196 STUDIES IN ADVANCED PHYSIOLOGY. 



muscles freed from their regular control relax and expand 

 to their natural dimensions. 



Second: Different afferent nerves may inhibit it. When 

 food enters the stomach the sensory impulses reaching the 

 brain from the stomach inhibit that part of the vaso-con- 

 strictor center which governs the gastric arteries, and these 

 removed from the tonic stimulus to remain contracted di- 

 late, and so the mucous gastric membrane becomes red and 

 flushed with blood. 



Third: This vaso-constrictor center is very energetic- 

 ally inhibited by impulses reaching it from the depressor 

 nerves of the heart. As stated in a former paragraph, this 

 tierve is probably normally stimulated when the pressure of 

 the blood in the heart (and so, of course, elsewhere), be- 

 comes too great. Such impulses on reaching the vaso- 

 constrictor center in the medulla forcibly inhibit it, and an 

 immediate dilatation of arteries results, the consequence 

 of which is that the blood pressure at once sinks. Such a 

 general sinking of blood pressure is usually accomplished 

 by having that part of the constrictor center inhibited which 

 governs the abdominal viscera, and which at once arrests 

 the 'tonic action of the splanchnic nerve. As a result of 

 this the many large and small arteries through stomach 

 and intestines, etc., enlarge, the blood streams through, 

 and the arterial pressure is relieved. On the other hand, 

 to stimulate the splanchnic nerve, that is, to make this tonic 

 action stronger and so produce an increased contraction, 

 may diminish the size of the abdominal blood-vessels to 

 such an extent as to press out of them as much as twenty- 

 seven per cent, of their contained blood supply. The ef- 

 ficiency of and the necessity for such a nicely regulated 

 control of the blood supply for the various organs is, 

 of course, quite apparent. 



Under some circumstances this constrictor center may 

 be actually stimulated. Thus, turning pale with fright is 

 due to the flurried stimulation of its activity. 



