ASPIRATION OF THE HEART. 



129 



BR 



mm. Hg. prevails (H. Jacobson). Hence, the lymph-stream can flow unhindered. 

 As the distance of the veins from the heart increases, there is a gradual increase 

 of the lateral pressure ; in the external facial vein (sheep) = + 3 mm. ; brachial, 

 4 # 1 mm., and in its branches 9 mm. ; crural, 11*4 mm. [The pressure is said to 

 be negative when it is less than that of the atmosphere. The gradual fall of the 

 blood-pressure from the capillary area (C) to the venous area (V) is shown in fig. 

 108, while within the thorax, where the veins terminate in the right auricle, the 

 pressure is negative.] 



Modifying Conditions. (1) All conditions which diminish the difference of 

 pressure between the arterial and venous systems increase the venous j^essure, and 

 vice versa. 



(2) General plethora of blood increases it ; anaemia diminishes it. 



(3) Respiration, or the aspiration of the thorax, affects specially the pressure 

 in the veins near the heart ; during inspiration, owing to the diminished tension, 

 blood flows towards the chest, while during expiration it is retarded. The 

 effects are greater, the deeper the respiratory movement, and these may be very 

 great when the respiratory passages are closed ( 60). 



[When a vein is exposed at the root of the neck, it collapses during inspiration, and fills 

 during expiration. The respiratory movements do not affect the venous stream in peripheral 

 veins. The veins of the neck and face become distended with blood during crying, and on 

 making violent expiratory efforts, as in blowing upon a wind instrument. Every surgeon is 

 acquainted with the fact that air is particularly liable to be sucked into the veins, especially in 

 operations near the root of the neck. This is due to the negative intra-thoracic pressure occur- 

 ring during inspiration.] 



(4) Aspiration of the Heart. Blood is sucked or aspirated into the auricles 

 when they dilate (p. 59), so that there is a double aspiration one synchronous 

 with inspiration, and the other, which is but slight, 

 synchronous with the heart-beat. There is a cor- 

 responding retardation of the blood-stream in the 

 venae cavae, caused by the contraction of the auricle 

 (p. 58, a). The respiratory and cardiac undula- 

 tions are occasionally observable in the jugular 

 vein of a healthy person ( 99). 



(5) Change in the position of the limbs or of 

 the body, for hydrostatic reasons, greatly alters the 

 venous pressure. The veins of the lower extrem- 

 ity bear the greatest pressure, while at the same 

 time they contain most muscle (K. Bardeleben, 

 65). Hence, when these muscles from any cause 

 become insufficient, dilatations occur in the veins, 

 giving rise to the production of varicose veins. 



[Braune showed that the femoral vein under Poupart's 

 ligament collapsed when the lower limb was rotated out- 

 wards and backwards, but filled again when the limb was 

 restored to its former position. All the veins which open 

 into the femoral vein nave valves, which permit blood to 

 pass into the femoral vein, but prevent its reflux. This 

 mechanism acts to a slight degree as a kind of suction and 

 pressure apparatus when a person walks, and thus favours the onward movement of the blood.] 

 [(6) Muscular Movements. Veins which lie between muscles are compressed 

 when these muscles contract, and as valves exist in the veins, the flow of blood is 

 accelerated towards the heart ; if the outflow of the blood be obstructed in any 

 way, then the venous pressure on the distal side of the obstruction may be greatly 

 increased. When a fillet is tied on the upper-arm, and the person moves the 

 muscles of the fore-arm, the superficial veins become turgid, and can be distinctly 

 traced on the surface of the limb.] 



I 



Fig. 110. 

 Scheme of the blood-pressure. H, 

 heart ; a, auricle ; v, ventricle ; A, 

 arterial ; C, capillary ; and V, ven- 

 ous areas. The circle indicates the 

 parts within the thorax ; B. P., 

 pressure in the aorta. 



