VESSELS IX MUSCLES. 13 



Uhi stimulus ihi affliixus is an old doctrine and expresses a 

 great truth. Wherever the need for increased nourishment or 

 increased supply of oxygen exists in the healthy body, thither 

 does the blood flow in larger quantities than usual. If the 

 glands are active, their blood supply is greatly increased, as was 

 shown by Bernard, and a similar occurrence takes place in the con- 

 tracting muscle, as lias been shown by Ludwig and his scholars. 

 The vessels of the intestines and skin, as well as their numerous 

 glands, have their calibre regulated by the vaso-motor nerves 

 which proceed from the centre in the medulla oblongata. This 

 centre acts most readily upon the vessels of the intestine, and 

 rather less readily on those of the skin. In consequence of 

 this, when the centre is irritated, the vessels of the intestine 

 contract and drive the blood through the skin, so that it is 

 warmer than before, and it is only when the stimulation is very 

 great that the vessels of both contract, so that the skin receives 

 less blood than normal, and becomes colder than before. But 

 if the vessels of the skin and intestine are both contracted, 

 where does the blood go ? This question was put by Ludwig, 

 and answered by the experiments which he made with Hafiz. 

 It is evident that if the heart be stopped while the blood 

 pressure is being measured in the artery of an animal, the 

 pressure will fall regularly and steadily, because the blood is 

 flowing out all the time through the arterioles and capillaries 

 into the veins. One would naturally expect that if the arterioles 

 were contracted by irritation of the vaso-motor centre in the 

 medulla, the fall of blood pressure would either not take place 

 at all, or would be very much slower than before ; but on trying 

 the experiment, Ludwig and Hafiz found, to their surprise, that 

 the blood pressure fell almost as quickly as when the vaso- 

 motor centre was left alone, and the vessels of the skin and 

 intestine therefore remained uncontracted. In other words, the 

 vessels which supply the muscles of the body and limbs are 

 capable of such extension that when fully dilated they will 

 allow the arterial blood to pour through them alone nearly as 

 quickly as it usually does through the vessels of the skin, intes- 

 tine and muscles together. This observation, it seems to me. 



