Media 



INJECTIONS 



163 



the same result is to inject some material 

 which can, with ease, be sulisceiuently 

 stained differential!}'. Thus Fischer 1902 

 (23681, 13:277) recommended the injec- 

 tion of milk, the fat in which could sub- 

 sequently be stained by any of the fat- 

 staining "methods (Chapter 21, DS 22.4). 

 The objection to this type of mount, how- 

 ever, is that it cannot be dehydrated with- 

 out the removal of fat, and must, there- 

 fore, be mounted in one of the aqueous 

 mounting media discussed in Chapters 4 

 and 5, few of which ha\'e an index of re- 

 fraction high enough to show the speci- 

 mens properly. There is also the objection 

 that, if the specimens are to be sectioned, 

 a freezing technique must be used rather 

 than the more con\-entional parafhn or 

 nitrocellulose methods. A far better ap- 

 proach to this prol)lem is that of Altmann 

 1878 (23632, 10:191) who suggested the 

 injection of dilute olive oil which could 

 be stained, before embedding, with osmic 

 acid. Various methods liave from time to 

 time been proposed for the injections of 

 solutions of stains of such low penetrating 

 power that they will not pass beyond the 

 walls of the capillaries. Robin 1871, p. 40 

 and Hoyer 1882 (2981, 2:19) suggested 

 the injection of media containing silver 

 nitrate which was subsequently "devel- 

 oped" with one of the solutions intended 

 for the development of silver by metal- 

 staining techniques (see Chapter 23, 

 AMS21.1). 



The problem of ensuring that the blood 

 vessels remain open while the injection 

 medium is inserted into them is more diffi- 

 cult to solve. Glycerol in particular causes 

 the blood vessels to contract, but this 

 difficulty can be overcome in two waj^s. 

 Either the animal may be killed with some 

 substance which causes the greatest pos- 

 sible vasodilation, such as alcohol or amyl 

 nitrite, or blood can be washed from the 

 vessels with a normal citrate-saline, and 

 then this fluid followed by some material, 

 such as foimaldehyde, which will fix and 

 harden the blood \'essels in an open condi- 

 tion. This was the universal practice of 

 the older workeis Init is rarely done today. 



The final difficulty is that of persuading 

 materials to stay in place after they have 

 been injected. This is not difficult in the 



preparation of wholemounts, in which cut 

 surfaces of blood vessels are unlikely to be 

 exposed. It is very difficult, however, in 

 the case of materials which have to be sec- 

 tioned and from the cut blood vessels of 

 which material is likely to bo washed, in 

 the course of manipulation, if it is not 

 specifically held in position. It is, there- 

 fore, desirable that the injection pigment 

 should be incorporated in some mass 

 which may be hardened in position. This 

 again points up one of the advantages of 

 using milk, which may be readily coag- 

 ulated. Egg white, which may also be 

 easily coagulated in position, is unsatis- 

 factory for the reason that it is difficult to 

 incorporate pigments with it. Gelatin, 

 which may be set in position by coohng, 

 and subsequently hardened by formalde- 

 hyde to a condition which cuts readily, is 

 of almost universal employment. 



An interesting method for obviating all 

 of these difficulties is one of the oldest in 

 microtomy. This consists of first injecting 

 a solution of some material and, second, 

 injecting after it a material which will 

 cause a precipitation of a relatively gross 

 pigment in situ. This method is of great 

 antiquity and was apparently discovered 

 by Doyere sometime before 1839 (see 

 Cooper 1847, 156). Doyere himself recom- 

 mended several solutions, but the author 

 has only seen successful preparations pre- 

 pared by the injection of 2% potassium 

 dichromate followed by 2% lead acetate. 

 This method, which is described in detail 

 in one of the typical preparations follow- 

 ing this chapter, has recently been re- 

 discovered by various authors and yields 

 finer injections of capillaries than are ob- 

 tainable by any other method. It is not, of 

 course, satisfactory for thin sections from 

 which the material would be readily 

 washed, but a thick section of a human 

 kidney in the writer's collection, prepared 

 by this method sometime before 1847, 

 shows a better demonstration of the 

 glomeruli than is obtainable by any other 

 method. 



Methods of Injection 



There are two metliods Ijy which injec- 

 tion media may be inserted into fine blood 

 vessels. In the first method an inert mate- 



