126 MANUAL OF HISTOLOGY. 



members the rouleaux, in that the several cells swell out and assume the 

 spherical form, and thus again separate from one another. On this account 

 the roundish corpuscles of the blood of the hepatic and splenic vein show 

 no such columnar grouping. 



The cause of this formation of columns is still unknown. The explana- 

 tion of the phenomenon through the adhesiveness of the intercellular 

 fluid or surfaces of the cells does not suffice. 



At all events, it favours the descent of the coloured cells essentially, 

 for the little structures thus united must be able to overcome better than 

 when isolated the resistance offered to their gravitation by the fluid. 

 If rouleaux have once been formed, the same settling down makes itself 

 again rapidly evident in blood which has been re -agitated. 



REMARKS. It is a striking fact that, on the addition of anything which renders 

 the intercellular substance more dense, as, for instance, of concentrated solution of 

 sugar, the settling down of the blood-cells is accelerated, although just the contrary 

 might be expected. 



79. 



Coagulation of the Hood. The consistence of the blood begins very 

 rapidly to change a few minutes after it has been obtained from the 

 vessels it coagulates, namely. This process commences much more 

 slowly within the vessels of the corpse, or in sanguineous effusions in 

 the interior of the living body. The latter may preserve their original 

 consistence for many weeks. 



Now, as regards the phenomenon itself first of all, we remark the com- 

 mencement of this change in blood taken from the living body in from 

 two to five minutes. The first step in the process is the formation of a 

 thin pellicle of the greatest delicacy on the surface of the fluid, which 

 soon acquires greater thickness and solidity, so that it may be at length 

 lifted off with- the point of a needle. 



Commencing thus on the surface of the fluid, this formation of mem- 

 brane extends itself gradually along the sides and down to the bottom of 

 the vessel in fact, at every point at which our sample of blood comes in 

 contact with the latter. The consistence of the blood so enclosed then 

 begins to change ; it becomes firstly somewhat thickish, like a half-cooled 

 solution of glue, attaining not long after the consistence of stiff jelly, or 

 of a saturated cold solution of glue. Then, at the end of from seven to 

 fourteen minutes the blood has lost all its fluidity, and has been trans- 

 formed into a thoroughly solid mass, whose form is determined by that 

 of the vessel in which it is contained. 



This is, however, by no means the end of the process. The solid jelly, 

 overcoming the adhesion to the walls of the vessel, contracts subsequently 

 more and more, pressing out a part of the fluid which has been entangled 

 in it by the coagulation. The commencement of this contraction takes 

 place tolerably early, but it only reaches its termination after a compara- 

 tively long period, ranging from twelve to forty-eight hours. At first 

 there appear on the surface of the coagulum a few drops of a transparent 

 fluid ; the number of these soon increases, upon which they coalesce, form- 

 ing larger drops, and at last run together into a layer of fluid which covers 

 the surface of the coagulated mass. Whilst the coagulum thus pro- 

 gressively contracts to a smaller volume, similar layers of fluid to that 

 on the surface collect under the latter, as well as along the edges and 

 floor of the vessel, until the mass which at first adhered closely to the 



