393 THE HUMAN BODY. 



largely under reflex control; a dash of cold watei on the- 

 skin, the irritation of the nasal mucous membrane by 

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

 modification in the respiratory movements a long indrawn 

 breath, a sneeze, or a cough. But, although thus subject 

 to influences reaching it by afferent nerves, the respiratory 

 centre seems essentially independent of such. In many 

 animals, as rabbits, (and in some men,) marked breathing 

 movements take place in the nostrils, which dilate during 

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

 to the medulla, thus cutting off all afferent nervous im- 

 pulses to the respiratory centre except such as may reach 

 it through cranial nerves, the respiratory movements of the 

 nostrils still continue until death. The movements of the 

 ribs and diaphragm 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, 

 all afferent cranial nerves be divided, so as to cut off the 

 respiratory centre from all possible afferent nervous im- 

 pulses, the regular breathing movements of the nostrils 

 continue. It is, therefore, obvious that the activity of the 

 respiratory centre, however much it may be capable of 

 modification through sensory nerves, is essentially inde- 

 pendent 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 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 dyspnoea. Normal quiet breathing is eupncea. 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 



