396 COMPARATIVE PHYSIOLOGY. 



of the middle line ; also as made up of an inspiratory and ex- 

 piratory part; automatic essentially, but greatly modified by 

 afferent impulses, especially those ascending the vagi nerves ; 

 while the latter may be considered as containing both inhib- 

 itory and augmenting fibers for the center, the whole process 

 will be clearer. Respiration on this view would be self-regula- 

 tive ; the deeper the inspiration, the stronger the inhibitory in- 

 fluence, so the greater the tendency to arrest of inspiration; 

 hence either expiration or apncea. 



Is it, then, the excessive accumulation of carbon dioxide or 

 the deficiency of oxygen that induces dyspnoea ? Considering 

 that the former gas acts as a narcotic, and does not induce con- 

 vulsions, even when it constitutes a large percentage of the 

 atmosphere breathed, and that the need of oxygen for the tis- 

 sues is constant, it certainly seems most reasonable to conclude 

 that the phenomena of dyspnoea are owing to the lack of oxy- 

 gen, chiefly at least ; thougli the presence of an excess of car- 

 bonic anhydride may take some share in arousing that vigorous 

 effort on the part of the nervous system, to restore the func- 

 tional equilibrium, so evident under the circumstances. 



THE INFLUENCE OF RESPIRATION ON THE 

 CIRCULATION. 



An examination of tracings of the intra-thoracic and blood- 

 pressure, taken simultaneously, shows (1) that during inspira- 

 tion the blood-pressure rises and the intra-thoracic pressure 

 falls ; (2) that during expiration the reverse is true ; and (3) 

 that the heart-beat is slowed, and has a decided effect on the 

 form of the pulse. But it also appears that the period of high- 

 est blood-pressure is just after expiration has begun. 



We must now attempt to explain how these changes are 

 brought about. By intra-thoracic pressure is meant the press- 

 ure the lungs exert on the costal pleura or any organ within 

 the chest, which must differ from intra-pulmonary pressure 

 and the pressure of the atmosphere, because of the resistance of 

 the lungs by virtue of their own elasticity. 



It has been noted that even in death the lungs remain par- 

 tially distended ; and that when the thorax is opened the pul- 

 monary collapse which follows demonstrates that their elas- 

 ticity amounts to about five millimetres of mercury, which 

 must, of course, represent but a small portion of that elasticity 



