PHYSIOLOGY IN THE STUDY OF DISEASE 91 



in its power of working or to a deficient supply of 

 blood reaching it. That its power of action is not 

 impaired is shown by the fact that if more fluid 

 is supplied to it, it can respond at once by increased 

 activity. Hence we conclude that the low pressure 

 is not due to failure of the heart. 



Is it due to a relaxation of the little arteries ? 

 These are controlled by the nervous system so that 

 if the pressure in the arteries falls they contract 

 and thus keep up the head of blood so essential 

 for the circulation. Has this mechanism got out of 

 action ? It is very probable that, in the early 

 shock or fainting which may immediately follow 

 a wound, it is thrown out of action, but in these 

 later cases no such failure seems to exist. 



Why then is there this fall in the pressure ? In 

 some cases it is due to loss of blood from wounds, 

 but in many others the loss of blood has been small, 

 and we know from physiological experiments that 

 such a loss is soon made good by the passage of 

 fluid from the tissues to the blood. Of course there 

 might be some interference with this passage of 

 fluid, but that this is not so is indicated by the 

 result obtained on administering fluid which passes 

 into the vessels and raises the pressure for a time. 

 The only possibility left then seems to be that 

 a considerable amount of blood has gone out of 

 effective circulation, that it must have got side- 

 tracked and stored up somewhere. 



