80 CIRCULATION OF THE BLOOD 



they do not exist in the cavities, where the venous trunks are not sub- 

 ject to compression. In the action of muscles, the skin frequently is 

 stretched over the part, and the cutaneous veins are somewhat com- 

 pressed. This may be seen in the hand, by letting it hang by the side 

 until the veins become swollen, and then contracting the muscles, when 

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

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

 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. 



Conditions that impede Venous Circulation. Expiration, in its influ- 

 ence on the circulation in veins near the thorax is directly opposed to 

 inspiration. As inspiration has a tendency to draw the blood from these 

 vessels into the chest, 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 quite marked, 

 being followed by congestion of the veins of the face and neck and a 

 sense of fulness in the head, which may become distressing. The op- 

 position to the venous current usually 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 confined to a very 

 restricted circuit of vessels, the obstructive influence of violent and 

 prolonged expiration may be extended much farther, as is seen when 

 the vessels of the neck, face and conjunctiva become congested in pro- 

 longed vocal efforts, blowing etc. This is not simply a reflux from the 

 large trunks of the thoracic cavity ; for in this case, it would be 

 necessary to assume an insufficiency of certain valves, which does not 

 exist. 



It is in the internal jugular that the influence of expiration is most 

 important, both on account of its great size in the human subject, as 

 compared with the other vessels, and the importance and delicacy of the 

 parts from which it collects the blood. At the opening 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. 

 When the act of expiration arrests the onward flow in the veins near the 

 thorax, these valves are closed and protect the brain from congestion 

 by regurgitation. The blood accumulates behind the valves, but the 

 free communication of the internal jugular with the other veins of the 

 neck relieves the brain from congestion, unless the effort is extraordi- 

 narily violent and prolonged. 



It is evident that there are other conditions that may impede the 



