92 HOW TO WORK 



solution of ammonia (liquor ammonioe) to the carmine, and diluting 

 the mixture until the proper colour is obtained, or it may be diluted 

 with size. The latter by adding a persalt of iron to a solution of 

 ferrocyanide of potassium. 



177. Advantages or Employing Prussian Blue. In order to inject 

 satisfactorily the most minute vessels of a tissue, and at the same 

 time to demonstrate their relation to adjacent structures, we must be 

 provided with an injecting medium which possesses the following 

 properties : The fluid should be of such a consistence that it will 

 run readily through the smallest vessel. It must contain a certain 

 amount of colouring matter to render the arrangement of the vessels 

 distinct, but must be sufficiently transparent to admit of the examina- 

 tion of the specimen by transmitted light. The colouring matter 

 must not be soluble, for in this case it would permeate the tissues 

 indiscriminately, and would thus prevent the vessels from being dis- 

 tinguished from other textures. Though insoluble, the particles of 

 which the colouring matter is composed should be so minute as not 

 to exhibit distinct granules when examined with the highest powers, 

 for if this were so, the specimen would have a confused appearance. 

 The fluid in which the colouring matter is suspended, must be capable 

 of permeating the walls of the vessels with tolerable facility. It 

 must possess a certain refractive power, and a density approaching 

 to that of the fluid which surrounds the tissues in the natural con- 

 dition. It must be of such composition that it may be employed 

 without the application of heat. 



The injecting fluid must not escape too readily from the numerous 

 open vessels necessarily exposed in cutting a thin section of the tissue 

 for examination, and particles accidentally escaping ought not to 

 adhere intimately to the surface of the section, for this would render 

 the specimen confused and indistinct, when subjected to examination, 

 especially if high magnifying powers are required. The fluid employed 

 must not interfere with the preservation of the specimen. The inject- 

 ing fluid ought not to undergo any alteration by being kept for some 

 time, and it should be cheap and capable of being readily prepared. 



The Prussian blue fluid which consists of an insoluble precipitate, 

 so minutely divided, that it appears like a solution to the eye, 

 fulfils all these requisitions. The particles of freshly prepared 

 Prussian blue are very much smaller than those of any of the 

 colouring matters employed for making opaque injections. For many 

 years I have employed Prussian blue as the injecting fluid, and 

 according to my experience it possesses advantages over every other 

 colouring matter. It is inexpensive. It may be injected cold. The 

 preparation does not require to be warmed. No size is necessary. 



