INTRODUCTION. 



conformation of the individual tissues. In accordance with this 

 view, we have endeavoured to indicate, in the shortest possible 

 terms, the morphological relations of each elementary tissue, its 

 admixture with other textural elements, and its occurrence in the 

 various organs of the animal body. 



While it cannot be denied that chemical investigations of the 

 tissues, if they are to lead to any valuable result, must be inti- 

 mately associated with histological examinations, so on the other 

 hand the first step towards a chemical recognition of the textural 

 elements can only be taken through the aid of micro-chemical 

 reactions. We will, therefore, begin the consideration of " histo- 

 chemistry proper/' with a description of the changes which we 

 observe with the microscope in the texture of each tissue after the 

 application of various reagents, limiting ourselves, for the most 

 part, to the results obtained by personal observation. There is no 

 department of physical science in which personal observation is 

 more necessary for the purpose of forming a correct judgment than 

 in micro-chemistry. Our judgment regarding surrounding objects 

 or phenomena, of which we obtain cognizance only through the 

 sense of sight, is exposed to numerous sources of error ; we know 

 that in making observations with the microscope we are deprived 

 of many aids which, under other circumstances, would assist us in 

 forming our opinion regarding the objects we perceive, and we 

 especially miss this assistance in forming our judgment regarding 

 the changes which microscopical objects undergo under the influ- 

 ence of chemical reagents ; thus, for example, many histological 

 elements swell and become as imperceptible to the eye as if they 

 were actually dissolved, whilst in reality they are simply reduced 

 by the reagent to a gelatinous condition in which their refractive 

 power corresponds with that of the surrounding fluid; and it not 

 unfrequently happens that the membrane or fibre that had become 

 invisible may be again brought into view by repeated washing with 

 water, or by careful saturation of the acid or base that had been 

 employed, thus affording evidence that it had not been dissolved 

 in the reagent. After the application of other reagents, parts 

 often become visible which previously could not be perceived ; it 

 is then often impossible to distinguish whether these objects 

 actually existed previously and were only very transparent, or 

 whether they were produced by the application of the reagent. 

 In such cases it is often impossible to arrive at any certain con- 

 clusion ; we know, for instance, that histologists are not yet fully 

 agreed as to the nuclei which appear so abundantly in the corpuscles 



