386 RESPIRATION. 



higher animal without blood, and even without any special air-pas- 

 sages ; the interchange of gases and the formation of the carbonic 

 acid originate here directly from the organ from which the carbonic 

 acid is otherwise conveyed to the atmosphere by various indirect 

 means (and necessarily through the blood and lungs). As, more- 

 over, these experiments show that the muscles cannot retain 

 their activity without an access of free oxygen, at all events, a 

 large portion of this gas must, after its absorption by the lungs, be 

 conveyed in a free state through the blood and the walls of the 

 capillaries into the muscles. The blood is, therefore, quite as well 

 adapted to convey to the muscles the free oxygen necessary for the 

 accomplishment of their functions as to carry off the carbonic acid 

 formed by this function ; the first interchange of gases is, there- 

 fore, effected in the parenchyma of the organs themselves or if we 

 regard the interchange of gases between two different media as 

 the process of respiration, the first act of this process the first 

 interchange is effected between the parenchymatous juice and the 

 blood in the capillaries. This act may be compared to the respira- 

 tion of water-breathing animals; the difference consisting almost 

 solely in this, that the medium conveying the oxygen is on an ave- 

 rage denser than the medium which is destined to absorb oxygen, 

 or that the difference in the density of both is on the whole very 

 small, whilst in the true water-breathing animals the density of the 

 receiving medium exceeds that of the water very considerably, but 

 principally in the circumstance that the blood differs essentially 

 from water in its great capacity for the absorption of oxygen and 

 carbonic acid. Notwithstanding this difference, we may hope that 

 when the respiration of animals breathing through gills has been 

 sufficiently elucidated (and this subject is at present occupying the 

 attention of Valenciennes), we may succeed in bringing this inter- 

 change of gases, which is effected through the walls of the capil- 

 laries of the greater circulation, into accordance with the laws of 

 the transfusion of the gases absorbed by fluids. Then only can we 

 establish the theory of this portion of the respiration on physical 

 grounds. 



Before we proceed to consider the second act of respiration in 

 the higher animals, that is to say, the interchange of gases in the 

 capillaries of the lesser circulation and the pulmonary vesicles 

 the interchange between the blood and the air we must not omit 

 to inquire whether all the oxygen in the arterial blood is free, and 

 whether all the carbonic acid in the venous blood is only mechani- 

 cally combined. The previous experiments, as well as the obser- 



