HISTOLOGICAL METHODS. 177 



Tissues that have been slightly silvered may be stained 

 with logwood or carmine ; the ammonia of the latter, how- 

 ever should be neutralised with acetic acid, and the tissue 

 washed in acidulated water ( 321). 



329. Silver process for blood-vessels. If a frog be taken, 

 kill it by stunning it on the head : expose the heart, snip 

 off its apex, and allow it to bleed thoroughly. Push a glass 

 or brass cannula from the ventricle into one of the aortae, 

 and inject a stream of distilled water to wash out the blood 

 with its chlorides : inject J per cent silver nitrate solution, 

 allow it to remain for 8 or 10 minutes, and then wash it 

 out with distilled water. The most convenient parts to 

 take are the mesentery and bladder. These are therefore 

 removed and exposed to the light, as stated in 328. If, 

 however, it is desirable to silver also the epithelium covering 

 the mesentery, this is done after the injection, as described 

 in 328. In the case of a warm-blooded animal, such as 

 a guinea-pig, the water and the silver solution are both 

 heated to 39 C. The fluids are injected into the aorta. 



A solution of silver nitrate may also be injected into 

 lymphatic gland, areolar tissue, testis, lung, etc., for the 

 fixing and staining of tissue elements. The necessary 

 apparatus for injection is described in 344. 



330. Chloride of Gold (auric chloride) is of much 

 service for staining nerve fibrils, connective tissue corpuscles, 

 cartilage cells, etc. It is not merely a staining, but, like 

 silver nitrate, is also a hardening agent. Unfortunately its 

 action as a dye is somewhat uncertain. 



a. Ordinary Method. Place the tissue, not later than 

 fifteen minutes after its removal from the body, in \ per 

 cent solution of auric chloride in distilled water, until it 

 becomes of a pale yellow colour. From fifteen minutes to 

 two hours will be required, according to the thickness of 

 the tissue. Transfer to very dilute acetic acid (i or 2 per 

 cent) for five or ten minutes. Lastly, place the tissue in 

 distilled water, and expose to diffuse daylight, until a steel- 

 gray or violet tint appear. This results from the reduction 

 of the gold salt. The reduction is hastened by the acetic 

 acid. The tissue may also be exposed to the light in dilute 



N 



