064 THEORY OF NUTRITION. 



no combination of its proximate constituents 

 with the membraneous, bony, muscular, and 

 nervous tissues. Nor could chyle and venous 

 blood be converted into the arterial state, without 

 giving off carbon and hydrogen in the lungs, by 

 which caloric is evolved, the temperature of the 

 blood elevated, and its composition renewed.* 



Should it be urged, that arterial contains more 

 oxygen than venous blood, I reply that the differ- 

 ence is very small, and wholly disappears in 

 the lungs, on giving off carbon and hydrogen, 

 where venous becomes arterial blood, that al- 

 though it be true that the latter has been found 

 to contain a little more oxygen, and the former a 

 little more carbonic acid, as observed by Magnus, 



* It is therefore not true, as maintained by Dr. John Davy, that 

 " the object of respiration is to introduce oxygen into the blood, 

 and separate carbonic acid from the blood" that " animal heat 

 is owing to the fixation of oxygen in the blood in the lungs, and 

 to the combinations into which it enters in the circulation, in con- 

 nexion with the different secretions and changes essential to ani- 

 mal life." For if it be a fact, which he has clearly demonstrated, 

 that the temperature of blood is elevated while passing through 

 the lungs, and diminished while passing through the general sys- 

 tem, animal heat must be evolved in the pulmonary air cells, 

 where the carbonic acid is generated, and there unite with the 

 blood, from which it is given off to the different organs. Nor is 

 it true, as maintained by Dr. Stephens, that arterial blood is 

 changed to the venous state, by acquiring carbonic acid in the 

 systemic capillaries, and again arterialized by the attraction of 

 atmospheric oxygen for carbonic acid, and the abstraction of the 

 latter in the lungs much less, that arterial blood owes its colour 

 and vital properties to the salts contained in the serum; because 

 they exist in chyme and chyle, which are white. 



