TISSUES OF THE BODY. 



327 



cup ; so that the latter could "be turned upside-down without its falling 

 out, commences to float in the expressed liquid. 



From this on, the process only undergoes a quantitative alteration that 

 is, a continuous contraction of the lump causes it to decrease more and 

 more in size, at the same time that an ever-increasing quantity of fluid 

 is being pressed out of its interstices. When the whole process is at an 

 end, we have a larger or smaller coagulum, sometimes soft and sometimes 

 hard, floating in a varying amount of transparent fluid, which has, like 

 the plasma, a slight yellowish tint. The coagulated mass having con- 

 tracted uniformly, preserves the figure of the vessel, and forms a diminu- 

 tive cast of the same, appearing in an ordinary porcelain basin plano- 

 convex, and in a test-tube cylindrical. Its colour is that of the blood, 

 of a darker red, however, at the lower and internal portions than on the 

 surface, where it is light. 



This red lump has received the name of the crassamentum or placenta 

 sanguinis, while the fluid in which it swims is known as the serum, or 

 serum sanguinis. 



Now, how are these two portions of the coagulated blood related to 

 that which circulates in the living body, to its cells and intercellular 

 substance ? 



We must remember, in the first place, that the latter is a fluid con- 

 taining the two constituents of fibrin in solution. And as in other cases, 

 so also after withdrawal of the blood from the system, these combine to 

 form coagulating fibrin, by which, in that the fibrinogen is. sufficient, 

 the whole fluid, together with its 

 cells, is entangled by the solidify- 

 ing mass ; just as a solution of 

 glue retains, on cooling, any par- 

 ticles which may have been sus- 

 pended in it to make use again 

 of this ordinary simile. By the 

 progressive contraction of the 

 gelatinous mass, a part of this now 

 defibrinated intercellular fluid is 

 expressed in ever-increasing pro- 

 portion from its meshes, whilst the 

 blood-cells remain behind en- 

 tangled. From this we see that the 

 liquor sanguinis consists of inter- 

 cellular fluid deprived of its fibrin, 

 or is, in other words, defibrinated 



plasma. The crassamentum must Fig. 123. Human blood-cells ; coagulated fibrin at <?, 



consequently be formed of the with included corpuscles, 



blood-corpuscles entangled in coagulated fibrin. And, in fact, microscopical 

 examination of thin sections of the placenta sanguinis shows us the 

 unchanged cells embedded in a homogenous, fibrinous, or plaited sub- 

 stance (fig. 123,cZ). 



Of course a more or less considerable quantity of the intercellular fluid 

 must still remain entangled in the cake of blood. 



According to what has just been remarked, serum shares with the 

 plasma its transparency, light yellowish colour, and chemical characters. 

 Its specific gravity must, however, be somewhat lower, and may be 

 stated at between 1-026 and T029. It not unfrequently happens that a 



