4 o8 APPENDIX. 



stance is demonstrated as dark lines. The method is of further use in bring- 

 ing to view the lymph-spaces within the dense connective tissues, as the cor- 

 nea, in which the spaces then appear as irregular light figures surrounded 

 by the brown ground-substance. 



When the mesothelium, covering for instance the mesentery, is to be 

 displayed, the tissue is cut from the recently killed animal and carefully 

 transferred by glass rods to i per cent, aqueous solution of argentic nitrate. 

 After immersion for 5-10 minutes, according to the thickness of the object, 

 the tissue is rinsed in distilled water, placed in a porcelain dish containing 

 distilled water, and stood in the direct sunlight until reduction of the silver 

 is completed. This is indicated by a decided reddish-brown tint and usually 

 requires 8-15 minutes. The tissue is then transferred to a small dish of dis- 

 tilled water to which a few granules of sodium chloride have been added, the 

 purpose of the latter being to arrest the action of the silver. After 10 min- 

 utes, the stained tissue is placed for 6-10 hours in 70 alcohol, in the dark, 

 followed by dehydration in alcohol, clearing and mounting in balsam. After 

 the reduction of the silver, staining with hematoxylin adds to the interest of the 

 preparation by bringing out the nuclei, which otherwise are only faintly seen. 



When the endothelium of the blood-vessels is to be stained, the method 

 recommended by Huber may be followed with advantage. After the escape 

 of the blood following incision of the exposed heart of an anesthetized animal, 

 a glass or hard rubber canula is inserted into the thoracic aorta and the ves- 

 sels injected with a i per cent, solution of argentic nitrate. After fifteen min- 

 utes, the inferior vena cava is cut immediately below the heart and a 4 per 

 cent, solution of formalin ( 10 parts commercial formalin to 90 parts of dis- 

 tilled water) is injected into the aorta through the same canula. The injec- 

 tion of the formalin washes out the superfluous silver solution, thereby 

 avoiding disturbing precipitates, and fixes the vessels while distended. The 

 desired tissue is then cut from the animal, care being taken to remove the 

 structures to be examined in pieces sufficiently supported to prevent undue 

 distortion, immersed in 4 per cent, formalin and exposed to direct sunlight. 

 While the latter is not necessary, reduction of the silver taking place slowly 

 in diffuse daylight, the rapid reduction effected by sunlight is favorable to 

 sharp and well differentiated histological pictures ; it should, therefore, be 

 employed whenever possible. After dehydration, small flat pieces of the 

 tissue are cleared in carbol-xylol and mounted in balsam. If protected from 

 strong daylight, such preparations may be preserved for years with little 

 deterioration. 



Injecting Blood-Vessels. In order to demonstrate the distribution 

 of the smaller blood-vessels and the capillaries, advantage is taken of some 

 means to fill the blood-channels with a colored substance. The injection- 

 mass must meet two requirements be transparent and not diffuse through 

 the walls of the smallest vessels. Successful, that is complete, injection of 

 the capillaries requires considerable experience and, even at best, is attended 

 with an element of uncertainty, since the condition of the tissues, particularly 

 of the vessels, influences the freedom with which the injecting fluid runs. 



Two injecting masses are commonly employed, carmine-gelatin and 

 Berlin blue. When successful the former yields very beautiful preparations, 

 but has the disadvantage of requiring to be used while hot and with heated 

 tissue, in order to prevent untimely solidification. The beginner will find 

 Berlin blue more convenient, since it is used cold, runs well in unwarmed 

 tissues, and does not extravasate. The results, moreover, are equally in- 

 structive, although perhaps less striking. The injection fluid is readily pre- 



