10 METHODS. 



soaking in distilled water. The exposure to this re-agent may 

 vary from fifteen to sixty minutes, or even over this time. After 

 the re- agent is washed off with distilled water, especially for 

 staining nerves, a few drops of acetic, tartaric, or formic acid 

 may be added. Such specimens are mounted in glycerine. 



Absolute alcohol (Spina) is a re-agent which has recently 

 become of importance for bringing to view certain features 

 in the varieties of connective tissue. The tissue is kept for only 

 two or three days in alcohol, cut and examined in alcohol, but 

 cannot be preserved. 



Injections. In order to render the vessels of a tissue plainly 

 visible, stained liquids are driven into them. The best liquid is 

 fine melted gelatine, stained red with carmine, or blue with 

 soluble Prussian blue. As a rule, the injection is made into a 

 larger artery, whence the liquid spreads through capillaries and 

 veins. The artery is fastened to a small metal or glass tube 

 fitting at one end the caliber of the artery, at the other end the 

 caliber of the tube of the syringe or other apparatus. All other 

 vessels must be ligated except one vein, which, by emptying 

 the injected liquid, indicates a complete filling of the vascular 

 system. Both the gelatine and the tissue must be kept at a 

 temperature preventing coagulation. The injection is made by 

 a syringe, or by more complicated pressure apparatus, which 

 latter, by their slow action, yield better results than the former. 

 Injected specimens are placed and kept in alcohol, as the chromic 

 acid solution destroys the colors added to the gelatine. Spon- 

 taneous injection has been used on frogs. So-called parenchy- 

 matous injections, in which colored liquids are driven with a 

 pointed syringe directly into the tissue, were thought to be of 

 great value at one time, but they are abandoned nowadays. 



How to Work with the Microscope. After a specimen is trans- 

 ferred to the table of the stage of the microscope, and by the 

 coarse adjustment or by pushing the tube is brought into focus, 

 one hand is placed on the fine adjustment, the micrometer screw, 

 and should not be removed during the examination. Both eyes 

 must be kept open, and no ocular accommodative power exercised, 

 as the careful handling of the micrometer screw renders accommo- 

 dation superfluous. Every specimen should be examined at first 

 with low powers of the microscope, and a gradual increase of 

 the power is accomplished by changing the systems of the 

 objectives. For illumination of the object we use dispersed day- 

 light or kerosene-light, which latter is far superior to gas-light. 



