RESPIRATION 129 



is wholly internal respiration. The respiratory mode of mammalian 

 embryos is practically the same as that of fishes, for although the former 

 have no proper gills, still the gases interchange (through layers of maternal 

 and fetal epithelium) between the liquid blood of the embryo and the liquid 

 environment. In this case the environment is the circulation in the 

 placenta of the mother. The fecal chorionic villi and the blood-filled 

 sinuses of the decidua serotina of the placenta, extending inward from 

 the uterine fundus, interknit with the greatest closeness. Thus, the villi 

 of the fetal circulation, made up largely of capillaries, are quite surrounded 

 by the large blood -sinuses filled with maternal blood . The circulation of 

 the fetus, then, exchanges its two respiratory gases through the walls of 

 capillaries just as does the adult circulation, save that instead of exchang- 

 ing them with the alveoli the fetal blood takes its oxygen and excretes 

 its carbon dioxide second-hand, as it were, from and into the maternal 

 circulation. The essence of these facts was understood by Mayow in 

 England as early as 1674. 



The sinuses of the maternal circulation in the placenta are large, and 

 the blood-current through them correspondingly slow, thus allowing 

 ample time for the respiratory exchange. Diffusion would partly account 

 for the interchange, but probably here as elsewhere the protoplasmic 

 powers of the septa have more or less to do with the passage through. 

 Unlike the adult's hemoglobin, that of the fetus is never saturated with 

 oxygen. An estimate of the oxygen consumed by the human fetus was 

 0.169 gm. daily per kilo of body-weight, compared with the 14 gm. or 

 so used by the adult per kilo, or about 1.2 per cent, of the adult's 

 requirement. This low demand for oxygen depends on a correspond- 

 ingly low metabolism, which in turn is due to the relative inactivity of 

 the fetus, its protection from loss of heat, etc. 



Respiration through the Skin. The lungs are not the only organs 

 by which oxygen is absorbed into the organism and carbon dioxide 

 given off, for the skin and the intestines are also media for a small amount 

 of this interchange, even in mammals. This is not difficult to understand, 

 for a part of the tissues of the body, constantly producing carbon dioxide 

 and requiring oxygen, are separated from the atmosphere, the great 

 reservoir of both these gases, only by the thin upper layers of the skin. 

 This important organ, the skin, is in some respects an admirable osmotic 

 membrane, being moist, thin, and vascular. The structure of its outer 

 layer, the epidermis, however, is not so favorable to respiration, the 

 epidermis being, indeed, evolved to be negative and protective. 



Cruikshank more than a century ago proved that a clean hand or foot 

 immersed in lime-water (a solution of calcium oxide) for an hour ren- 

 dered the water milky by the production of the insoluble calcium car- 

 bonate the ordinary test for carbon dioxide. Abernethy thereupon 

 showed that in ordinary air oxygen was absorbed and carbon dioxide 

 discharged by a hand as readily as in pure oxygen. Gerlach, from 

 experiments on a part of a man's skin, calculated that 8.4 gm. of carbon 

 dioxide were given off in twenty-four hours from the entire bodily sur- 

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