22 Proceedings of Royal Society of Edinburgh. [sess. 
It is advisable to apply both of these reagents for the detection 
of iron in the splenic elements ; for although the Prussian blue 
method yields the more brilliant results, yet there are some 
structures which give with it a doubtful reaction, and yet, when 
treated with ammonium sulphide and no counterstain applied, are 
seen to be distinctly darkened. 
Up to the present time these observations have been made only 
on the spleens of sheep and of rabbits, and it may be stated that 
iron-containing elements have been observed in every case. The 
amount of iron present and its histological position vary, however, 
a good deal in different animals. By the position of the iron is 
meant whether it is intra- or extra-cellular, both of which con- 
ditions are to be observed, and both usually side by side in the 
same section. 
I. Of the extra-cellular forms there are several varieties. 
(a) Small globules lying free in the spaces of the spleen pulp. 
They vary in size from very minute points to globules approaching, 
and in rare cases even exceeding, in size that of the red blood 
corpuscles. Some of the larger globules seem to be formed by 
coalescence of the smaller ones, and in all cases the coloration 
both with potassium ferrocyanide and ammonium sulphide is of 
great intensity. 
(b) It is quite common to find in sections of spleen dark brown 
masses which frequently appear crystalline. These seem to contain 
iron also, although its presence is not so readily demonstrated as 
in the previous case, probably owing to the already deep colour 
of the crystals, which obscures the effect of the ferrocyanide. 
Occasionally, however, some of these bodies are surrounded by a 
blue halo ; and in places where a large mass has been shattered by 
the impact of the knife during cutting, the finer portions of the 
debris exhibit the dark blue reaction, and around the fine particles 
there is usually a good deal of diffuse blue staining. 
(c) This third form is one regarding which I was for some time 
in doubt as to whether it really contained iron or not. Some of 
the bodies, however, undoubtedly darkened with ammonium 
sulphide, while any development of Prussian blue was frequently 
doubtful. These bodies are of variable shape and size, highly 
