340 CIRCULATION OF NUTRITIVE FLUID. 



606. The movement of the Blood through the Veins is, without doubt, 

 chiefly effected by the vis a t'ergo, or propulsive force, which results from 

 the contractile power of the heart and arteries, aided by the power 

 generated in the capillary vessels. The intermittent flow, which is 

 caused by the interrupted action of the former, is usually so far equalized 

 during the passage of the blood through the capillary network, that no 

 pulsation can be shown to exist in the veins ; but instances occasionally 

 present themselves, in which a venous pulse may be clearly perceived. 

 The Venous Circulation is affected, however, by certain other causes, 

 which exert little influence on the movement of blood in the Arteries. 

 One of these is the frequently recurring action of Muscles, to which the 

 Veins are peculiarly subject, on account of their position. In every 

 instance in which Muscular movement takes place, a portion of the 

 Veins of the part will undergo compression ; and as the blood is pre- 

 vented by the valves in the veins, from being driven back into the small 

 vessels, it is necessarily forced onwards towards the heart. As each 

 set of muscles is relaxed, the veins that were compressed by it fill out 

 again, to be again compressed on the renewal of the force. Thus we 

 see how the general Muscular movements of the body have an important 

 influence in maintaining the Venous Circulation, how continued exer- 

 cise, involving the alternate contraction and relaxation of several groups 

 of muscles, must send the blood more rapidly towards the heart, and 

 thus increase the rapidity of its pulsations, and how the sudden and 

 simultaneous action of a large number of muscles after repose (as when 

 we rise up from the sitting or lying, to the standing posture), may drive 

 the blood to the heart with 1 so violent an impetus, as to produce even 

 fatal results, if, by any diseased condition of that organ, it should be 

 rendered unable to dispose with sufficient rapidity of the quantity of 

 blood thus transmitted to it. 



607. The Respiratory movements exert a slight influence upon the 

 flow of blood through the large veins near the heart ; for as the chest is 

 a closed cavity, in which a partial vacuum is produced by the act of In- 

 spiration, whilst its contents are compressed by the act of Expiration, 

 the former state will favour the movement of blood from the large veins 

 on the exterior of the cavity, towards the heart, whilst the latter condi- 

 tion will retard it. This produces the phenomenon termed the respira- 

 tory pulse ; which may be seen in the veins of the neck and shoulders in 

 thin persons, and especially in those who are suffering from pulmonary 

 diseases. The influence of the Respiratory movements is made evident 

 by introducing a tube into the Jugular vein, the lower end of which 

 dips into water ; for an alternate elevation and depression of the water 

 into the tube is then witnessed, showing the suction power of the Inspi- 

 ratory movement, and the expellent force of the Expiratory act. On 

 the other hand, the Expiratory movement, while it directly tends to 

 cause accumulation in the veins, will assist the heart in propelling the 

 blood in the Arteries ; and by the combined action of these two causes 

 is produced, among other effects, the rising and sinking of the Brain, 

 synchronously with expiration and inspiration, which are observed when 

 a portion of the cranium is removed. 



608. A pulsatory movement may be occasioned in the great veins 



