662 THEORY OF NUTRITION. 



different organs, as the greatest mystery of life. 

 And Sir Charles Morgan observes, that " the 

 conversion of arterial into venous blood is a pro- 

 cess involved in the deepest obscurity" that 

 " whether any single element is abstracted where 

 the change takes place, or whether the alteration 

 depends upon a change of disposition in all the 

 elements among themselves, cannot even be 

 guessed." (Philosophy of Life, p. 145.) 



But we have already seen, that during the pas- 

 sage of dark venous blood through the lungs, in 

 combination with chyle and lymph, variable pro- 

 portions of carbon and hydrogen are given off, 

 and unite with atmospheric oxygen, in exchange 

 for which caloric is received, with more or less 

 nitrogen, according as the aliment contains more 

 or less of that element, that during the tran- 

 sition of blood through the pulmonary tissue, its 

 temperature is elevated from 1 to 3, its colour 

 changed to a bright florid hue,* its specific gravity 

 diminished, the proportion of its organic particles 

 augmented, and all its vital properties exalted. 

 In this state, it excites the left ventricle of the 



* Hippocrates seems to have been fully aware, that the tem- 

 perature is higher in the left ventricle of the heart and aorta, than 

 in the right ventricle or vena cava. He also observes, that the 

 heart is the well known fountain of the aorta, which is connected 

 with the lungs by the trachea, which he regarded as the upper 

 extremity of the aorta. (irept ^Xe^ov, i. ii. Trepi ApKov, vi. 

 Trepi Rapine, vii. viii.) But so little did this greatest of the an- 

 cient Physicians know in regard to the true theory of respiration 



