A textbook of human physiology / translated from [the] 6th German edition by W. Stirling.

  • Landois, Leonard
Date:
1888
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Licence: Public Domain Mark

Credit: A textbook of human physiology / translated from [the] 6th German edition by W. Stirling. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

    181/980 page 129
    nun. Hg. prevails (//. Jacobson). Hence, the lympli-streani can How 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, 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 diminuli the difference of pressure between the arterial and venous systems increase the venous pressure, and ifice versa. (2) General plethora of blood increases it ; anannia 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 tlie root of the neck, it collapsef; durin<f inspiration, and fills during expiration. The respiratory movements do not aH'ect the venous stream in ])eripheral veins. The veins of the neck and face become distended with blood during crying, and on making violent expiratory etlbrts, as in blowing ujion a wind instrument. Every surgeon is acquainted with the tact that air is particnlarlj- liable to be sucked into the veins, especially in operations near tlie root of the neck. This is due to the negative intra-thoracic pi'essure 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 venie cav<i?, 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 (A'. Bardelehen, §65). Hence, when these muscles from any cause become insufficient, dilatations occur in the veins, giving rise to the production of varicose veins. [Brauue showed that the femoral vein under Poupart's . . ligament collapsed when the lower limb was rotated out- j^f^^.^^ wards and backwards, but filled again when the limb was restored to its former j)osition. All the veins wliich open into the femoral vein have 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 heart; « arterial; ous areas Fig. 110. the blood-pressure. H, auricle ; v, ventricle ; A> C, capillary; and V, ven- The circle indicates the parts within the thorax; ]i.P., pressure in the aorta. 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 hllet 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
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