THROMBOSIS. 223 



Layer upon layer is deposited, and by this continuous coagulafAon^ 

 1st, a lateral thrombus may easily be converted into an obstruct- 

 ing one ; and 2nd, the coagulation may spread from one vessel 

 to its neighbours. In the latter event, the intruding clot will 

 at first be lateral, then obstructing, &c. The size and shape 

 which the thrombus may ultimately assume cannot be determined 

 a priori^ and can only be predicted with some degree of certainty 

 in a few cases, c.a. after lio;ature of arteries. AVith reference to 

 these we have the following law to guide us : the coagulation 

 never extends beyond the nearest pervious collateral branch, 

 whether in a peripheral or a central direction. The peripheral 

 coagulation is not as a rule of much moment ; since, owing to the 

 complete contraction of the arterial walls it usually forms a 

 slender and barely perceptible thread. 



§ 189. Wq may now proceed to consider the farther changes 

 which occur in thrombi. A few preliminary remarks are indis- 

 pensable however, about certain diiferences in their 'primary 

 structure. 3Iuch depends on whether a thrombus has been 

 formed rapidly, by some sudden accident, as e.g. when a vessel 

 is tied and a certain quantity of blood, suddenly cut off from the 

 current, coagulates forthwith ; or whether it has been produced 

 more gradually. In the former case the coloT:rless and red 

 corpuscles are so equally distributed throughout the clot, that 

 on examinino; sections of it we find the colourless elements 

 distributed at regular intervals throuo^hout the mass of red ones. 

 It is far otherwise wdiere the coagulation begins and goes on 

 gradually and slowly. As examples of this latter mode, we have 

 the coagulation which begins in the superficial vessels of an 

 amputated stump affected by diphtheritic inflammation, and 

 which gradually spreads upwards into the veins ; the thrombosis 

 which occurs in the left auricle in cases of mitral contraction ; 

 thrombosis in varicose veins and aneurisms. In all these cases, 

 with which we shall become more familiar when we study the 

 diseases of the vessels, what is known as the "viscosity'' of the 

 colourless corpuscles comes into play, giving rise to a concentric 

 lamination of the clot. The colourless corpuscles are glutinous, 

 i.e. their protoplasm tends to adhere to solid bodies, to penetrate 

 into their pores. Most of all does it incline to unite with the 

 protoplasm of other leucocytes ; so in the comparatively sluggish 

 current of the veins we not unfrequently find the white blood- 



