204 CHAPTER XVI. 



study, it must be re-blued by oxidation (see last §). It is 

 therefore tlie usual practice to dissect out the tissues to be 

 examined, and leave them for some time exposed to the air. 

 This is done in order that they may take up from the air 

 the necessary oxygen. Another consideration that justifies 

 the practice is that by exposure to air the preparations take 

 up a trace of ammonia, and Apathy has experimentally 

 established that this is an important factor in the sharpness 

 of the stain. Bhrlioh also {op. oil.) holds that an alkaline 

 reaction of the tissues is a necessary condition to the stain. 

 Apathy further holds that the staiu is a regressive one, easily 

 washed out by the suri-ounding liquid ; and in order to 

 prevent this washing-out being excessively rapid, it is 

 desirable to have it go on in presence of as little liquid as 

 possible. 



341. The Moies of Staiaingf. — The practice of the earlier 

 workers at this subject was (following Ehrlich) to inject 

 methylen blue into the vascular system or body-cavity of a 

 living animal, wait a sufficient time, then remove the organ 

 for further preparation and study. And there appears to 

 have been a belief with some workers that it was essential 

 that the stain should have been brought about by injection 

 of the colouring matter into the entire imimal. It is now 

 known that the reaction can often be equally well obtained 

 by removing an organ and subjecting it to a hath of the 

 colouring matter in the usual way. But in some cases it 

 seems that injection is preferable, if not necessary. 



342. The Solutions employed.— The solutions used for injec- 

 tion are generally made in salt solution (physiological, or a 

 little weaker) ; those for staining by immersion, either in 

 salt solution or other "indifferent" liquid, or in pure water. 

 The earlier workers generally took concentrated solutions. 

 Thus Aenstein [Anat. Ana., 1887, p. 125) injected 1 c.c. of 

 saturated {i. e. about 4 per cent.) solution into the vena cutanea 

 magna of frogs, and removed the organ to be investigated 

 after the lapse of an hour. Biedermann [Sitzh. Akad. Wiss. 

 Wien, Math. Nut. CI, 1888, p. 8) injected 0-5 to 1 o.c. of a 

 nearly saturated solution in 0'6 per cent, salt solution into 

 the thorax of crayfishes, and left the animals for from two 



