CONDITIONS WHICH IMPEDE THE VENOUS CIRCULATION. 105 



neous veins are somewhat compressed. This may be seen in the hand, by letting it hang 

 by the side until the veins become somewhat swollen, and then contracting the muscles, 

 when the skin will become tense and the veins are very much less prominent. Here the 

 valves have an important action. The compression of the veins is much greater in the 

 substance of and between the muscles than in the skin ; but the blood is forced from the 

 muscles into the skin, and the valves act to prevent it from taking a retrograde course. 

 The fact that the contraction of muscles forces blood into the veins of the skin may be 

 seen by surrounding the upper part of the forearm with a moderately-tight ligature, 

 which will distend the cutaneous veins below. If we now contract the muscles vigor- 

 ously, the veins below will become sensibly more distended and knotted ; showing, at 

 once, the passage of blood into the skin and the action of the valves. 



When a vein is distended by the injection of air or a liquid forced against the valves, 

 it is observed that, at the point where the convex borders of the valves are attached, the 

 vessel is not dilated as much as at other parts. This is due to the fact that the valves 

 are bordered with a fibrous ring which strengthens the vessel and prevents distention 

 at that point, which would otherwise separate the free borders of the valves and render 

 them insufficient. 



A full consideration of the venous anastomoses belongs to descriptive anatomy. 

 Suffice it to say, in this connection, that they are very numerous and provide for a 

 return of the blood to the heart by a number of channels. The azygos vein, the veins 

 of the spinal canal, and veins in the walls of the abdomen and thorax, connect the infe- 

 rior with the superior vena cava. Even the portal vein has lately been shown to have 

 its communications with the general venous system. Thus, 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 requirements 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 opposed 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 

 assists in forcing the blood out from the vessels of the thorax 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 pro- 

 longed expiration may be extended very much farther, as is seen when the vessels of the 

 neck, face, and conjunctiva become congested 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 necessary to assume an insufficiency of certain valves, which does not exist. In 

 extreme congestion, reflux of blood may take place to a certain extent in the external 

 jugular, for this vessel has but two valves, which are not competent to prevent regurgi- 

 tation ; the chief cause of congestion, however, is due, not to regurgitation, but to accumu- 

 lation from the periphery and an obstruction to the flow of blood into the great vessels. 



It is in the internal jugular that the influence of expiration is most important, both 

 from its great size in the human subject, as compared with the other vessels, and from 

 the importance and delicacy of the parts from which it collects the blood. At the open- 

 ing of this vessel into the innominate vein, is a pair of strong and perfect valves, which 

 effectually close the orifice when there is a tendency to regurgitation. These valves have 



