95 



other share than, a mere concurrence which is compelled by the laws 

 of affinity subsisting bet ween those properties and the chymicaland 

 material' constituents. It is inferred, also, that blood being made, 

 oxygen is capable of altering its colour. Dark-coloured blood in 

 the chick is conveyed by the umbilical arteries to the membranes 

 covered by the shell, and it is returned florid blood by the umbili- 

 cal veins; the same thing takes place in the human foetus, with 

 this difference, viz. that the dark-coloured blood is conveyed by 

 tlie umbilical arteries to a thick placentary mass, from whence ft 

 is returned by the veins of a bright vermilion colour, &c. This 

 effect upon the blood, this change, is perpetual. It has been re- 

 marked (at 76), that in the case of the egg there is no source 

 which admits an adequate oxygenation by atmospherical air. To 

 imagine that air passes at all througli the shell is a mere imagina- 

 tion; and to suppose that it passes through the shell so freely as 

 the argument requires that it should, is too absurd to demand a 

 refutation. The want of a source of air in the human placenta 

 has also been remarked in the same place. We find then that 

 oxygen is not only incapable of converting nutritious fluids into 

 red blood, but even that the colour characteristic of venous blood 

 may be changed to that characteristic of arterial blood, without 

 any thing like an atmospherical oxygtnation. 



81. If the blood of the foetus does not undergo this change, 

 which has been attributed to oxygen, the foetus dies. But there 

 is no atmospherical oxygen, or only an inadequate quantity, at the 

 place where this change is accomplished: by what then is this 

 change, viz. the conversion of venous into arterial blood, ac- 

 complished? 



82. The placenta is a production of the ovum, it is governed 

 by its life, are all other growths and structures; its life extinct, 

 its fabric falls to decay; its chymical properties, also, are depen- 

 dent upon its organic spirit. It has properties of life, but no 

 others for which it is not indebted to life. Its properties of life 

 are related with those of the embryon, its function is connected 

 with all the parts of the organic spirit of the foetus; but its rela- 

 tion is not direct but mediate, and not through the medium of the 

 textures, but through that of the blood. 



83. The relations of properties of the spirits in different seats, 

 the effects, &c. of their reciprocal agency, how complicated the 

 concurrence is, &c. have been sufficiently spoken of. This is an 

 instance of that concurrence. The vital properties of the em- 

 bryon are not capable of themselves of maintaining life, or of 

 developing the structures; they want the influence of allied pro- 

 perties: these properties are in the placenta, which are not the 

 identical properties of life, but are made such by their relation 

 with thcs ' of the erabryon. 



84. These properties of the placenta influence those of the 

 embryon by the change they produce in the blood : this change is 

 indicated by an alteration in its colour. The influence which the 



