TISSUES OF THE BODY. 129 



we see that very soft fragile clots may subsequently become again fluid. 

 Finally, in some portions of the system the blood does not undergo this 

 change at all, as, for instance, in the hepatic vein, and also possibly the 

 menstrual blood of the female (p. 121). The blood of persons struck by 

 lightning, and of those dead of asphyxia, has been found fluid in toto. 



If in the moment of coagulation the coloured cells have already disap- 

 peared from the uppermost layers of the fluid, the coagulum does not 

 present the usual red colour, but is yellowish white in its superior por- 

 tion, then known as the crusta phlogistica s., wflammatoria. Micro- 

 scopic examination of the latter shows the absence of red corpuscles in the 

 coagulated fibrin, and, on the other hand, the lymph-corpuscles which are 

 specifically lighter imbedded in the lower part of this light coloured 

 stratum. And in that a quantity of cells usually hinders the contraction 

 of the fibrin, the latter shrinks with much more energy in this uppermost 

 layer, which is poor in the former, than in the parts of the cake which 

 are of a deeper red. This explains the fact, that the crusta phlogistica 

 generally forms a concave disk depressed in the centre, and smaller than 

 the red part of the placenta lying underneath it. 



This buffed portion then is produced, on the one hand, by the more than 

 usually rapid gravitation of the red cells, or on the other by delay in the 

 coagulation of the fibrin. Thus we meet it as a normal appearance in the 

 blood of horses. It is met with frequently, likewise, in the human being 

 as a pathological phenomenon, and especially during inflammatory dis- 

 eases of the respiratory apparatus; but also under more normal conditions, 

 as, for instance, in the blood of pregnant women. 



Owing to our ignorance in regard to the protein substances, this pheno- 

 menon of coagulation cannot be at present explained. Since the earliest 

 days of medicine, however, there has been naturally no lack of efforts to 

 do so. The cooling of the mass of blood, its coming to a state of rest, 

 or the action of oxygen on it, have all been looked upon as the causes of 

 the process. Eecently Briiclte has entered the lists in defence of an old 

 theory formerly held by A. Cooper and Thackrah, namely, that the blood 

 is retained in a fluid state by contact with the internal surface of the 

 living heart and blood-vessels. A. Schmidt also ascribes to these surfaces 

 the property of retarding coagulation. 



This is the present (but let us hope temporary) state of our knowledge 

 on this point. 



81. 



If we now ask, at the conclusion of this long inquiry into the nature of 

 the blood, How much is known at the present day of the conditions 

 during life of its two species of cells? we must allow that the results of 

 all research so far are but very unsatisfactory. 



The red cells are the vehicles for the oxygen of respiration, and appear 

 to generate haemoglobin within them, and to contain fibrinoplastic matter. 



The physiological destruction of these cells takes place, firstly, in the 

 blood passing through the vessels of the liver, taking a part there in 

 the production of the bile, as is indicated by the solvent power of the 

 alkaline salts of the biliary acids (p. 108), as also the near relationship 

 between haemotoidin ( 35) and bilirubine ( 37). 



Again, we meet with another species of decay of the blood-cells in the 

 more quiescent blood of the spleen, where they form small aggregations, 

 which are transformed gradually into dark pigmentary masses. Forced 



