22 THEORY AND PRACTICE 



turition approaches, the mammary glands become congested in 

 order to stimulate the cells to secrete the milk. Otherwise 

 agalactia would be the result. 



The causes of active congestion are: 



1. Increased blood pressure. 



2. Diminished arterial resistance. 



The Brst is due to an obstruction in one part causing an in- 

 creased volume of blood to another, thus raising the pressure 

 in the vessels. At first their walls remain practically of the 

 same diameter, but later on become dilated by the increased 

 volume of blood. 



The second is caused by the nerves ; the vaso-constrictors 

 become depressed and as a result the arterioles dilate. 



Whatever the irritating factor causing congestion, the imme- 

 diate effect of it is a spasmodic contraction of the blood vessels. 

 Dilatation then follows ; the red blood corpuscles pile up in the 

 enlarged lumen of the blood vessel, forming rouleaux, while the 

 leucocytes take up their position along the vessel wall to which 

 they apply themselves very closely. By means of their ameboid 

 movements and on account of the increased pressure they work 

 their way through the vessel into the surrounding tissues. At 

 the same time a considerable amount of blood plasma has exuded 

 into the. tissues. The blood current has slowed down so that 

 there is almost complete stasis, the blood is thicker and more 

 viscid, and the red blood corpuscles may become pressed out into 

 the tissues either by diapedesis or rhexis. 



In hemorrhage by diapedesis the blood passes out into the 

 tissues through the pores of the vessels, — the plasma, the leu- 

 cocytes, and some few red blood corpuscles. In hemorrhage by 

 rhexis the pressure is so great that a rupture occurs in the vessel 

 wall, and the whole number of the blood elements passes through 

 the rupture. As a result we get an extravasation of blood, a 

 patch of tissue outside a vessel infiltrated with blood. Oedema 

 is produced by hemorrhage by diapedesis. 



If the congestion in the vessel is severe enough to plug it, 

 an infarction may result. The stasis of blood in the vessel cuts 

 off the nutrition from the surroundins: tissues and thev die. This 

 is most apt to occur where there is no collateral circulation. 



