IODISED SERUM. 233 



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 composition 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 proportion 

 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 invariably 

 to contain both crystalloids and colloids. Thus (as stated by 

 Frey) vitreous 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. 



359. Salt Solution (" normal salt solution/' "physiolo- 

 gical salt solution"). 0*75 per cent, sodium chloride in water. 

 Carnoy recommends the addition of a trace of osmic acid. 



360. 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 amniotio 

 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 



