416 THE HUMAN BODY. 



largely under reflex control; a dash of cold water on the 

 skin, the irritation of the nasal mucous membrane by snuff, 

 or of the larynx by a foreign body, will each cause a modifi- 

 cation in the respiratory movements a long indrawn breath, 

 a sneeze, or a cough. But, although thus very subject to 

 influences reaching it by afferent nerves, the respiratory 

 centre seems essentially independent of such. In many ani- 

 mals, as rabbits (and in some men), marked breathing move- 

 ments take place in the nostrils, which dilate during inspira- 

 tion ; and when the spinal cord of a rabbit is cut close to the 

 medulla, thus cutting off all afferent nervous impulses to the 

 respiratory centre except such as may reach it through cranial 

 nerves, the respiratory movements of the nostrils still con- 

 tinue until death. The movements of the ribs and dia- 

 phragm of course cease, and so the animal dies very soon 

 unless artificial respiration be maintained. Moreover, if 

 after cutting the spinal cord as above described, the chief 

 sensory cranial nerves be divided, so as to cut off the respira- 

 tory centre from almost all possible afferent nervous im- 

 pulses, the regular breathing movements of the nostrils con- 

 tinue. It is, therefore, nearly certain that the activity of the 

 respiratory centre, however much it may be capable of modi- 

 fication through sensory nerves, is essentially independent of 

 them ; in other words the normal respiratory movements are 

 not reflex. 



What it is that Excites the Respiratory Centre. The 

 thing that, above all others, influences the respiratory centre 

 is the greater or less venosity of the blood flowing through it. 

 If this blood be very rich in oxygen and comparatively poor 

 in carbon dioxide the respiratory centre acts but feebly, and 

 the respirations are shallow. If, on the other hand, this 

 blood be highly venous the respiratory movements are more 

 rapid than normal, and are forced, the extraordinary muscles 

 of respiration being called into play; this state of violent 

 labored respiration, due to deficient aeration of the blood is 

 called dyspnma. Normal quiet breathing is eupnosa. If 

 active artificial respiration be kept up on an animal for a 

 short time, it is found, on its cessation, that the creature 

 (dog or rabbit) makes no attempt to breathe for a period 

 which may extend to one and a half minutes. TMs breath- 

 less condition, in which an animal with no hindrance opposed 

 to its breathing makes no respiratory movement is apncea. 



