170 PRACTICAL HISTOLOGY. 



in which they will soon become hard enough for the 

 preparation of sections. The object of trms gradu- 

 ally increasing the strength of the spirit is to prevent 

 as much as possible the shrinking of the gelatine 

 which otherwise results. 



The blue injection possesses the material disad- 

 vantage that it is apt to become temporarily reduced 

 in the smaller vessels and rendered almost entirely 

 colorless, so that it is difficult to determine whether 

 a successful injection has been made or not. The 

 color may, however, be readily restored by pouring 

 some oxidizing fluid, such as a weak solution of 

 peroxide of hydrogen, over any part about which 

 there is doubt ; and in the ordinary course of pre- 

 paring and mounting sections, the blue color is 

 always brought back, especially if turpentine is sub- 

 stituted for oil of cloves. 



Most other forms of apparatus which are used for 

 injecting are more or less like the one above 

 described, the chief modification being in the mode 

 in which the pressure is produced, this being effected 

 in one form by allowing water to flow from a tap 

 into the pressure-bottle (which in such cases is gene- 

 rally made of metal), and thus compressing the air ; 

 in another by allowing mercury to flow from one 

 vessel into another. But the latter method, al- 

 though useful when small quantities of injecting 

 fluid only are required, as with the injection of the 

 lymphatics (with which the apparatus will be 

 described), is costly for large quantities ; indeed it 

 will be found that none are more simple and efficient 

 in working than the one here recommended. 



If a condensing syringe is not at hand, sufficient 

 pressure may be got in many cases merely by blovring 

 air into the pressure-bottle through an India-rubber 

 tube, its escape being prevented by subsequently 

 clipping the tube. 



Injections which are fluid in the cold (of which 

 the best is a one or two per cent, solution of Berlin 

 blue) are sometimes used for the bloodvessels, espe- 



