HARDENING 649 



The pressure required for injection may be obtained by the 

 gentle use of a hand syringe ; by the displacement of the confined 

 air in a large bottle or carboy by tap water ; or much better by the 

 use of a water blast, of which the small glass type is relatively inex- 

 pensive and will furnish a pressure for injection about equal to 

 180 mm. of mercury. The air outflow of the water blast is con- 

 nected by rubber tubing with a glass canula of proper size to fit the 

 vessel injected, a Wolff bottle containing the warm injection mass 

 being interposed. If a manometer is connected, by means of a 

 T-canula, on the proximal side of the Wolff bottle, a relatively even 

 and accurately measured pressure is assured. The amount of pres- 

 sure should be at first low (20 to 40 mm. of mercury), and should 

 be gradually increased up to, but not much beyond, the normal 

 blood pressure in the vessel injected. 



The injected organ is cooled rapidly in a refrigerator, or by 

 being packed in ice or immersed in ice water. After solidification 

 small pieces are immediately placed in 95 per cent, alcohol for fixa- 

 tion, dehydration, and hardening. 



HARDENING 



After proper fixation nearly all tissues require to be hardened 

 before satisfactory sections can be cut. This is accomplished by 

 immersion in alcohol until dehydration is complete. The process 

 requires from a day to a week, according to the size of the tissue 

 and the volume and strength of the fluid. Various strengths of 

 alcohol are advised. For general use the proceeding recommended 

 by Gage * is found to be very satisfactory. The tissues after fixa- 

 tion are successively placed for one or two days in each of the fol- 

 lowing strengths of alcohol 67, 82, and 95 per cent. Tissues can 

 remain indefinitely in the 95 per cent, alcohol, but are improved 

 by being embedded for sectioning without great delay ; this is espe- 

 cially true of tissue which has been fixed with Zenker's solution. 



EMBEDDING 



Thick sections may be obtained from the firmer tissues by free- 

 hand sectioning with a razor, but for the satisfactory preparation 

 of thin sections a microtome is a necessity and the tissues must 

 be previously embedded to render them sufficiently firm. This is 

 accomplished by infiltrating the tissue with celloidin or parafin, 

 either of which yields a firm, waxy consistence. 



* The Microscope. 



