198 ON THE NATURE AND PRINCIPLES OF 



the other contractile structures of the body ; and, 

 though the vascular tunics, being subservient to 

 functions different from those of the higher forms of 

 contractile tissue, do undoubtedly exhibit a corre- 

 sponding peculiarity in their action, it does appear 

 to me that the pathological distinction which now 

 separates these varieties of the same vital structure 

 has been too hastily assumed, and has never been 

 satisfactorily established by facts or reasoning. For 

 the explanation of the phenomena now under con- 

 sideration, it is not necessary to claim for the minute 

 vessels a liability to actual spasm ; it will suffice for 

 my present purpose that they be allowed to possess, 

 in common with every other contractile tube or 

 cavity, the power or property of resisting any attempt 

 at their sudden and forcible dilatation ; and it must 

 remain for those who are inclined to dispute this 

 proposition to adduce their reasons for considering 

 these vessels as exempt from the operation of a 

 general and well-established law.* This principle, 

 then, operating in the contractile capillaries, as in all 



* Many surgical operations demonstrate the necessity of attending 

 to the influence of time in modifying the action of dilating or ex- 

 tending forces upon contractile tissue. Thus, in the reduction of a 

 dislocated limb, or in the injection of the bladder, or any other 

 muscular cavity, it is found that while a slowly-applied elongating 

 power acts without experiencing any resistance from the contractility 

 of the yielding tissues, the abrupt application of a similar force 

 becomes, in fact, </ mechanical stimuli/ \, and, as such, instantly causes 

 an extreme contraction of the muscular structures. It is from the 

 operation of a similar exciting cause that the capillaries, under the 

 circumstances above mentioned, resist the influx of the aru-ria 



