378 THE CIRCULATION OF THE BLOOD. [CHAP. XXVIII. 



David Barry illustrated the influence of inspiration in favouring 

 the centripetal flow of blood. He introduced one end of a bent 

 glass tube into the jugular vein of a horse, the vein being tied above 

 the point at which the tube was inserted : the other end of the tube 

 was immersed in a coloured fluid. At each inspiration the fluid 

 rose in the tube, being drawn towards the chest; whilst during 

 expiration it sank, or remained stationary. 



Forced expiratory efforts, on the other hand, retard the venous 

 circulation, as may be well illustrated by holding the breath for a 

 few seconds, or straining strongly, when the veins, especially those 

 of the neck and chest, will swell up and become distended ; but as 

 soon as the breathing is restored, they return immediately to their 

 former size. Hence it is that persons subject to frequent distur- 

 bances of the respiratory actions, as in asthma or dyspnoea of any 

 kind, exhibit, after a time, more or less enlargement of the venous 

 system. 



Disturbances in the respiratory actions seem to affect the circula- 

 tion, and especially that in the veins more extensively in another 

 way ; namely, through the pulmonary capillaries. The imperfect 

 respiratory changes, consequent upon the disturbed breathing, 

 retard the flow of blood through the capillary plexus, which under- 

 goes, by the dilatation and rupture of the air-cells, considerable 

 stretching and widening of its meshes, and even becomes oblite- 

 rated in parts. These changes also create additional obstacles to 

 the pulmonary circulation, which impede the flow from the right 

 ventricle, and increase the backward pressure of the blood on the 

 walls of that cavity, causing it to become dilated and hypertrophied. 

 It is in this way that are produced the hypertrophy and dilatation 

 of the right cavities of the heart, which, to a greater or less extent, 

 are invariable consequents of frequent attacks of asthma or bron- 

 chitis. 



The influence of the respiratory movements upon the venous 

 circulation is shewn in the clearest manner by the use of the 

 hsemadynamometer, as in the experiments of Poiseuille, Magendie, 

 Ludwig, Valentin, and Mogk, the column of mercury rising in 

 expiration, and falling in inspiration ; and these experiments like- 

 wise prove that this influence is only felt in the large veins near 

 the chest, and not in the more distant ones. The influence of ex- 

 piration in retarding is much more powerful than that of inspiration, 

 in promoting the venous circulation; for the same physical condition 

 of the chest which exists at the commencement of inspiration, arid 

 which favours the rush of blood to it, tends rather to delay the 



