124 VETERINARY rATHOLOGV. 



capillaries. They occur more frequently in the heart and veins 

 because of the presence of valves. 



Varieties of Thrombi. — Thrombi may be classified on the 

 basis of color and extent; 



1. Color. Thrombi are variable in color according to their 

 structure and may be red, white or mixed. A thrombus formed 

 in a vessel in which there is complete stasis of blood will oc- 

 cupy practically the entire vessel-lumen and be red. On the 

 other hand a thrombus formed gradually by the deposition of 

 fibrin upon a roughened endothelial surface of a blood vessel 

 will be white. A mixed thrombus may be formed as a result 

 of blood-stasis in a vessel in which there was a white throm- 

 bus, or bv a red thrombus becoming partially detached from 

 the \-essel wall and contracting, thus allowing the blood to 

 pass through the partiallv obstructed vessel and depositing 

 fibrin (a white thr(inibus) upon the red thrombus. 



2. Extent. A thrombus may be complete or obstruutive, 

 i. e.. occup-s' the entire lumen of tlie vessels, or it may be partial. 

 A partial thrombus may be lateral, i. e., be found along one side 

 cif a vessel. It niav also be parietal, i. e., extend around the 

 entire lumen of a \-cssel. 



Extension of Thrombi. — A throm1)US may form as a plug 

 in a vessel rir nia\- e.xtcnd a considerable distance in the ves- 

 sel, the extension usually 1)cing in the direction of the bhiod 

 stream. Thus a Ijicuspid \-alve thrombus may extend, by 

 continued deposit, out into the posterior aorta until it has 

 reached the iliac arteries, or a thrombus arising in the metatar- 

 sal region mav extend up through the metatarsal and continu- 

 ing veins until it reaches the posterior vena cava. The exten- 

 sion mav be the result of direct growth or deposit upon the 

 original tlirombus or it may be the result of fragments becom- 

 ing detached (emboli) and floating in the blood stream until 

 thev arri\c at the junction of blood vessels too small to allow 

 them to pass and so form secondary thrombi or produce embol- 

 ism. 



Appearance of Thrombi.. — Macroscopic. — A red thrombus 

 appears similar tri a blood clot but is usually a little more dense. 

 It is red, jelly-like and quite easily broken and may be partial 

 or complete. K mixed thrombus is practically the same as the 

 red except in color. A white thrombus is usually a little more 

 brittle than the red, is yellowish white in color and if formed at 

 different peririds, strata mav be observed. 



Microscopic. A red thrombus is practically identical with a 

 blood clot, i. e., it is composed af fibrin in which white and red 



