RESPIRATION. 453 



inspiratory rise, or it may cause a fall. If, upon the respiration of compressed 

 air, the pressure of the air be above that exerted by the elastic tension of the 

 lungs, no effort of the inspiratory muscles is required, the chest being expanded 

 by the pressure of the air. Therefore, instead of an increase of negative intra- 

 thoracic pressure, as in normal inspiration, there is a decrease, and negative 

 intrathoracic pressure is replaced by positive pressure. As a result, the blood- 

 vessels and the heart, instead of being dilated by an aspiratory action, are 

 pressed upon, forcing the blood into the general circulation, and thus causing a 

 transient rise of pressure, which is, however, succeeded by a fall due to obstruc- 

 tion to the flow of blood through the heart and the pulmonary vessels. Ex- 

 piration into compressed air causes at first a transient increase of blood-pressure 

 followed by a fall, the former being due to the forcing of some of the blood 

 from the intrathoracic and intrapulmonary vessels into the general circulation, 

 and the latter to obstruction to the blood-flow through the heart and the pul- 

 monary circulation. 



When individuals are exposed to compressed- air, as in a pneumatic cabinet, 

 or to rarefied air, as in ballooning, the effects on the circulation become of a 

 very complex character, owing chiefly to the additional influences of the 

 abnormal pressure upon the peripheral circulation ; moreover, the effects of 

 breathing against obstructions or of respiring rarefied or compressed air may 

 be materially influenced by secondary effects resulting from excitation of the 

 cardiac and vaso-motor mechanisms. 



In artificial respiration, as ordinarily performed in the laboratory, air is 

 periodically forced into the lungs by a pair of bellows or a pump, and is ex- 

 pelled from the lungs by the normal elastic and mechanical factors of expira- 

 tion. When the lungs are inflated the pulmonary capillaries are subjected to 

 opposing forces — the positive pressure of the air within the lungs on one hand, 

 and the resistance of the thoracic -walls on the other — so that the blood is 

 squeezed out, thus momentarily increasing the blood-pressure, but subsequently 

 retarding the current and consequently lowering the pressure. During expira- 

 tion the pressure is removed and the blood-flow is encouraged : there is, there- 

 fore, a temporary fall during the filling of the pulmonary vessels, followed by 

 a rise due to the removal of the obstruction. If the air is aspirated from the 

 lungs, the rise of the pressure is augmented, owing to the further dilatation of 

 the intrapulmonary capillaries ; hence, in artificial respiration, during the in- 

 spiratory phase the blood-pressure curves arc reversed, there being a primary 

 transient rise followed by a fall, and during the expiratory phase a transienl 

 fall followed by a rise. In normal respiration the oscillations are due essen- 

 tially to the changes in capacity of the intrapulmonary vessels caused essen- 

 tially by the alterations in their length, while in artificial respiration the 

 effects of these alterations are opposed and superseded by those due directly 

 to positive intrapulmonary pressure. 



J. Special Respiratory Movements. 

 The rhythmical expansion- and contractions of the thorax which we under- 

 stand as respiratory movements have for their object the ventilation of the 



