84 VERTEBRATE RESPIRATION 



which would be at least sufficient to produce very marked excita- 

 tion of the respiratory centre and may even be high enough to 

 have a depressant effect. 



The carbon dioxide has a definite effect on the medullary 

 respiratory centres - a notable observation being that a medulla 

 that has been completely cut off from all sensory input and 

 which is more or less electrically silent shows well-defined 

 respiratory potentials when the air mixture ventilating the 

 animal is raised in CO 2 content by 6-5 %. Chemical stimulation 

 can also be obtained by the direct injection of sodium bicar- 

 bonate and CO 2 into the medulla. The localisation and precise 

 nature of the responses to these injections vary not only in 

 a mammal but also in a teleost fish. Whether the CO 2 has its 

 cfiQCtperse or because of the resulting changes in hydrogen ion 

 concentration has long been discussed. A recent view is that the 

 effects are the result of both of these. Injection of acid alone 

 certainly stimulates respiration, but the effects on respiratory 

 activity of equal increases in acidity produced by CO 2 inhalation 

 and acid injection show that CO 2 has a greater effect than that 

 resulting from the change in pH alone. It is concluded, there- 

 fore, that CO2 has its effect as a result of both factors. 



Chemoreceptors are also present in the carotid and aortic 

 bodies or glomi. These are to be distinguished from the pressor 

 receptors in similar regions (carotid sinus) which also have an 

 effect on the respiratory rhythm but not so marked. Both the 

 carotid sinus and glomus lie close to the bifurcation of the 

 common carotid in mammals ; the glomus lies on the occipital 

 artery and the sinus on the internal carotid. The sensory fibres 

 from both pairs of receptors pass to the medulla in the glosso- 

 pharyngeal (IX) nerves. The aortic glomus is single and lies on 

 the concave side of the aorta. It has similar receptors to the 

 carotid glomi but its afferent fibres enter the depressor branches 

 of the vagus. 



Perfusion of the carotid glomus with Ringer solution at con- 

 stant pressure showed that an increase in respiration followed 

 a decrease in oxygen tension, increase in carbon dioxide 

 tension, or an increase in hydrogen ion concentration of the 



