410 LECTURE XVIII. 



replaced all the blood in the various vessels by this saline solution. 

 This animal was then placed in an atmosphere of pure oxygen, and con- 

 sumed as much of the gas and evolved as much carbonic acid gas as a 

 normal frog. 



To-day there is not the slightest possible doubt that oxygen diffuses 

 into the tissues, and that the cells themselves obtain their energy by the 

 combustion of their nutriment, which takes place in their immediate 

 vicinity. We know a great many facts which are in harmony only 

 with this assumption. One of the principal proofs is that the blood 

 itself possesses no oxidizing properties. 1 If, for example, salts of lactic 

 acid, acetic acid, etc., are placed in the blood they remain unchanged, 

 whereas in their passage through the organism they are quickly and 

 completely oxidized. This experiment becomes more convincing if 

 carried out with surviving organs. If, for example, blood is conducted 

 through the liver of a dead animal by the portal vein, it can be shown that 

 ammonium formate introduced into the blood disappears and in its place 

 urea is formed. This is never the case, however, if the formate is merely 

 exposed to the action of the blood without coming in contact with the 

 liver-cells; in such cases the ammonium formate remains unchanged. 

 Evidently there is a mutual action between the blood and the cells of the 

 liver which is necessary to cause this complicated reaction to take place. 

 This is merely one example out of many. 



The fact that oxygen actually passes through the walls of the blood- 

 vessels is strikingly shown by the way the fcetus is provided with this 

 element. It is well known that there is no direct connection between 

 the vascular system of the mother and that of the child. The circu- 

 lation of the foetus is isolated. The umbilical arteries carry the blood 

 rich in carbon dioxide and poor in oxygen from the fcetus through the 

 umbilical cord to the placenta. Here these arteries break down into 

 extremely fine branches. They change into the form of chorionic villi in 

 the enlarged capillaries of the mucous membrane, in the intravillous 

 spaces of the decidua. To this region the organism of the mother sends 

 blood rich in oxygen. In order that this oxygen may enter the fcetal 

 circulation i.e., that it may enter into the umbilical veins, it must first 

 penetrate the epithelium and vascular walls of the chorionic villi, and 

 conversely the venous fcetal blood of the umbilical arteries gives up its 

 carbon dioxide in the same way. 



Another proof that the oxygen passes through the walls of the blood 

 capillaries lies in the fact that the saliva contains a constant amount of 

 free oxygen. According to E. Pfliiger, 2 it contains 0.5 per cent by volume 



1 Cf. E. Pfliiger: Pfliiger's Arch. 6, 43 (1872); Hoppe-Seyler: ibid. 7, 407 (1873). 



2 Pfliiger's Arch. 1, 686 (1868). See also R. Kiilz: Z. Biol. 23, 321 (1887). J. L. 

 Bancroft: Biochem. J. 1, 1 (1906). 



