4G ON DIGITALIS, WITH SOME OBSERVATIONS ON THE UFJNE. 



closing one nostril, and thus obstructing the passage of air, the 

 arterial tension rises with citpiration, avA falls with inspiration. 

 Bat if respiration be freely performed as with open mouth, the 

 rension rises witli aspiration, and falls with expiration. This 

 difference is due to the pressure upon the aorta and large vessels 

 of thorax and abdomen. 



The enlargement of the thoracic cavity during inspiration 

 causes a vacuum which is at once filled by the external aii", if 

 there be no obstacle to its entrance, and thus no great suction 

 is exerted. If the air enter with difficulty, the enlargement 

 of the thoracic cavity by the respiratory muscles acting like the 

 withdrawal of the piston of an exhausting syringe, greatly 

 lessens the internal pressure, and causes much suction, the blood 

 is drawn to the aorta and large vessels of the thorax from the 

 periphery, and the tension is much diminished. During ex- 

 piration, tlie contractility of the lung and the expiratory 

 muscles combine to expel the air ; but this being done with 

 difficulty, the aorta and large vessels are much compressed, the 

 ])lood driven towards the periphery, and the tension raised. 

 While this is going on in the thorax, the diaphragm descends 

 into the abdomen during inspiration, lessening its cavity and 

 pressing on the abdominal aorta and increasing the tension, 

 while, during expiration, it ascends, lessening the pressure, and„ 

 consequently, the tension. In a normal state, the thoracic and 

 p,bdominal pressures counteract each other ; but when there is 

 any obstacle to the passage of air, the thoracic influence predo- 

 minates, and the abdominal, when there is any hindrance to- 

 expansion of the abdominal parietes. In the normal state of 

 the animal economy, the number of the pulse and the arterial 

 tension are in inverse ratio to each other, the law given by Marey 

 on this point being, " The heart beats so much the more fre- 

 quently the less the difficulty it has in emptying itself."* 



I have entered at length on this question of arterial tension^ 

 as a knowledge of the facts I have stated is necessary in order 

 to understand fully the way in which the circulation is modi- 

 lied by digitalis. 



On the False. — When given in small doses, digitalis first 



* Marey, op. cit. 



