XXVlll INTRODUCTION. 



cacy to allow of examination with moderately high powers. 

 Teeth, bone, and cartilage constitute, however, exceptions to 

 this statement. Bone can, even when fresh, be cut into thin 

 disks with saws, which may then be rubbed down with emery 

 on a roughened glass plate, and polished on a hone. Cartilage 

 requires no preparation, as thin sections may be readily cut 

 from it with a sharp knife. The teeth are too brittle for the 

 application of a saw. They should be attached to a cork by 

 means of shellac, and rubbed down upon a whetstone. As a 

 general rule, artificial methods of hardening the tissues must 

 be employed. The simplest and most elegant mode is that of 

 refrigeration. The tissue to be examined is placed in a little 

 platinum capsule, and imbedded in the freezing mixture ; then, 

 as soon as it has become hard, sections may be made with a 

 cold knife. A second method of hardening that is in constant 

 use is that by means of alcohol. The tissue, divided into small 

 pieces, is placed in a flask containing absolute alcohol, which 

 is renewed every few days, according to the amount of water 

 present in the object. For membranous tissues, boiling in 

 vinegar was at one time adopted, but so many better plans are 

 now known, that it has with good reason fallen into disuse. 

 If it be desired to harden the tissues by boiling, the best fluid 

 is one which consists of eight parts of water, one part of 

 creosote, and one part of vinegar ; in this the tissues should 

 be allowed to boil for two or three minutes, and be then laid 

 out to dry. After two, or at most three, days it acquires a 

 consistence which is admirably adapted for permitting sections 

 to be made. The thin sections should then be treated with a 

 little dilute acetic acid, in which the tissues again increase in 

 volume, and they can then be examined either in water or in 

 glycerine. If boiled preparations remain for a long time uncut, 

 they gradually acquire such consistence that they are no longer 

 appropriate for obtaining sections. This inconvenience has led 

 to the method of drying. It is, indeed, much more advan- 

 tageous to dry fragments of tissue. It is to be remembered, 

 however, that the morphological elements of the tissues, in all 

 these modes of hardening, are not so perfectly preserved as 

 when they are kept in fluids. A means of hardening, of very 

 general value and application, is found in chromic acid. This 



