100 CIRCULATION OF THE BLOOD IN THE VESSELS. 



branches, which soon become sufficient to meet all the requirements of the 

 circulation. 



CONDITIONS WHICH IMPEDE THE VENOUS CIKCULATION. 



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 quite marked, these being fol- 

 lowed by congestion of the veins of the face and neck and a sense of fullness 

 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 ves- 

 sels, the obstructive influence of very violent and prolonged 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 a mere reflux from the large trunks of the 

 thoracic cavity. Were this the case, it would be necessary to assume an insuf- 

 ficiency 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 regurgitation. 

 The chief cause of congestion, however, is due, not to regurgitation, but to 

 accumulation 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 on account of its great size in the human subject, as com- 

 pared 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 expi- 

 ration arrests the onward flow in the veins near the thorax, these valves are 

 closed and effectually 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 con- 

 gestion, unless the effort be extraordinarily violent and prolonged. 



The above remarks with regard to the influence of expiration are appli- 

 cable to vocal efforts, violent coughing or sneezing, or any unusual muscular 

 efforts, such as straining, in which the glottis is closed. 



Regurgitant Venous Pulse. In the inferior animals, such as the dog, if the 

 external jugular be exposed, a distention of the vessel is seen to accompany 

 each expiratory act. This is sometimes observed in the human subject when 

 respiration is exaggerated, and has been called improperly the venous pulse. 

 There is no sufficient obstacle to the regurgitation of blood from the thorax 



