292 THE CIRCULATION. 



1. The force, of aspiration of the thorax. When the chest expands 

 by the lifting of the ribs and the descent of the diaphragm, its 

 movement, of course, tends to diminish the pressure exerted upon 

 its contents, and so has the effect of drawing into the thoracic cavity 

 all the fluids which can gain access to it. The expanded cavity is 

 principally filled by the air, which passes in through the trachea 

 and fills the bronchial tubes and the pulmonary vesicles. But the 

 blood in the veins is also drawn into the chest at the same time and 

 by the same force. This force of aspiration, exerted by the expan- 

 sion of the chest, is gentle and uniform in character, like the move- 

 ments of respiration themselves. Accordingly its influence is ex- 

 tended, without doubt, to the farthest extremities of the venous 

 system, the blood being gently solicited toward the heart,' at each 

 expansion of the chest, without any visible alteration in the size of 

 the veins, which are filled up from behind as fast as they are emptied 

 in front. 



But if the movement of inspiration be sudden and violent, instead 

 of gentle and easy, a different effect is produced. For then the walls 

 of the veins, which are thin and flaccid, cannot retain their position, 

 but collapse under the external pressure too rapidly to allow the 

 blood to flow in from behind. In this case, therefore, the vein is 

 simply emptied in the immediate neighborhood of the chest, but 

 the entire venous circulation is not assisted by the movement. 



The same difference in the effect of an easy and a violent suction 

 movement, may be readily shown by attaching to the nozzle of an 

 air-tight syringe a flexible elastic tube with thin walls, and placing 

 the other extremity of the tube under water. If the piston of the 

 syringe be now withdrawn with a gentle and gradual motion, the 

 water will be readily drawn up into the tube, while the tube itself 

 suffers no visible change ; but if the suction movement be made 

 rapid and violent, the tube will collapse instantly under the pres- 

 sure of the air, and will fail to draw the water into its cavity. 



A similar effect shows itself in the living body. If the jugular 

 or subclavian vein be exposed in a dog or cat, it will be seen that 

 while the movements of respiration are natural and easy no fluc- 

 tuation in the vein can be perceived. But as soon as the respira- 

 tion becomes disturbed and laborious, then at each inspiration the 

 vein is collapsed and emptied ; while during expiration, the chest 

 being strongly compressed and the inward flow of the blood arrested, 

 the vein becomes turgid with blood which accumulates in it from 

 behind. In young children, also, the spasmodic movements of res- 



