328 CIECULATIOX. 



veins of the spinal canal, and veins in the walls of the abdo- 

 men and thorax, connect the inferior with the superior vena 

 cava. Even the portal vein has lately been shown to have 

 its communications with the general venous system. Tims, 

 in all parts of the organism, temporary compression of a vein 

 only diverts the current into some other vessel, and permanent 

 obliteration of a vein produces enlargement of communicating 

 branches, which soon become sufficient to meet all the require- 

 ments of the circulation. 



Conditions which impede the Venous Circulation. 



Influence of Expiration. The influence of expiration on 

 the circulation in the veins near the thorax, is directly oppo- 

 site to that of inspiration. As the act of inspiration has a 

 tendency to draw the blood from these vessels into the chest, 

 the act of expiration has a tendency to force the blood out 

 from the vessels of the thorax, as the air is forced out by the 

 trachea, and opposes a flow in the opposite direction. The 

 effect of prolonged and violent expiratory efforts is very 

 marked ; being followed by deep congestion of the veins of 

 the face and neck, and a sense of fulness in the head, which 

 may become very distressing. The opposition to the venous 

 current generally extends only to vessels in the immediate 

 vicinity of the thorax, or, it may be stated in general terms, 

 to those veins in which the flow of blood is assisted by the 

 movements of inspiration ; but, while the inspiratory influence 

 is absolutely confined to a very restricted circuit of vessels, 

 the obstructive influence of very violent and prolonged expi- 

 ration may be extended very much further, as is seen when 

 the vessels of the neck, face, and conjunctiva become con- 

 gested in prolonged vocal efforts, blowing, etc. 



The mechanism of this is not what we might at first be 

 led to suppose ; namely, a mere reflux from the large trunks 

 of the thoracic cavity. "Were this the case, it would be ne- 

 cessary to assume an insufficiency of certain valves, which 



