WITH THE MICROSCOPE. 293 



the medium fulfilling the former condition, another can be readily 

 substituted which fulfils the latter requirements. 



5. In investigations upon the changes which structure undergoes 

 in the organism, it is necessary to distinguish between that part of 

 the texture which is the oldest, and that which has just been pro- 

 duced between matter in which active changes are going on, and 

 matter which is in a passive state. It is only by fulfilling this require- 

 ment that the direction in which growth takes place, and the point 

 where new matter is added, can be ascertained. 



6. It is most important in many investigations, that we should be 

 r.ble to distinguish positively the vessels from the other constituents 

 of the tissue, and it is necessary that the process by which this is 

 effected should not interfere with the demonstration of all the tissues 

 m the immediate vicinity of the vessels. 



7. It is of the utmost importance, that the medium employed 

 for demonstration should have the property of preserving the speci- 

 mens, so that observers should be able to exhibit their preparations 

 to others. 



Glycerine and syrup fulfil the requirements mentioned in the 

 foregoing paragraphs. 



363. Action of Glycerine and Syrup on Tissues. Strong syrup 

 may be made by dissolving with the aid of heat, lump sugar in dis- 

 tilled water, in the proportion of about three pounds to a pint. 



Glycerine may be used diluted or undiluted. It is necessary 

 in many cases to employ the strongest glycerine. In this country 

 we have had the advantage of the beautiful preparation called 

 Price's glycerine, which is made of specific gravity 1240. It has been 

 said that glycerine and strong syrup are not adapted for preserving 

 soft tissues, because the tissues shrink, and soft cells collapse in con- 

 sequence of exosmose of their fluid contents. But I have mam- 

 hundred specimens preserved in the strongest glycerine I could pro- 

 cure, and I should be glad if glycerine could be made of still 

 greater density. There would be no difficulty in impregnating even 

 very soft tissues with it. In fact, the objections urged are theoretical, 

 and result from ignorance of some properties of the tissues on the 

 part of those who have advanced them. If objectors had simply 

 tried the experiment, they would have found no difficulty whatever 

 in cany ing out the process. Tissues possess a considerable elastic 

 property, and although they shrink when immersed in a medium of 

 considerable density, they gradually regain their original volume if 

 left in it for some time. In practice, the specimen is first immersed 

 in weak glycerine or syrup, and the density of the fluid is gradually 

 increased. In this way, in the course of two or three days, the 



