ON THE ARTERIES. 349 



XXIV. 



ON THE ARTERIES. 



There is perhaps no finer proof of the adapta- 

 tion of the apparatus of circulation to the princi- 

 ples of hydraulics than the fact of the increasing 

 diameters of the arteries as they recede from the 

 heart. Mr. John Hunter took great pains to 

 prove this : and he did demonstrate that when a 

 great artery divided into tw^o branches, the united 

 areas of the branches were greater than the area 

 of the trunk ; that when the branches subdivided, 

 the united areas of the subdivisions were greater 

 than the areas of the vessels from which they were 

 derived, and so on to the extreme vessels. Re- 

 flecting on this, it is interesting to find that the 

 engineer in laying down pipes, comes practically 

 to the conclusion, that a pipe dividing into two 

 branches, whose united areas are exactly equal to 

 the area of that from which they proceed, will not 

 deliver the same quantity of water that would 

 have flowed through the greater tube. He dis- 

 covers that he must take into account the attrac- 

 tion and friction of the fluid upon the sohd, and 

 that the smaller the caliber of the tube, the sur- 

 face of attraction or friction will be proportionably 

 the greater. Does not this fact coming out in 

 31 



