DISSECTING TISSUES. 



93 



to cut very minute objects, because they dissected them 

 equally, whereas knives, if ever so fine and sharp, are apt 

 to disorder delicate substances. His knives, lancets, and 

 styles were so fine, that he could not see to sharpen them 

 without a magnifying glass." (Fig. 65.) 



The mode adopted for breaking up tissues into very 

 small pieces is usually conducted, as represented at fig. 62, 



Fig. 62. Teasing-out membi one. 



with a pair of the small needles held firmly between the 

 fore-finger and thumb. The structure must be teased out ; 

 an operation which requires care and perseverance, as most 

 of the animal tissues are very difficult of separation. All 

 substances should be carefully separated from dust and 

 other impurities which renders their structure indistinct 

 or confusing. With very delicate membranes, and with 

 those of the nervous system of the smaller animals, in- 

 sects, &c., it becomes necessary that the investigation 

 should be carried on under water, or in fluid of some sort, 

 in a glass cell, and having a strong light thrown down 

 upon it by the aid of the condensing lens, as represented 

 in fig. 63. A certain amount of change of structure must 

 be expected and allowed for ; as nearly all membranes 

 imbibe some portion of the fluid. Delicate structures are 

 often advantageously wetted with dilute solutions of sugar 

 or common salt, to prevent the changes from endosmosis, 

 which result from the use of pure water. The contents of 

 bodies are frequently rendered more distinct by the addi- 

 tion of re-agents referred to further on. 



Cells or troughs are made out of pieces of stout plate- 

 glass, their edges being accurately ground, and cemented 



