MECHANICAL PHENOMENA OF RESPIRATION. 315 



repeated, causes dilatation of the veins, varices, vascular 

 hypertrophy of the thyroid gland, &c. 



This influence of expiration produces equally marked effects 

 in the arterial cone. At the top of this cone, the pressure 

 produced by the ventricle is -j 2 ^. If we assume the pressure 

 in the thorax at T \/^, in the arterial cone it will be 1 *$ F ; this 

 causes the arterial blood to flow much faster, there being here 

 nothing which can counteract or delay the effect of this 

 increase of pressure ; and the fluid is forced into the arteries 

 by two pumps, the heart and the thorax. It is true that the 

 slackening of the flow of the blood in the veins has a ten- 

 dency to counterbalance its increased rapidity in the arteries, 

 but, in spite of this, immense pressure is produced on the 

 entire current of the circulation, accompanied by a strong 

 tendency to hemorrhage, ruptures of aneurisms, varicose 

 dilatations, &C. 1 



The phenomena which follow a diminution of pressure in 

 the thorax, produced by a violent inspiratory movement, are 

 entirely different from the above. The pressure at the top of 

 the venous cone then becomes less than 0, or, in fact, aspira- 

 tion of blood by the veins, an increased acceleration of the 

 circulation of the venous blood ; if the blood does not flow 

 in sufficient quantity to satisfy this aspiratory demand, the 

 coats of the veins become relaxed, and show a tendency to 

 collapse. In the veins which are near the thorax, and are 

 especially under the influence of this aspiration, the rela- 

 tions between the coats of the veins and the aponeuroses 

 are such that these vessels remain constantly open : the 

 aspiration is thus continued to veins more remote from the 

 heart. In a surgical operation, therefore, if one of the veins 

 near the thorax be opened, the outer air, at the moment of 

 inspiration, may be drawn into the interior of the vessel, an 

 occurrence which is generally followed by speedy death. 



Under the influence of this inspiratory aspiration, the aortic 

 pressure, which is -f^ falls to T \fo, or ^%, causing a slackening 

 of the circulation, diminished tension of the vessels, feeble- 

 ness of the pulse, &c. But while the conditions of expiration 

 were favorable to hemorrhage, these resist it, and, in order to 

 arrest the flow of blood, it is sometimes only necessary to 

 cause the patient to make several deep inspirations. 



These results, at which we have arrived by simple reason- 



1 See F. Guyon, " Note sur 1'Arret de la Circulation Caroti- 

 dienne pendant T Effort." Archives de Physiologic, 1886. 



