DILATATION OF A'ENOUS SYSTEM. 481 



the heart, however numerous and however vipjorous, can send a 

 drop of blood into the aorta. 



It must be recollected that blood is only useful to the tissues 

 when it is in the arteries, just as water is only available for 

 household purposes while it is in the cistern or supply pipes. 

 Once the water gets into the sewer it is of no more use, unless 

 it can be filtered and again pumped back into the cistern ; and 

 once the blood has got into the veins, it is no more use unless 

 it can be purified by the lungs and pumped back into the aorta, 

 from which it may once again pass to nourish the tissues. The 

 only difference between blood still in the veins and blood which 

 has run out of them into a basin, is that, when in the basin, it 

 cannot get to the heart, and be pumped by it into the arteries ; 

 while, so long as it is in the veins, it generally reaches the heart 

 readily. But althougli it generally does reach the heart easily, 

 it does not always do so. Sometimes it accumulates in the veins 

 of the abdomen, and never reaches the auricle ; so that it might 

 just as well be in a basin for any use it is to the lieart or body. 

 This was shown by Professor Goltz, of Strassburg, by a remark- 

 able experiment. After exposing the heart of a frog he noticed 

 that it beat regularly, and at each beat sent a quantity of blood 

 into the aorta, again becoming full of blood in the interval 

 between the pulsations. The frog's heart is partially trans- 

 parent ; so that it is easy to see by its colour when it is empty 

 and when it contains blood. He now struck the frog's intestines 

 pretty hard, and found that the heart stopped. The irritation 

 produced by the blow had been conducted up to the medulla 

 oblongata ; and, being reflected down the vagus nerves to the 

 heart, had stopped it. After a little while, the heart seemed to 

 recover, and began to pulsate again. But there was a very 

 remarkable difference between its appearance now and its 

 appearance before the blow had been given. Instead of becom- 

 ing filled with blood during each diastole, and assuming a deep 

 red colour in consequence, it remained quite pale and empty ; 

 and, although it contracted vigorously, the circulation stopped, 

 for the heart had no blood to propel. On looking at the vena 

 cava, Goltz found the cause of this phenomenon. The frog was 

 hanging with its legs downwards, and the vena cava was not 



