136 CIRCULATING FLUIDS : 



In the air after expiration we always find a greater or less 

 amount of watery vapour. According to Menzies, an adult 

 man^ in the course of twenty -four hours, gets rid, in this man- 

 ner, of 2880 grains of water. Abernethy fixes the amount at 

 4320; Thomson at 9120; Hales at 9792; and Lavoisier at as 

 much as 13,704 grains. This water exhales from the blood 

 which is circulating in the bronchi and cavity of the throat, 

 and contains some animal matter which causes it to decompose 

 speedily. Alcohol, ether, and substances of that nature are 

 removed from the blood by the lungs, at least in part ; for after 

 they have been taken, their odour may be distinctly recognized 

 in the breath. Sulphuretted and phosphoretted hydrogen, if 

 injected into a vein, are easily recognized in the breath by the 

 odour ; and if phosphoretted oil is applied in a similar manner, 

 dense white vapours of phosphorous acid are speedily exhaled. 



Resjnration of the foetus and of animals. 



As the function of respiration in the embryo of the mam- 

 malia cannot be carried on by the lungs, an equivalent is 

 supplied to them by the influence of the maternal fluids on 

 those of the foetus, in the placenta. Anatomical investigations 

 have shown, that it is impossible for the blood of the mother 

 to be transmitted unchanged into the foetus ; nutriment and 

 arterial blood can only make their way into the foetal system 

 through the medium of cells. 



In the umbilical cord there are two vessels which convey 

 venous blood from the foetus to the placenta, and there is one 

 that conducts arterial blood from the placenta to the foetus. 

 The changes which are efi'ected in this manner in the foetal 

 blood are not so obvious as if they had occurred in the ordinary 

 manner in the lungs ; in fact it is by no means easy, or in- 

 deed ahvays practicable, to detect anj^ difi'erence in the colour 

 of the arterial and venous foetal blood. The change, however, 

 such as it is, is of the highest importance to the foetus, since 

 it dies if the umbilical cord be tied before birth. The anato- 

 mical peculiarities in the circulating system of the foetus are 

 too well known to require any description. 



In the embryo of birds the respiration is carried on during 

 the later stages of development, by the allantois, an extremely 

 vascular membrane, over which the left umbilical arterv is 



