CAUSES OF THE VENOUS CIRCULATION. 321 



that this force from the thorax was felt "in the entire venous 

 system, an opinion which, as we shall see, the most simple 

 observations have shown to be entirely erroneous. 1 As this 

 force is not felt throughout the whole of the venous system, 

 it becomes a question of interest to determine how far its in- 

 fluence extends, and why it is restricted to certain vessels. 

 Like the action of the muscular system on certain veins, it is 

 simply superadded to the force of the heart, the latter being 

 entirely competent to keep up the venous circulation. A 

 proof that it is not essential is seen in the fact that the circu- 

 lation is effected in animals which do not inspire, but swallow 

 their air, 2 and in the foetus, before any movements of respi- 

 ration take place. 



Direct observations on the jugulars show conclusively that 

 the influence of inspiration cannot be felt much beyond these 

 vessels. They are seen to collapse with each inspiratory act, 

 a condition which limits this influence to the veins near the 

 heart. The flaccidity of the walls of the veins will not permit 

 the extended action of any suction force. If a portion of a vein 

 removed from the body be attached to the nozzle of a syringe, 

 and we attempt to draw a liquid through it, though the suc- 

 tion force be applied very gently, when the vessel has any 

 considerable length, its walls will be drawn together. In the 

 circulation, the veins are moderately distended with blood by 

 the vis a tergo, and,, to a certain extent, supported by con- 

 nections with surrounding tissues, so that the force of aspira- 

 tion is felt farther than in any experiment on vessels re- 

 moved from the body. The blood, as it approaches the 

 thorax, impelled by other forces, is considerably accelerated 

 in its flow ; but it is seen by direct observation, that beyond 



1 BARRY, Recherches Experimentales sur les Causes du Mouvement du Sang dans 

 les Veines, Paris, 1825, p. 12 et seq. 



2 In many animals that take the air into the lungs by an act like that of de- 

 glutition, there are regular pulsations in the veins near the heart, which are quite 

 abundantly provided with muscular fibres like those found in the heart. It is a 

 question whether this does not take the place of the suction force from the chest, 

 which operates in other animals. 



21 



