RESPIRATION. 



death of the animal. For this reason the term^'nceud vitaiy was 

 applied to it. In this area the respiratory center wa"s~tecal:ed. ^It has, 

 however, been shown by Gad that if this area be gradually destroyed 

 by cauterization the respiratory movements do not cease, but con- 

 tinue until the cauterization has reached a point far forward in 

 the formatio reticularis, in which the respiratory center was assumed 

 to lie. 



Though its existence has not been anatomically determined 

 beyond question, it is permissible to speak of the central mechanism 

 as a "center" located in the medulla oblongata. 



The activity of the respiratory center has long been described as 

 automatic or autochthonic (Gad) in character, expressive of the idea 

 that the rhythmic discharge of nerve impulses is conditioned by the 

 composition of the blood or lymph by which it is surrounded. Thus 

 so long as the blood retains its normal composition the respiratory 

 movements are normal. If, however, the blood becomes richer in 

 oxygen and poorer in carbon dioxid, the rate of discharge of nerve 

 impulses and the inspiratory movements diminish until the condition 

 of apnea results. If, on the contrary, the blood becomes poorer in 

 oxygen and richer in carbon dioxid, the reverse condition obtains: 

 viz., an increased rate of discharge of nerve impulses, increased fre- 

 quency of respiration, hyperpnea, and dyspnea. This view is ap- 

 parently supported by the fact that after division of the fifth and 

 vagus nerves the respiratory movements continue, though changed 

 in character, becoming less frequent and deeper. Whether they 

 would continue after division of all afferent nerves it is impossible 

 to state, since from the nature of the case such an experiment would 

 be most difficult to perform. 



The first inspiration aftpr hi^th JfLJillCEI^cL to be^due to the direct 

 stimulation of the respiratory center by the increase in the carbon 

 dioxid present in the blood, though it may be aided by the cooling 

 of the skin due to vaporization of the amniotic fluid. 



Whether the respiratory center is automatic in character or not, 

 k may be influenced directly by nerve impulses descending from the 

 brain in consequence of volitional acts or emotional states, and in- 

 directly by nerve impulses reflected to it from the general periphery 

 through various afferent nerves, in consequence of agencies acting 

 on their peripheral filaments : e. g., cold applied to the skin, irritating 

 gases to the nasal and bronchial mucous membrane, distention and 

 collapse of the pulmonary alveoli. 



V Of all afferent nerves, the vagus appears to be the most influential in 

 maintaining the rhythmic discharge ol nerve impulses from the respi- 

 ratory center.^ TTius7~tf~while the animal is breathing regularly and 

 quietly both vagi are cut, the respiratory movements become much 

 slower, falling perhaps to one-third their original number per minute. 



