THE BLOOD. 41 



indeed, impossible, to obtain aqueous humor or even an animal 

 fluid such as has been described, and microscopists have accord- 

 ingly made use of a substitute that can be prepared at any time 

 and kept indefinitely. This is a solution of common salt in 

 distilled water (1 400). Add a drop of fresh frog's blood to a 

 drop of the salt solution, mix them well, and it will be seen 

 that the delicate protoplasm of the red blood-corpuscle, most 

 susceptible of change, is not altered in appearance, though the 

 body itself will change in form from the elliptical to the spher- 

 ical. This salt solution has been found, in practice, an excellent 

 substitute Jfor blood-serum, and is very generally used in ex- 

 amining fresh specimens, where it is important to avoid any 

 material change in the corpuscle. 



Action of distilled water Irrigation. The effect of water 

 is also noteworthy, as it is a very important consideration in 

 both histological and pathological work, especially the latter. 

 Take a drop of frog's blood, add to it an equal quantity of 

 distilled water and apply a cover. The nucleus or central body 

 will now be readily seen, surrounded by a yellow border ; the 

 body of the corpuscle or peripheral part will at the same time 

 gradually become paler and larger. Now add distilled water 

 slowly, drop by drop, in the following way : Take a long strip 

 of tissue or filter paper about half the length of the slide and in 

 breadth equal to one-half the diameter of the cover. Apply 

 the water with an ordinary minim dropper, close to the edge of 

 the cover, on the side opposite to the paper strip. This latter 

 will now take up the excess of water and cause a stream to 

 pass through the specimen. This processes called irrigation. 

 Push the paper a short distance under the edge of the cover, 

 and the solid particles in the fluid will be carried to the edge 

 of the paper, where they will remain at rest and may be ob- 

 served at one's leisure. 



This plan is often nseful in other sorts of microscopic work, as in looking 

 for renal casts, urinary crystals, etc. It may save much valuable time. I first 

 learned it from my friend, Dr. Edward Curtis, of this city. 



Continued addition of water will cause the corpuscles to 

 swell and after a time burst, or, at any rate, become so expand- 

 ed that they can scarcely be seen. When water is applied 

 slowly to human blood, the corpuscles soon begin to lose their 



