THE CIRCULATION OF THE BLOOD. 269 



and laid on the ground, and after it ceased to beat it was 

 blown up and ordered to be dried. It was then hung up 

 when it began to move again and continued to beat, though 

 more slowly, for ten hours ; and it continued to contract till 

 it became so dry as to rustle iffith the motion. If the heart 

 of any animal be taken from the body immediately after 

 death and carefully washed, it will continue to act for some 

 time ; showing that this alternation of action is natural to 

 its irritable fibre, and results directly from its structure. 



33. The arteries have not an equal power of contraction, 

 with the heart, though they are generally found more or less 

 contracted after death. The pulse, which may be felt by 

 placing the fingers on the side of the wrist, takes place at 

 the very instant the heart contracts, and is not probably 

 owing to the contraction or dilitation of the artery, but 

 chiefly to the jet of blood, which is sent along the tube. 

 When arteries are changed into bone, the pulse is still felt. 

 No pulse exists in animals destitute of a heart. 



34. That the arteries are not only elastic, but contract so 

 as to assist the heart in circulating the blood, is evident from 

 the following facts. If an artery be laid bare, and two lig- 

 atures applied so as to cut off all communication, and then 

 a small opening made between the ligatures, the blood will 

 spirt out with considerable force, and the artery become 

 much contracted. When a person or animal is bleeding to 

 death, the arteries always contract in proportion to the loss 

 of blood ; after death they relax again. Arteries too will 

 contract by the application of stimulants or irritating sub- 

 stances. 



35. Besides these facts to prove that arteries contract, 

 We may mention the following. We read of cases of pal- 

 sy, where in the paralytic limb no pulse could be felt, al- 

 though the heart beat as strongly as ever. We read of oth- 

 er cases where the arteries continued to beat after the pul- 

 sations of the heart ceased. In diseases of the heart, we 

 sometimes have a weak pulse, although the heart beats very 



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