218 A MANUAL OF PHYSIOLOGY 



the blood circulating through the spinal bulb, as when the 

 carotid arteries of a dog are laid on metal boxes through 

 which hot water is kept flowing, also causes dyspnoea (heat- 

 dyspnoea), (p. 272). But if the temperature be too high, the 

 respiratory movements may be slowed, perhaps by a partial 

 paralysis or inhibition of the respiratory centre. When the 

 blood is cooled the respiration becomes deeper and slower, 

 but if the temperature is greatly and suddenly lowered, the 

 centre may be stimulated and the breathing quickened. In 

 man the increased temperature of the blood in fever is prob- 

 ably connected with the increase in the rate of respiration. 



The physiological opposite of dyspnoea is apncea. This 

 condition may be produced in an animal by rapid artificial 

 respiration. For some seconds, in a successful experiment, 

 after the artificial respiration is stopped, the animal remains 

 without breathing. The apnoeic state seems to be due 

 partly to an excess of oxygen in the arterial blood or in the 

 lungs, partly to some nervous effect produced through the 

 vagi on the respiratory centre. Possibly the pulmonary 

 nerve-endings of the vagi are affected mechanically by the 

 inflation ; for rapid and repeated inflation of the lungs with 

 hydrogen may cause apnoea (Traube). The venous blood in 

 apnoea is, if anything, poorer in oxygen than normal venous 

 blood. 



That poorly oxygenated blood produces dyspnoea by acting 

 on some portion of the brain may be shown in an interesting 

 manner by establishing what is called a cross-circulation in 

 two rabbits or dogs. The vertebral arteries and one carotid 

 are tied in both animals ; the remaining carotids are divided 

 and connected crosswise by glass tubes, so that the brain of 

 each is supplied by blood from the other (Bienfait and 

 Hogge). When the respiration is artificially hindered or 

 stopped in one of the animals, it shows no dyspnoea ; it 

 is in the other, whose brain is being fed with improperly 

 oxygenated blood, that the respiratory movements become 

 exaggerated. The point of attack of the ' venous ' blood 

 has been further localized in the spinal bulb by the observa- 

 tion that when the brain has been cut away above it, the 

 cord severed below the origin of the phrenics, and all other 



