IODISED SERUM. 193 



of osmosis, to which process the destructive action of pure 

 water is mainly due. Salt solution is a medium suggested 

 by this necessity. But salt solution by no means fulfils" 

 all the conditions implied in the notion of an " indifferent " 

 liquid. In so far as it possesses a density approaching to that 

 of the liquids of the tissues, one cause of osmosis is eliminated ; 

 but there remains another, due to the difference of composi- 

 tion of the liquids within the tissues and that without. Cell 

 contents are a mixture of colloids and crystalloids, salt solution 

 contains only a crystalloid, whose high diffusibility causes it 

 to diffuse over into the colloids of the tissues. In order to 

 reduce the consequent osmotic processes to a minimum, it is 

 necessary that the examination medium contain in addition to 

 a due proportion of salt or other crystalloid, also a due pro- 

 portion of colloids. By adding, for instance, white of egg to 

 salt solution, this end may be attained, and, as a matter of 

 fact, the liquids recommended as indifferent are found in- 

 variably to contain both crystalloids and colloids. Thus (as 

 stated by Frey) vitraous humour contains 987 parts of water 

 to'about 4*6 of colloid matters and 7*8 of crystalloids (common 

 salt). In 1000 parts of the juice of fruits are contained about 

 3'8 parts of colloid matter (albumen), 5*8 of salt, and 3*4 of 

 urea. In blood serum, 8' 5 of colloids and 1 of crystalloid 

 substance are found. 



345. Salt Solution. (" Normal salt solution," "physio- 

 logical salt solution") 0'75 per cent, sodium chloride in water. 

 Carnoy recommends the addition of a trace of osmic acid. 



346. Iodised Serum. Iodised serum was first recommended 

 by Max Schultze (Virchow's Archiv, xxx, 1864, p. 263). I 

 take the following instructions concerning it from Ranvier 

 (Traite, p. 76). 



The only serum that gives really good results is the amniotic 

 liquid of mammals. A gravid uterus of a sheep or cow 

 having been obtained (in large slaughter-houses such can be 

 obtained without difficulty), an incision is made through the 

 wall of the uterus and the foetal membranes. A jet of serum 

 issues from the incision, and is caught in a flask prepared for 

 the purpose. Flakes of iodine are then added, and the flask 

 is frequently agitated for some days. Two points should be 

 noted. A perfectly fresh amnios must be taken; for the 



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