CH. XXVI.] CARBONIC ACID IN THE BLOOD 375 



secretion is not impossible is shown by the fact that a similar 

 secretion of oxygen is known to occur in the swim-bladder of certain 

 fishes. The swim-bladder corresponds morphologically with the lungs 

 of- a mammal, and the oxygen stored in it is far in excess of anything 

 that can be explained by mere diffusion from the sea-water. This 

 storage of oxygen, moreover, ceases when the vagus nerves which 

 supply the swim-bladder are divided. 



2. Carbonic Acid. 



The tension of carbonic acid in the alveolar air is measured, like 

 that of oxygen, by the method of Haldane and Priestley, whilst the 

 tension in the blood is measured by the tonometer. 



The tension of the carbonic acid in the tissues is high, but one 

 cannot give exact figures ; we can measure the tension of the gas in 

 certain secretions : in the urine it is 9, in the bile 7 per cent. The 

 tension in the cells themselves must be higher still. 



The following figures (from Fredericq) give the tension of carbonic 

 dioxide in percentages of an atmosphere : 



Tissues 5 to 9 \ ^ 



Venous blood . . . . . . . 3*8 to 5*4 J- in dog. 



Alveolar air 2'8 ) 



External air 0'04 



The arrow indicates the direction in which the gas passes, namely, 

 in the direction of pressure from the tissues to the atmosphere. 



In view of the above figures and of such experiments as that of 

 Krogh, cited on the opposite page, and having regard to the very slight 

 changes in the tension of carbonic acid in the alveolar air, which are 

 capable of affecting the respiratory centre (a subject we shall 

 immediately pass to), we shall adhere to the view that diffusion 

 explains the passage of that gas from the blood to the alveolar 

 air, and that it is unnecessary to call to our assistance the hypothesis 

 that secretory activity of the alveolar epithelium is at work. 



Cause and Regulation of Respiration. 



There are three factors, each of which plays a part in maintaining 

 and regulating the rhythmic movements of respiration. They are 

 the respiratory centre, the vagus nerves, and the chemical condition 

 of the blood. 



1. The Respiratory Centre. 



In the central nervous system there is a specialised small district 

 called the respiratory centre. This gives out impulses which travel 

 down the spinal cord to the centres of the spinal nerves that 

 innervate the muscles of respiration. It also receives various afferent 

 fibres, the most important of which are contained in the trunk of the 



