16 



Injections 



General Principles 



Nature of the Process 



Injection is the art of filling cavities in 

 materials intended for microscopical ex- 

 amination, with some material which will 

 render them more visible. In materials in- 

 tended for gross dissection, the prepara- 

 tion of which is not described in this book, 

 onlj- the blood vessels are so treated and 

 the idea has, therefore, developed that the 

 process may only be applied to blood 

 vessels. From the point of view^ of the 

 microscopist any cavity may be filled 

 whether it be blood, lymph, or bile vessels, 

 or even the caniculi of bone (see Chapter 

 28, V 31.1 Altmann 1870). The material 

 to be injected, if it is to be readily visible, 

 must be either brightly colored or densely 

 opaque. Though the term injection usually 

 brings to mind the insertion of materials 

 under pressure, there are some cases, 

 as in the example just mentioned, in 

 which a vacuum may be used to displace a 

 fluid or air and then replaced with the 

 material being used. Injections may be 

 prepared either as sections or as whole- 

 mounts, but different methods and mate- 

 rials should be used for each. It will be 

 necessary, therefore, to discuss first the 

 selection of the injection medium, second 

 the methods by which this medium may 

 be inserted into the cavity which one 

 desires to demonstrate, and lastly the 

 method of preparing a permanent mount 

 from the tissues so injected. 



Before proceeding with this discussion, 

 however, it is necessary to point out that 

 injection is not the only, or even the best, 

 means of showing fine vessels in a micro- 

 scopic preparation. In Chapters 21 and 23 

 'y\'ill be found methods of demonstrating 



both blood capillaries and bile capillaries 

 by differential staining. These methods are 

 usually less laborious than is the process 

 of injection and should always be tried 

 before an injection is undertaken. 



« 



Selection of an Injection Medium 



The recjuirements of an injection mass 

 are that it shall run readily into the vessels 

 being injected, that it be readily visible 

 when it is in the \-essels, and that it remain 

 in place through subsequent manipula- 

 tions. The requirement that it should go 

 into the vessels easily is met by providing 

 a fluid medium of low surface tension 

 which does not itself cause contraction of 

 the muscular coats of the blood vessels. 

 The condition of low surface tension is 

 usually met by including in the medium a 

 certain quantity of glycerol, though Hag- 

 mann 1940 (2d540b, 15:115) has recom- 

 mended the inclusion of a wetting agent in 

 a medium intended for injection of insect 

 trachea. There is no reason why similar 

 wetting agents should not be included in 

 media intended for blood vessels. 



The recjuirement that the injection 

 should be visible when in position may be 

 met in two ways. First one may incorpo- 

 rate with the injection mass a pigment 

 fineh' enough divided to be apparently in- 

 visible but of a sufficiently large molecular 

 size not to pass through the wall of the 

 capillaries. The majority of injection 

 masses are of this type and the detailed 

 directions for preparation of them (Chap- 

 ter 28, V 31.1) are largely devoted to the 

 jirecipitation of a pigment of such fine 

 grain size that it fulfills the required con- 

 ditions. The second method of achieving 



162 



