1 2 o THE ME CHANISM OF THE CIR C ULA TION. 



results. The manometer employed for venous work is filled with a 

 semi-saturated solution of sodium or magnesium sulphate. 



Jacobson l found in the sheep, during normal quiet respiration, the 

 following venous pressures : — 



The pressure is lowest nearest the heart and thorax, and becomes higher 

 in proportion as the vein is placed more peripherally. The negativity in the 

 central veins is due to the suction of the thoracic cavity produced by the 

 elastic pull of the lungs. Owing to this negative pressure, when a large vein 

 is opened in the neighbourhood of the thorax, air can be sucked into the 

 circulation. Air that has thus obtained an entry has been observed to pass 

 right through the pulmonary circulation, and to enter the arteries. The danger 

 of air entering during operations has been over-rated. A large amount of air 

 can be slowly injected into a vein without killing an animal. 2 



A rapid injection, such as would be caused by blowing air into the venous 

 cannula, kills by causing frothing in the heart, embolism in the lungs, and 

 perhaps embolism in the coronary arteries. The danger of embolism from the 

 entry of air is much greater in a small than in a large animal, for the smaller 

 the heart the less the amount of air required to hinder its action by frothing. 3 



Effect of muscular movements. — -Muscular movement is a most 

 important agency which aids the venous flow. On account of the 

 valves the blood can only be expressed from those veins which are 

 compressed by muscular contraction, in one direction, viz. towards the 

 heart. Continued tetanic spasm stops the How, but clonic or repeated 

 movements, by the alternate filling and expression of the veins, materially 

 favours the flow. The pressure in the vena cava may be considerably 

 raised by general tetanic spasm of the whole body, as is seen after 

 injection of strychnine or essential oil of absinthe ; and this is so, in 

 spite of a general arterial constriction which diminishes the inflow into 

 the veins. The rise of venous pressure in such a condition of tetanus 

 is of mixed origin. It is due, first, to compression of the veins by the 

 spasm of the muscles ; and, secondly, to general vaso-constriction of 

 arteries and portal veins. Thus, while the capacity of the venous system 

 is diminished, the output of the heart is, owing to the high arterial 

 pressure, lessened, and the blood is congested into the veins. 



Pressure of the body against external objects influences the venous 

 flow in the same manner as muscular contraction. Sitting or lying in 

 one posture retards the venous circulation, while, on the other hand, 

 exercise and constant change of posture not only accelerate the circula- 

 tion on the arterial side, but also aid the flow in the veins. 



By rhythmic compression of the thigh muscles the blood can be 

 discharged in jets from the femoral vein of a dog, and the flow of blood 

 greatly increased. The rhythmic contractions of the spleen and the 



1 Arch./. Anat., Physiol, u. wissensch. Med., 1867, S. 226. 



2 Laborde and Muron, Compt. rend. Soc. de biol., Paris, 1873, tome lvii. pp. 84, 181 ; 

 Jnrgensen, Deutschcs Arch. f. Jclin. Med., Leipzig, 1882, Bd. xxxi. S. 453. 



3 Hill and Barnard, Joum. Physiol., Cambridge and London, 1897, vol. xxi. p. 335. 



