122 PHILOSOPHY OF ZOOLOGY. 



ra of molluscous animals, insects and worms, it is impos- 

 sible, without the use of water, to obtain any satisfactory 

 views of their position. The subjects may be kept steady 

 in the water, by having the bottom lined with cork or wax 

 to hold the pins. Where very transparent objects are ex- 

 amined, the bottom of the vessel may be of glass, covered 

 with wax, except a circular spot in the middle, on which ^ 

 the parts to be examined may be placed, and light trans- 

 mitted through them from below. 



The instruments employed in dissection, differ in their 

 kinds and form, according to the views or ingenuity of the 

 operator. Scalpels of various forms are used for the mere 

 cutting part of the operation. Fine pointed sharp scissars, 

 make a cleaner cut than any knife, and leave the parts 

 more nearly in their natural and relative position. In tra- 

 cing the course of vessels or muscular fibres, where the ob- 

 ject is not to cut, but to unravel or separate, a needle is 

 usually employed, or, in some cases, where the surround- 

 ing matter is very soft, the point of a hair-brush forms an 

 advantageous substitute. 



Besides the preparatory circumstances which have been 

 thus shortly noticed, a variety of processes of a chemical and 

 mechanical nature, are employed in the course of dissec- 

 tion, which, in many cases, are indispensably necessary to 

 ensure success. The following merit some notice in this 

 place. 



1 . Maceration. — Water is the fluid most extensively used 

 in this process. When the soft parts of animals are im- 

 mersed in it for a time, they swell and exhibit more clearly 

 their intimate structure. If the immersion is prolonged, 

 the more soluble parts are abstracted, and the framework 

 of the organ left more fully exposed to view. It is some- 

 times necessary to press out the water gently, after it has 



