CIRCULAR DISTURllANCKS. 123 



point of the injection, but the thrombi resulting therefrom are 

 usually of little consequence. 



Etiology. — The coagulation of blood is a complex chemi- 

 cal reaction and is not thoroughly understood. From the various 

 investigations it may be concluded that three factors are essential 

 in the coagulation of blood. 1st. There must be soluble albumins 

 from which fibrin is derived. 2nd. Fibrin ferment ( fibri.'iogen) 

 which is probably derived from leucocytes and blood plates. ( It 

 is probable that vascular endothelium may also liberate fibrinogen.) 

 3rd. Soluble lime salts. The following are the most frequent im- 

 mediate causes of thrombic formation. 



1. Injury of the vascular cndothcliuiii which may be caused 

 by: (A) Mechanical interference, as torsion bv artery forceps. 

 or ligation: (B) Extension of disease from other pcirtions of 

 vessel walls, as atheromatous degeneration: (C) Insufficient 

 nutrition the result of passive hyperemia: (U) I'lireign liodies. 

 Thrombic formation succeeding injury of the vascular endo- 

 thelium is apparently a reaction on the part of the injured 

 cells for protection and to prevent hemorrhage until the wound 

 is repaired. Many thrombi are thus formed and later removed 

 by phagocytes, without causing sufficient inconvenience to be 

 clinically recognized. 



2. Retarded flo7v of hlood which may be caused by a weak 

 heart or the relaxation of the blood vessels especially the veins. 

 Frequently the so called "ante-mortem clot", which is a throm- 

 bus, is observed in post-mortem examination of horses that 

 have died of pneumonia, pleurisy, peritonitis and other ex- 

 haustive diseases that are accompanied by a weak heart 

 action. 



3. Variations in the composition of blood, as increased number 

 of platelets, which increase its coagulability, (probably because of 

 the production of fibrinogen), contamination with bacteria, etc. 



Process of Formation. — The process of formation varies ac- 

 cording to the kind of thrombus formed. A red thrombus is 

 formed when there is vascular obstruction and it is formed be- 

 cause of the coagulation of the blood contained in the obstructed 

 vessel. Coagulation in thrombic formation does not differ from 

 extravascular coagulation. White thrombi are formed as a result 

 of leucocytic adhesion, and the deposition of the fibrin from the 

 blood plasma on an injured internal vascular surface. The leu- 

 cocytes and fibrin mav continue to accumulate until the vessel 

 is obstructed. 



Location.— Thrombi form in the heart, veins, arteries, and 



