HISTOLOGICAL METHODS. 149 



apices pointing towards each other at the centre of the 

 slide, leaving an interval of about a tenth of an inch between 

 them (a) for the reception of the object, which is covered 

 with thin glass in the usual way. 



Electricity may be readily transmitted through the 

 electrodes by clamping the slide at each end with an 

 ordinary brass mounting clip (c c), round which thin wire 

 is twisted. If induced electricity be employed, the wires 

 are attached to the secondary coil of Du Bois Reymond's 

 induction apparatus, with a key interposed in the secondary 

 circuit ; while in the case of voltaic electricity, the wires are 

 attached to the poles of one or more voltaic cells, with a 

 key introduced into the circuit. 



METHODS OF STUDYING LIVING TISSUES. 



264. In studying the normal characters of living tissues 

 when removed from the body of an animal, they are either 

 examined in the fluids that bathe them during life e.g., 

 blood, salivary, mucus, and pus corpuscles are investigated 

 in their own fluids, or the tissues are studied in other 

 fluids calculated to produce little or no change in their 

 vital properties. Such fluids are termed "neutral" or 

 " indifferent" 



265. Serous Fluids. The fluid which bathes most 

 tissues is lymph. Blood serum seeing that it has a com- 

 position resembling that of lymph is sometimes employed 

 for moistening living tissues ; but the blood corpuscles 

 that it contains are an inconvenience. Aqueous humour 

 is the most available serous fluid when only a small quantity 

 is desired. It can always be readily obtained from the eye 

 of a sheep or ox. It is somewhat difficult to puncture the 

 cornea without losing the aqueous humour. Beer's knife 

 (Fig. 57) is suitable for the purpose. 



266. Serous fluids are also employed in the examination 

 of non-living tissues when these are prone to undergo 

 change e.g., nerve fibrils, red marrow cells, spleen pulp, 

 etc. When they are required in considerable quantity, 

 amniotic fluid, pericardial fluid, iodised serum, or dilute 



