550 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



ratory discharges from the respiratory centre. This view receives further sup- 

 port in several facts : first, that the same number of inflations, whether of pure 

 O, of air, or of H, causes apnoea, the only difference being the length of the 

 apnoeic pause after the cessation of artificial respiration, which pause lasts for 

 the longest period when O is used, and for the shortest period, or not at all, 

 when H is employed ; second, that apnoea cannot be caused by inflation of the 

 lungs with H if the pneumogastric nerves be previously divided ; third, that the 

 arrest of respiration which occurs during swallowing (" deglutition-apnoea ") 

 is due to an inhibition of the respiratory centre by impulses generated in the 

 terminations of the glosso-pharyngeal nerves (p. 570). It therefore seems evi- 

 dent that apnoea may be due to either gaseous or mechanical factors, or to both, 

 the former being effective, not because of the blood being saturated with O, 

 but because of the increased amount of O in the alveoli a quantity sufficient 

 for a time to aerate the blood ; while the mechanical factors give rise to inhibi- 

 tory impulses which suspend for a longer or shorter period the rhythmical 

 inspiratory discharges from the respiratory centre, doubtless by depressing the 

 irritability of this centre (p. 563). From the experiment quoted it seems that 

 the first of these factors may alone be sufficient to cause apnoea, but that apnoea 

 is more easily produced, and lasts longer, when both factors act together, as is 

 usually the case. 



Polypncea, thermopolypnoea, and heat-dyspnoea are due to a direct excitation 

 of the respiratory centres through an increase of the temperature of the blood, 

 or reflexly by excitation of the cutaneous nerves by external heat. This con- 

 dition may be produced, as was done by Goldstein, by exposing the carotids 

 and placing them in warm tubes, thus heating the blood ; or, as was done by 

 Bichet and others, by subjecting the body to high external heat. Eichet in 

 employing this latter method found that dogs so exposed may have a respira- 

 tory rate as high as 400 per minute. Ott records marked polypnoea as a result 

 of direct irritation of the tuber cinereum. This form of hyperpnoea is entirely 

 independent of the gaseous composition of the blood ; moreover, an animal in 

 heat-dyspnoea cannot be rendered apnoeic, even though the blood be so thor- 

 oughly oxygenated that the venous blood is of a bright arterial hue. 



Dyspnoea is generally characterized by slow, deep, and labored respiratory 

 movements, although in some instances the rate may be increased. Several 

 distinct forms are observed : " O-dyspnoea," due to a deficiency of O ; 

 " CO 2 -dyspnoea," due to an excess of CO 2 in the blood ; a form of dyspnoea 

 due to substances imparted to the blood by the muscles during activity ; and 

 cardiac and hemorrhagic dyspnoeas, belonging to the O category. 



Dyspnoeas due to the gaseous composition of the blood may be caused either 

 by a deficiency of O or by an excess of CO 2 , but are generally due to both. 

 Dyspnoea from a deficit of O is observed when an animal is placed within a 

 small closed chamber, or when an indifferent gas, such as pure hydrogen or 

 nitrogen, is respired. Under the latter circumstances dyspnoea occurs even 

 though the quantity of CO 2 in the blood be below the normal. If, on the 

 contrary, the animal be compelled to breathe an atmosphere containing 10 vol- 



