166 



THE ART OF MAKING MICROSCOPE SLIDES 



Mounting 



provide too ready a means of egress for the 

 material inserted. One should, therefore, 

 always slit the artery, slide the injection 

 needle into the slit, and then ligate it in 

 two places, the proximal ligature being for 

 the purpose of holding the needle in place, 

 and the distal ligature being for the pur- 

 pose of sealing the other end of the slit. 



The second great cause for failure lies in 

 the stopping up of the blood vessels before 

 an injection has reached them. This stop- 

 page may be produced either by the 

 natural contractions of the vessels them- 

 selves, or by the presence in the injection 

 medium of some particle too coarse to 

 pass through the finer capillaries. The con- 

 traction of the vessels themselves may be 

 prevented by killing the animal with a 

 vasodilating material, or by waiting before 

 injection until rigor mortis has passed off. 

 If the latter course is adopted one must, 

 of course, wash the contained blood out of 

 the blood vessels with a saline solution in 

 order to prevent coagulation. 



The last, and most frequent, cause of 

 failure is that the blood vessels will burst 

 before the medium has penetrated them. 

 This is obviously due to the appUcation of 

 too great pressure, and is commonest 

 among workers wiio use a hypodermic 

 syringe and endeavor to push in the injec- 

 tion as if they were making a hypodermic 

 injection of a drug. Tlie rat shown in Fig. 

 87 was perfused, using the pressure shown, 

 for a period of six hours with a glycerol- 

 carmine mixture before the injection could 

 be considered complete. It is occasionally 

 possible, with very small organs, to com- 

 plete an injection in a few moments, but 

 it is usually far better to use a low pres- 

 sure and take a long time, not only to 



avoid bursting of the vessels but also to 

 l^revent a gross swelhng and distortion of 

 them. 



Mounting Injections 



Injections may either be mounted as a 

 wholemount in the manner described in 

 Chapter 6, or prepared as sections by the 

 methods described in Chapters 12, 13, and 

 14. In either case the material, after injec- 

 tion, should be fixed in a medium which 

 will coagulate the injection material, if 

 sections are to be cut, or which will remove 

 the unwanted portions of the injection 

 medium, such as glycerol, if wholemounts 

 are to be made. The processes of dehy- 

 drating, clearing, and mounting, or em- 

 bedding need not be described further 

 since they have already been dealt with in 

 the chapters indicated; but attention 

 should be drawn to the fact that sections, 

 if the}^ are to be cut, should be relatively 

 thick. The only purpose of an injection is 

 to show the course of blood vessels and 

 this cannot be done in an ordinary histo- 

 logical section of 10 or 12 microns in 

 thickness. It is much better to cut a sec- 

 tion of from }/[o- to J-^-millimeter in thick- 

 ness, and then to treat that as though it 

 were a wholemount, being particularly 

 careful that it is perfectly dehydrated and 

 perfectly cleared so that the uninjected 

 portions may be as transparent as glass. 

 In sections of this type one may easily 

 follow the course of even the finest blood 

 vessels with the aid of a binocular dissect- 

 ing microscope. 



The general principles discussed in this 

 chapter are illustrated by three quite 

 different injections which will now be 

 described. 



Specific Examples 



Injection of the Blood Vascular System of a 60-hour Chick Embryo with India Ink 



The trick of making these preparations, 

 which are so widely used for teaching pur- 

 poses, appears to be known to very few 

 people and Is, therefore, worthwhile de- 

 scribing here. The age of 60 hours for the 

 chick has been specified in this example 

 because this is the easiest size on which to 

 learn the technique; but the method to be 

 described can equally well, after some 



practice, be applied to any chicken embryo 

 between the time when the heart is first 

 formed and the end of the 96th hour, when 

 the specimen becomes too big to inject 

 conveniently. A description is given else- 

 where (Chapter 20) of the removal of an 

 embryo from the yolk and need not be 

 repeated. 



To prepare the medium, take any com- 



