RESPIRA TION. 56 1 



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

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

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

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

 tially to changes in negative intrathoracic pressure, while in artificial respira- 

 tion, as above noted, they are due to changes in positive intrapulmonary 

 pressure. 



J. SPECIAL RESPIRATORY MOVEMENTS. 



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

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

 lungs. There are, however, other movements which possess certain respiratory 

 characters, but which are for entirely different purposes, hence they are spoken 

 of as special or modified respiratory movements. Some of these movements 

 are purposeful in character, others are spasmodic; some are voluntary or in- 



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