xlii INTRODUCTION. 



are suspended molecules and granules. If these exhibit molecular motion, the material in 

 which they are suspended must be liquid. It sometimes becomes a question whether a 

 body enclosed within another is central or lateral. This is readily determined by causing 

 the body to revolve by inclining the stage of the microscope, when, if central and fixed, 

 the enclosed body will retain this position ; and if it be less than the cavity of the enclosing 

 structure, positive indication will be afforded that the latter is solid, or at least that it does 

 not consist simply of an outer coat with liquid contents and the enclosed body. But if it 

 be attached to the inner wall of the enclosing structure, the eccentricity of its motion 

 whilst revolving will be evident. 



Tbe contents of microscopic bodies are frequently rendered distinct by the addition of 

 reagents, and in some cases can only be distinguished by their use ; thus the nuclei of 

 animal cells are at once made evident by the addition of acetic acid, &c. 



The micro-spectroscope is often used in detecting small quantities of different substances 

 (Spectroscope). 



We frequently have to decide whether the interior of an object is solid or tubular. If 

 it consist of a firm substance, drying it, if in liquid, will cause the evaporation of the liquid 

 or other contents, and the entrance of air. A section of it will also show whether it is 

 solid or hollow. The effects of crushing it should also be observed. 



An important aid in describing the structure of many objects, the components of which 

 are of so nearly similar refractive power as to be undistinguishable by any variation in 

 illumination, is that of dyeing or staining. Different portions of a structure often have a 

 variable affinity for colouring-matters or dyes, and can thus be readily distinguished. 

 The various appropriate dyes are mentioned under the tissues, and the process under 

 Staining. 



B. Histological Analysis. 

 This consists in the resolution of the object into its component morphological elements, 

 and is usually effected by subjecting it to the action of various chemical reagents, continued 

 maceration, &c. It must never be attempted if inorganic matters be present in quantity, 

 until these have been previously removed. The reagent used should be one which exerts 

 a solvent action upon the substance of which the object is composed, the action being 

 interrupted at a certain stage by the addition of water, &c. In regard to those objects 

 whose morphological elements have become altered by individual growth, Szc, histological 

 analysis is of course useless, and the manner in which these have acquired their existing 

 structure can only be determined by tracing the gradual changes which their morpholo- 

 gical constituents undergo, from the earliest period of their existence to that at which 

 they form the object in question. This constitutes the study of development; or it might 

 be termed Histological synthesis. It can rarely be followed directly, but may often be 

 carried out indirectly by examining a number of the objects in all stages of their develop- 

 ment, and comparing the changes undergone by their constituents. It requires special 

 care in controlling the identity of the objects. 



C. Chemical Reactions, 



We cannot too strongly insist upon the necessity of investigating these in the case of 



all objects submitted to examination, the nature of which is at all doubtful — and this 



because in many instances the form or general appearance will afford no criterion by which 



the nature may be determined. Judgment founded simply upon the form, or upon the 



