52 THE CIRCULATION OF THE BLOOD 



Outside influences will cause the constrictors to act, as cold; 

 while alcohol will cause dilators to act and paralyzes the 

 constrictors. 



The chief vaso-motor center is in the medulla oblongata, 

 while subordinate centers exist in the cord. The vaso-motor 

 fibers reaching the vessels proceed from ganglia in the sympa- 

 thetic system, but these ganglia are influenced by the cells in the 

 vaso-motor center. 



FIG. 27. Capillaries. 



The outlines of the nucleated endothelial cells with the cement blackened by the 

 action of silver nitrate. (Landois.) 



Amount of Blood Important. When there are small losses 

 of blood from slight injuries the entire vascular system contracts 

 and the current supplying this diminished area is sufficient; 

 but at times the loss of blood is so great that the amount remain- 

 ing is not sufficient to carry on a complete circulation. Unless 

 remedied this results in death. In such cases of great loss the 

 deficit may be supplied by a normal salt solution, thus giving an 

 amount of fluid sufficient to maintain the heart action. But in 



