128 NORMAL HISTOLOGY. 



In Such a preparation, although we see the elongated 

 nuclei in the capillary wall, we cannot, as a rule, make 

 out the outlines of the cells. To accomplish this we have 

 recourse to the use of dilute solutions of nitrate of silver. 

 This can be applied by immersing some thin membrane, 

 such as the mesentery, for an hour in a solution of nitrate 

 of silver (1-500), brushing off the endothelium from the 

 surface, and exposing the specimen in water to the light, 

 until it becomes brown. Or, what is better, the entire 

 vascular system of a small animal, such as a frog, may be 

 first rinsed out with water through a canula introduced 

 into the aorta and attached to a syringe, and then in- 

 jected with the above silver solution. Thin membranes, 

 such as the mesentery or bladder, are removed from the 

 animal, exposed to the light for a sufficient time, and 

 then, either stained or unstained, mounted in glycerin. 



Silver Staining of Bladder. As the injection of an en- 

 tire frog is somewhat difficult without considerable prac : 

 tice, the following procedure may be substituted for it : 

 The bladder is exposed in a freshly-killed frog, and a 

 canula being passed into it, the organ is moderately dis- 

 tended with air and ligated in this condition. It is then 

 cut out, rinsed in water, and laid for twenty minutes in 

 one-half-per-cent. solution of silver nitrate. It is then 

 rinsed and exposed to the light, and treated as above. 

 The pictures are more distinct if the epithelium be 

 scraped from the inner surface before mounting. 



Arteries and Veins. Very small vessels can be studied 

 entire, since by careful focussing we can bring one por- 

 tion after another into view, obtaining thus what are 

 called optical sections. 



