VERTEBRATED ANIMALS. 293 



or transparent, each method having its special advantages. The former 

 is mosfc suitable where solid form and inequalities of surf ace are especially 

 to be displayed, as in Figs. 479 and 485; the latter is preferable where 

 the injected tissue is so thin as to be transparent (as in the case of the 

 retina and other membranes of the eye), or where the distribution of its 

 blood-vessels and their relation to other parts may be displayed by sec- 

 tions thin enough to be made transparent by mounting either in Canada 

 balsam or Dammar (Plate xxv.). The injection is usually thrown into 

 the vessels by means of a brass syringe expressly constructed for the pur- 

 pose, which has several jet-pipes of different sizes, adapted to the differ- 

 ent dimensions of the vessels to be injected; and these should either be 

 furnished with a stop-cock to prevent the return of the injection when 

 the syringe is withdrawn, or a set of small corks of different sizes should 

 be kept in readiness, with which they may be plugged. The pipe should 

 be inserted into the cut end of the trunk which is to be injected, and 

 should be tied therein by a silk thread. In injecting the vessels of Fish, 

 Mollusks, etc., the softness of the vessels renders them liable to break in 

 the attempt to tie them; and it is therefore better for the operator to 

 satisfy himself with introducing a pipe as large as he can insert, and with 

 passing it into the vessel as far as he can without violence. All the ves- 

 sels from which the injection might escape should be tied, and sometimes 

 it is better to put a ligature round a part of the organ or tissue itself; 

 thus, for example, when a portion of the Intestinal tube is to be injected 

 through its branch of the Mesenteric artery, not only should ligatures be 

 put round any divided vessels of the mesentery, but the cut ends of the 

 intestinal tube should be firmly tied. For making those minute injec- 

 tions, however, which are needed for the purposes of anatomical investi- 

 gation, rather than to furnish ' preparations ' to be looked-at, the Author 

 has found the glass-syringe (Fig. 106), so frequently alluded-to, the most 

 efficient instrument; since the Microscopist can himself draw its point to 

 the utmost fineness that will admit of the passage of the injection, and 

 can push this point without ligature, under the Simple Microscope, into 

 the narrowest orifice, or into the substance of the part into which the in- 

 jection is to be thrown. Save in the cases in which the operation has to 

 be practised on living animals, it should either be performed when the 

 body or organ is as fresh as possible, or after the expiry of sufficient time 

 to allow the rigor mortis to pass-off; the presence of this being very ini- 

 mical to the success of the injection. The part should be thoroughly 

 warmed, by soaking in warm water for a time proportionate to its bulk; 

 and the injection, the syringe, and the pipes should also have been sub- 

 jected to a temperature sufficiently high to insure the free flow of the 

 liquid. The force used in pressing-down the piston should be very mod- 

 erate at first, but should be gradually increased as the vessels become 

 filled; and it is better to keep up a steady pressure for some time, than to 

 attempt to distend them by a more powerful pressure, which will be cer- 

 tain to cause extravasation. This pressure should be maintained 1 until 

 the injection begins to flow from the large veins, and the tissue is thor- 

 oughly reddened, and if one syringeful of injection after another be re- 

 quired for this purpose, the return of the injection should be prevented 



Microscope;" the " Handbook to the Physiological Laboratory;" and Rutherford's 

 and Schafer's treatises on " Practical Histology." 



1 Simple mechanical arrangements for this purpose, by which the fatigue of 

 maintaining this pressure with his hand is saved to the operator, are described in 

 the works referred-to in the preceding note. 



