FORMATION OF TISSUE FROM BLOOD. 93 



physiological facts which render that view untenable in 

 these days. It would almost seem as if by the " tendency" 

 of scientific thought, a demand for certain theories of a 

 certain tendency was from time to time excited. If that 

 be so, no wonder there should be a good supply of new 

 fancy facts and observations, for without some such support 

 the tendency itself would soon lose its vitality. 



I propose now to refer briefly to the vital process of 

 nutrition as it occurs in man and the higher animals. It 

 has been said that the life of the body is the blood, and it 

 has been surmised that from this fluid the tissues derive 

 not only the elements of their nutrition, but the life or the 

 properties which we call by that name. But it is certain 

 that the material nutrient pabulum adapted for the nutrition 

 of the tissues, which the blood contains, is like all nu- 

 trient matter, lifeless, not living. The actual nutrition, 

 the act of conversion of the pabulum that was in the blood 

 into the tissue, is due to actions which occur outside the 

 vessels, and is altogether independent of the passive nu- 

 trient fluid. As little supported by facts as the opinion 

 above alluded to is the doctrine that arterial blood is very 

 highly nutritious, although a student reading any of our 

 text books would be led to believe that the highly nutritive 

 properties of arterial blood had been proved beyond all 

 question, and that every tissue to be nourished must have 

 its nutritive artery. The very active nutrition going on in 

 the lower animals and plants under conditions not favourable 

 to free oxidation, and the fact that in man and the higher 

 animals during the early periods of life when nutritive 

 activity is most remarkable, the blood is not so highly 



