174 APPLIED PHYSIOLOGY 



the action of the pump as an auxiliary in maintaining 

 the circulation. In cases of pericardial effusion the 

 heart, instead of being placed in a vacuum, is subjected 

 to a constant positive pressure which interferes greatly 

 with its proper filling; hence the large veins become 

 overdistended, whilst the output of the left ventricle 

 falls. 



Owing to the aspiration of blood into the chest at each 

 inspiration, a certain degree of ' pooling ' of it takes place 

 in the lungs at the commencement of the act, and in 

 consequence the output from the left heart is diminished 

 at first, and the arterial blood-pressure falls. So soon, 

 however, as the blood has had time to find its way round 

 to the left ventricle the output rises again above the 

 average, and the blood-pressure rises in proportion. If 

 there be any obstruction to the free entry of air into the 

 lungs, the amount of blood which can be accommodated 

 in the thorax is all the greater, and in such circumstances 

 the output from the left ventricle may fall so low that 

 the pulse disappears for a time. This result is all the 

 more likely to ensue if the peripheral arterioles be dilated, 

 so that blood can readily pass out of the arteries into 

 the veins i.e., in conditions of low arterial pressure. 

 Such a temporary disappearance of the pulse during 

 inspiration is spoken of clinically as the pulsus para- 

 downs. 



The Cerebral Circulation. 



The circulation in the brain is so peculiar, and at the 

 same time its disturbances are of such clinical importance, 

 that it demands a special description. The peculiarity 



