6 HISTO-CHEMISTRY. 



of frogs' blood that has stood for some time, some maintaining 

 that they exist preformed in the fresh circulating blood, and others 

 holding the opposite view. 



Again, many reagents induce coagulation of the interstitial 

 juice in mixed tissues, and in this way not only obscure the form, 

 but occasion such contractions and alterations of outline of one or 

 other of the parts, that it often becomes extremely difficult to dis- 

 tinguish to which of the tissues that are thus intermingled the 

 filaments or granules, that are observed,, belong. These, and similar 

 relations, make it extremely difficult to form a judgment regarding 

 the effect of the action of chemical reagents on the tissues, and, 

 indeed, often render it impossible to arrive at any definite result. 

 We might adduce many examples of apparently very simple ques- 

 tions regarding which the best histologists are even now at 

 variance (as, for instance, whether nucleoli are or are not contained 

 in the nuclei of certain cells, and whether these nucleoli, when 

 they are unquestionably present, consist of fat or of some other 

 matter). To show that we have not overrated the difficulties of 

 micro-chemical investigation, are there not many who even now 

 deny the pre-existence of an axis-cylinder in the nerve-tubes ? and 

 do we not find such distinguished observers as Mulder and 

 Donders holding the apparently very erroneous view that the axis- 

 cylinder visible within nerve-tubes that have undergone change 

 consists essentially of fat ? 



It is hardly necessary to remark that perfect familiarity with 

 the microscope, and an accurate acquaintance with all the auxi- 

 liaries employed in its use, and with all its sources of fallacy, are 

 indispensably requisite for the successful pursuit of micro-chemical 

 investigations. The various means of checking error which have 

 been recommended by such experienced observers as Jul. Vogel,* 

 Schleiden,f Hugo Mohl,J Purkinje. and others, in ordinary 

 microscopical inquiries, are required in a still higher degree in 

 micro-chemical researches. 



It must also be borne in mind that the application of chemical 

 reagents demands the observance of many precautions, the neces- 

 sity of which has been only recently perceived. Formerly it was 

 the ordinary practice to allow the chemical reagent to flow on the 

 microscopic preparation, and to observe its direct action on the 



* Anleitung z. Gebrauch des Mikroskops u. s. w. Leipz. 1841. 



f Botanik, Methodol. Einleitung. Leipz. 1849. 



J Mikrographie u. s. w. Tubingen, 1846. 



Wagner's Handworterb. d. Physiol. Bd. 2, S. 411-448. 



