CUTICLE, NAILS, HAIR AND LEATHERS. 27§ 



adheres to the parietes. This attraction is not, in- 

 deed, incompatible with a relative motion among 

 the particles. A drop of fluid adhering by capil- 

 lary attraction to the end of a glass rod often ex- 

 hibits intestine motions from the admixture of mi- 

 nute particles of dust, or from containing animalcu- 

 la. But it retards and restrains such motions ; and 

 there undoubtedly is a minute distance from the 

 surface of the solid, at which motion becomes im- 

 possible. The motions of fluids encounter greatest 

 impediments from this cause in the smallest vessels 

 of any animal. This circumstance counteracts the 

 effect of the more ample space provided for the 

 blood in that set of vessels, the sum of the areas of 

 which always much exceeds that of the trunks and 

 larger branches. Capillary adhesion contracts that 

 part of the calibre in which the motion is free, and 

 must always bear the greatest proportion where the 

 calibre itself is smallest. Hence the motions of the 

 blood form only a minute stream in the axes of these 

 vessels. There is unquestionably a point of minute- 

 ness at which the practicability of circulation ceases ; 

 and nothing except the want of attention to this 

 consideration, or the absence of any other hypothesis, 

 could have led physiologists .to believe in the exis- 

 tence of a circulation in situations in which neither 

 vessels nor fluids can be discovered. 



It has been supposed, that a circulation is con- 

 ducted in the minutest animals, exactly similar to 



