468 MODERN DEVELOPMENTS OF HARVEY's WORK. 



gata. This center acts most readily upon the vessels of the intestine, 

 and rather less readily on those of the skin. In consequence of this, 

 when the center 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 stimulati<m 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 ques- 

 tion was put by Ludwig, and answered by. the experiments which 

 he made with Haliz. 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 vasomotor centers in the medulla the fiill of blood 

 pressure would either not take place at all or would be very much 

 slower than before; but on trying tlie experiment, Ludwig and Ilafiz 

 found to their surprise that the blood pressure fell almost as quickly as 

 when the vasomotor center 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 tliat 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, intestine, and muscles together. This obser- 

 vation, it seems to me, is one of the greatest importance, and one that 

 has hardly received as yet the attention which it merits. 



It is obvious that contraction of the cutaneous vessels, such as occurs 

 upon exposure to cold, will drive more blood through the muscles, and 

 as oxidation goes on more rapidly in them the result will be increased 

 production of heat. 



The experiments I have just nicntioued show that the vessels of the 

 muscles are not controlled by the vasomotor center in the medulla 

 oblongata in the same way as those of the intestine and skin. IIow fsir 

 their vascular centers may be associated with those for voluntary move- 

 ments, which have been so admirably localized by Terrier in the cerebral 

 cortex, still remains to be made out. The circulation through the mus- 

 cles is indeed a complex phenomenon, and it was shown by Ludwig and 

 Sadler to depend upon at least two factors having an antagonistic action. 

 When a muscle is thrown into action, it mechanically compresses the 

 blood vessels within it, and thus tends to lessen the circulation through 

 it, but at the same time the stimulus which is sent down through the 

 motor nerve, and which calls it into action, brings about a dilatation 

 of the vascular walls, and thus increases the circulation through the 

 muscle. 



When the amount of blood is measured before, during, and after 

 stimulation of its motor nerves, it is sometimes found that the flow is 



