68 



FIXATION 



coagulates. Mercuric chloride is convenient in this respect, for it 

 does not greatly alter the volume of mammalian liver (see p. 76), 

 The results with this fixative are shown in fig. 7. The approxima- 

 tion of the graph to a straight line is remarkably close. It is clear 

 that the rate of penetration falls oiT with time exactly as it does 

 when fixatives enter clotted blood-serum or gelatine/albumin gel. 

 The rate of entry is, however, considerably slower. Thus mercuric 



FIG. 7. Graph showing the thickness of rabbit-liver fixed by 

 a saturated aqueous solution of mercuric chloride in various 



times. ^^ 



chloride penetrates 4-2 mm into liver in 25 hours (K = 0-84), but 

 1 1 mm into gelatine/albumin gel (K = 2-2). A survey of the work 

 done in this field shows that the rate of penetration of coagulant 

 fixatives into gelatine/albumin gels is in general agreement with 

 their rate of penetration into tissues, except that the latter is much 

 slower. Picric acid, however, goes more slowly than chromium 

 trioxide into the gel, but more quickly than the trioxide into liver 

 (fig. 5, a). (The reader will remember that in all such experiments, 

 the concentrations of the fixatives were those listed on p. 24; see 

 p. 32.) It is not obvious why fixatives should go so much more 

 slowly into tissues than into the gel, for the latter contains about 

 the same concentration of protein as ordinary protoplasm (p. 33). 



