292 NORMAL HISTOLOGY. 



liable to slight maceration in the alcohol, diluted with water from 

 the specimen. They can be kept off the bottom by means of a 

 little crumpled filter-paper. Specimens that have been fixed in a 

 chromatic solution should be kept in the dark while being hard- 

 ened ; those that have been fixed in corrosive sublimate should be 

 hardened in alcohols to which a little tincture of iodine (sufficient to 

 give them a sherry color) has been added. When absolute alcohol 

 is used, its strength should be maintained by contact with quick- 

 lime (see directions for fixing tissues in absolute alcohol). 



Methods of Impregnation. 



When tissues are so porous or friable that sections are likely to 

 tear or disintegrate it is desirable to impregnate them with some 

 embedding-material. The most useful substances for this purpose 

 are collodion, or celloidin, and paraffin. Whichever of these is 

 used, it is necessary to remove the water from the specimen before 

 the impregnation can be accomplished, for both collodion and 

 paraffin are insoluble in water. Tissues that have been hardened 

 in alcohol are to a certain extent already dehydrated. The residual 

 water may be removed or reduced to a trace by treatment with 

 absolute alcohol, in which collodion is soluble. 



The "celloidin" manufactured by Sobering is an excellent prep- 

 aration of gun-cotton, but almost equally good results may be 

 obtained by using the more economical soluble cottons employed by 

 photographers. Two solutions in a mixture of equal volumes of 

 ether and absolute alcohol (both, if possible, of Squibb's prepara- 

 tion) should be kept in stock : one, a weaker solution, having about 

 the consistency of thin mucilage ; the other, a stronger solution, 

 resembling a syrup. 



Collodion is soluble in absolute alcohol, so that tissues containing 

 only that fluid are ready for impregnation without further prelim- 

 inary treatment. When thorough impregnation is desired the tissues 

 should be immersed in equal parts of ether and absolute alcohol for 

 a few days, and then in the weaker solution of celloidin or collodion 

 for a number of days or weeks — the longer the better; 1 but such 

 complete impregnation is often unnecessary, and soaking for a day 

 or two will often suffice if the sections to be made need not be very 

 thin. It is not possible, in any event, to make very thin sections 



1 Impregnation may be greatly hastened if done at the body-temperature in a 

 hermetically closed vessel. 



