PHENOMENA. 9 



pcrienced by the current of blood. In tbe smaller vessels tbe 

 contraction is rapidly succeeded by dilatation, vrhich continues 

 for a longer or shorter period. When an artery is really in 

 action it gives rise to no hyperoemia ; on the contrary, the more 

 active the vessel, the less blood wlII pass through it. The con- 

 traction of the vessels is very irregulai, as will be seen from the 

 preceding engraving (Fig. 2). 



If contraction of the vessel be the first change observed, it is 

 succeeded by dilatation, and, as already stated, the streafia of 

 blood may become faster. Then, without any change being 

 observed in the size of the vessels, it becomes slower, oscillates, 

 and finally completely stagnant, constituting stasis or passive 

 congestion. 



The contraction of the vessels can be produced by the appli- 

 cation of a stimulus to the nerves supplying them, and the 

 relaxation by cutting these nerves.* From this we conclude — ' 

 and the fact is an important one — that relaxation of the mus-, 

 cular fibres of the vessels can be produced by paralysis of the 

 nerves that supply them, or by an interruption of the nervous 

 influence, from whatever cause it may proceed. That the capil-, 

 laries have the power of contraction has been demonstrated by 

 "Professor Lister, who has shown that this contractility is de- 

 pendent on fusiform (spindle-shaped) cells, which have the powei; 

 of shortening themselves, and which run transversely round the 

 delicate membrane that forms the walls of the capillaries. 



The stoppage of the blood (or stasis) has now to be accounted 

 for, and this is difficult of explanation, as no mechanical 

 obstruction has ever been seen to present itself. If there is nQ 

 obstruction, why should there be a stoppage ? 



Some pathologists have asserted that the vessels become 

 mechanically plugged up by the red corpuscles adhering to the 

 sides of them (Boerhave) ; by multiplication of the colourless 

 corpuscles, and by change in the specific gravity or Viscidity of 

 the blood in a part (Whabton Jones), or of the corpuscles in 

 particvdar (Bruecke) ; obstruction of the venous circulation (G. 

 Robinson); or .'wihesiveness of the blood corpuscles (Lister). 

 These opinions have been combated by Professor Bennett, who 

 says, "That the blood corpuscles indeed have a tendency to 

 aggregate together is certain, and I have frequently seen what 

 Mr. Lister has described, namely, a roll of them projecting from. 



