Oxon., 1674. 
