xxxii INTRODUCTION. 



II.— GENERAL METHOD OF DETERMINING THE STRUCTURE OF MICRO- 

 SCOPIC OBJECTS FROM THE APPEARANCES WHICH THEY PRESENT 

 UNDER VARIOUS CONDITIONS. 



Microscopic and histological Appearance, Structure, and Analysis, — Before proceeding to 

 this, let us define -what is meant by the structure of a microscopic object. If we take a 

 piece of the free end of the finger-nail, and examine a thin transverse section of it under 

 the microscope, Ave find it to present numerous shorter or longer dark and somewhat 

 irregular lines running nearly parallel to the surfaces. These appearances do not vary 

 essentially -whether it be examined in the dry state, or immersed in water or oil of 

 turpentine. 



But when it is moistened with solution of potash, and allowed to remain so for some 

 time, or the slide is gently heated, it becomes entirely resolved into a number of nucleated 

 cells ; and by watching the gradual action of the potash, it is easily seen that the cells 

 were originally flattened and arranged in layers, which layers produced the lined appearance 

 mentioned above (see the article Nails). Now which is to be considered as representing 

 the structure of the nail ? the first or the second of the above results P Undoubtedly the 

 second. The expressions microscopic structure and histological structure are used very 

 indefinitely, and often synonymously ; the former may very conveniently be restricted 

 to signify the apparent structure as determined with the aid of ordinary mechanical means ; 

 whilst the latter may designate the true structure in relation to development. It may at 

 first sight appear very unnecessary to make any distinction between the two ; but it is 

 really very important, and many of the descriptions of the structure of bodies, given in 

 books, refer only to their microscopic structure. 



The determination of the histological or true structure is often very difficult. Frequently 

 a week or a month must be devoted to the determination of a single point. Take the 

 instance of a hard structure — a piece of the skeleton of one of the Invertebrata. A few 

 sections may exhibit cells, laminas or fibres, according to the preconceived notions of the 

 observer ; whilst the histologist will not express an opinion until the inorganic matters 

 have been removed by long maceration in acid, the calcareous salts thoroughly washed 

 away, and attempts have been made to resolve the organic basis into its histological elements 

 by appropriate means. This may require very many experiments to be made, and no mean 

 knowledge of particular branches of science for guidance in the selection of appropriate 

 agents requisite for their performance. We shall have frequent occasion to use the above 

 Avoids in the restricted sense ; hence this shoidd not be forgotten. The word analysis will 

 have the same meaning as that generally attributed to it, the ultimate products being 

 morphological. 



A general method of determining the structure of objects can hardly be laid doAvn ; it 

 must vary so greatly according to the nature of the objects and their size. The first point 

 is to render them transparent, if not already so. This may frequently be done by immer- 

 sion or maceration, if dry, in water, glycerine, or oil of turpentine. But the solvent poAver 

 of the liquid must be borne in mind ; for the organic principle aleurone was overlooked for 

 years from its being soluble in water, in which the sections of the albumen of seeds con- 

 taining it were immersed to render them transparent. Sometimes the aid of heat is 

 necessary ; and objects may even require to be boiled in these liquids, either upon a slide 

 placed upon the brass table over the flame of a spirit-lamp, or in a small tube. Sometimes 



