BLOOD VESSELS 471 



In sustained muscular strain the thorax is fixed, and 

 hence, (a) at first (1) the pressure on the heart and thoracic 

 ortJ^ans is raised, and the increased pressure in the thorax 

 helps to support the heart and to prevent over-distension. 

 (2) The rigid thorax prevents the blood being sucked into 

 the heart by the respiratory movements. (3) The abdominal 

 vessels are pressed upon by the contraction of the abdominal 

 muscles, and the blood is pressed on to the heart, while the 

 sustained contraction of the limb muscles tends to prevent 

 the free flow of blood through the capillaries. 



Arterial pressure is thus raised, and the blood is forced 

 to the central nervous system in which the pressure rises, 

 and, if a weak spot in the vessels is present, rupture is 

 apt to occur. 



(b) Later, if the strain is still further sustained, the high 

 intra-thoracic pressure tends to prevent proper diastolic 

 filling of the heart, and the pumping action of respiration is 

 in abeyance. The abdominal vessels being pressed upon 

 prevents the free flow of blood through them to the heart, 

 and the venous inflow fails and the force of contraction of 

 the heart decreases. Thus, less blood is sent to the arteries 

 and the arterial pressure falls, less blood goes to the brain, 

 and fainting may result (see below). 



E. INFLUENCE OF POSTURE ON CIRCULATION. 



In the "head down" position, as in the horse in drinking, 

 the accumulation of blood in the head is prevented by the 

 vessels being packed inside the skull, and in the right side 

 of the heart by the supporting pericardium. 



In man, in the erect posture, the position of the abdominal 

 reservoir of blood at a lower level than the heart increases 

 the work of that organ. Especially is this the case with 

 animals, in which the abdominal wall is lax, so that the blood 

 can accumulate in the abdominal vessels, e.g. rabbits bred in 

 confinement. In these, failure of the heart or fainting may 

 occur when they are placed in the " head up " position. In 

 the normal position of quadrupeds the work is much easier, 

 for the reservoir is on the same level as the pump. 



