Method 



INJECTIONS 



165 



vessel and held there firmly in spite of 

 pressure being applied. An ordinary hyiio- 

 dermic needle cannot be used because it 

 cannot be held firmly in the blood vessel; 

 it is too smooth, even when tied in place, 

 to withstand the pressure of tlie injection. 

 The older books are filled with descrip- 

 tions of injection "pipes" which had a 

 little ridge of metal turned on their ends, 

 ai'ound which a thread might be tied to 

 hold them in position. These pipes were, 

 however, mostly very large and could 

 rarely be used with anything smaller than 

 tlie dorsal aorta of a rabbit. It is very easy 

 to turn a hypodermic needle into a good 

 injection pipe by putting a httle ridge of 

 fine silk on it immediately liehind the 

 orifice. The only silk which is suitable is 

 that used for tying trout flies, and is sold 

 under the designation six 0. The hypo- 

 dermic should be held in a convenient 

 holder, the end of the silk attached with a 

 half-hitch just behind the orifice, wrapped 

 around a half-dozen times, and then built 

 uj) into a little ridge of four or five layers 

 before being finished with a tohip finish. 

 A drop of any good lacquer or varnish is 

 then used to impregnate the silk, and the 

 minute injection pipe is read}'. If the 

 reader cannot understand this description 

 it is recommended that he apply to the 

 nearest tier of trout flies who will be able 

 in ten minutes to prepare for him a dozen 

 needles. 



The next difficulty which must be faced 

 is to attach to the other end of the needle 

 some apparatus through which pressure 

 ma}' be maintained in order to drive in the 

 injection medium. The ordinary h3'po- 

 dermic syringe, for which these needles 

 are made, is singularly ill-adapted to the 

 purpose. Pressure will have to be applied 

 for a considerable time, and various de- 

 vices have from time to time been pro- 

 posed for leaning a weight of some kind 

 upon the upper end of the i)lunger. The 

 writer, however, has always had more 

 success with the device shown in Fig. 87 in 

 which a bottle containing the injection 

 medium is suspended, at a height which 

 may be varied, above the object being in- 

 jected. The difficulty here is to secure 

 a satisfactory attachment between the 

 bottle and the hypodermic needle. In the 



illustration, a piece of heavy-walled glass 

 tube lias l)een di'awn out to a])proxi- 

 mately the right size and then ground at 

 the tip, in the manner in which the tip of a 

 hypodermic syringe is ground, to fit the 

 needle. This can easil}' be done l)v any 

 competent glass blower, but the process is 

 beyond the facilities of most laboratories. 

 It is, however, so necessary and so desir- 

 al)le, that the worker who intends to con- 

 duct a number of injections is well advised 

 to have some of these glass tubes made. It 

 must be emphasized that a method of de- 

 taching the needle from the supply must 

 be available, since in almost all injection 

 methods two or more fluids must be suc- 

 cessively used. It is highly desirable to 

 wash the excess blood from the animal by 

 running a saline solution through the 

 lilood vessels before the insertion of the 

 injection medium, and this obviously can- 

 not be done unless the attachment to the 

 needle may be readily changed. Equip- 

 ment of the type shown is used, however, 

 only when one desires to inject most of the 

 capillaries of an entire animal and it can- 

 not be used satisfactorily with a gelatin 

 medium which has to be kept molten 

 throughout the course of the injection. If 

 such a warm injection is to be made, one 

 must provide a heating jacket to surround 

 the bottle containing the medium, and also 

 immerse the entire animal in a tray of hot 

 water until the injection is completed. 

 Gelatin media, in fact, are far better 

 adapted for the injection of small parts of 

 animals (as described in one of the specific 

 examples following this chapter) in which 

 a hypodermic syringe may be used under 

 the surface of warm water. 



There are three ver}^ common causes of 

 the failure which attends the first attempts 

 of almost every individual to make an in- 

 jection. The first of these is the failure to 

 provide some means for the blood to get 

 out when the injection medium is inserted. 

 It is ridiculous to insert a needle into an 

 arter}' and to expect an injection medium 

 to be pushed through, unless some pro- 

 vision is made for the removal of tlie 

 ])lood through the corresponding vein. 

 One cannot, moreover, merely sever an 

 artery and insert the needle into its end, 

 or the bleeding from the other cut end will 



