112 Disturbances of Ci)'Ciilatio)i. 



were thrown into tetanic contraction. (Internal organs like the 

 brain or intestine may tlms be relieved of excess of blood by 

 muscular movements.) When the stomach and intestine are filled 

 with food during digestion they are much richer in blood than 

 when empty. The influences which give rise to this stimulation 

 are variable and multiple, acting sometimes directly upon the 

 muscular coats of the blood vessel wall and the nervous apparatus 

 inherent in the wall, sometimes upon the vascular centre in the 

 medulla oblongata (mechanical, thermic, electric, chemical irri- 

 tants). 



It is but a stc]) from the physiological to the pathological, and 

 there is no sharp line of differentiation ; change in the amount 

 of blood in a part becomes a pathological condition when it occurs 

 in an improper place, . or at a wrong time, or if it exceeds the 

 normal variations. 



Increase in the quantity of blood in a part of the system, local 

 engorgement, hypcrcoiiia, may be of one or other of two types, 

 the result either of an excessive influx of blood to the part, or of 

 a diminished escape of blood from the part. 



The first form, the basic feature of which consists of excessive 

 entrance of arterial blood into the tissues, is spoken of as active 

 hyperaemia, or, synonymously, as irrifafion-hyperccmia, congestive 

 hypcrccmia^ fluxion of blood or arterial hypcrcvnua. Increased 

 supplv of arterial blood may be caused by excessive blood pressure. 

 This is met where an artery is occluded or some part of the body is 

 for some reason prevented from receiving its proper supply : under 

 which circumstances the blood propelled from the heart passes 

 into the adjacent structures {collateral hypcrccmia), that is it 

 rushes into the pervious branches of the artery near the obstruction, 

 under the influence of the higher pressure. As a rule, however, 

 instead of the blood being forced into the part in question, there is a 

 lowering of the vascular tension, spoken of as a relaxation in 

 the arterial distribution (relaxation-hyperccmia), with a resultant 

 widening of the channels. The blood freely pours into the dilated 

 arterial branches and capillaries, and with increased force ; in a 

 w^ord it is drawn into the part. The degree of relaxation depends 

 largely upon the lack of tone of the smooth muscle of the vascular 

 wall and the congestion is therefore in such instance a myoparalytic 

 one. Every muscle being inseparable from its innervation, nervous 

 influences must finally be regarded as responsible for changes in 

 the vascular tension (tone). It is assumed that the vascular 



