292 VETERINARY PHYSIOLOGY 



ments of the thorax are increased in the panting which accom- 

 panies it, the blood is partly pressed, and partly sucked from 

 the abdomen into the heart, and so forced on into the arteries. 

 Even expiration helps in this, for the blood which has filled 

 the vessels of the lungs in inspiration is driven on into the 

 left side of the heart in expiration. The blood is thus forced 

 on into the arteries and so to the muscles, and they, by their 

 alternate contraction and relaxation, further help to drive it 

 on to the veins where the valves prevent any back flow during 

 relaxation and thus accelerate the circulation. The high 

 arterial tension thus produced tends to drive the blood through 

 the cranial vessels. The benefit of intermittent muscular 

 exercise on the circulation is thus manifest. 



When, on the other hand, some sustained muscular strain 

 has to be undergone, the thorax is fixed, and hence (1st) the 

 pressure on the heart and thoracic organs is raised, and the 

 increased pressure in the thorax helps to support the heart 

 and to prevent over-distension. The abdominal vessels are 

 also pressed upon, and the sustained contraction of the limb 

 muscles tends to prevent the blood flowing through them. 

 It is thus 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. (2nd) But, if the effort is still further 

 sustained, the high intra-thoracic pressure tends to prevent 

 proper diastolic filling of the heart; blood is therefore not 

 sent on from the veins into the arteries, the veins become 

 congested and the arterial pressure falls, less blood goes to 

 the brain, and thus fainting may result. 



In the " head down position," the accumulation of blood in 

 the dependent parts is prevented in the head by the vessels 

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

 heart by the supporting pericardium. 



In man the position of the abdominal reservoir of blood 

 at a lower level than the heart increases the work of that 

 organ. In the horizontal position, when the reservoir is on the 

 same level as the pump, the work is much easier. 



Fainting is a sudden loss of consciousness produced by 

 failure in the supply of blood to the brain. It is accompanied 



