OF MICROSCOPIC OBJECTS. 109 



mens are often taken from ihefrog and the pig, as being amongst 

 the best, Goadbifs solution being generally used in mounting them. 

 The muscle of insects also shows the striae very perfectly. 



NERVE-TISSUE. This is seldom mounted; as Dr. Carpenter 

 observes, "no method of preserving the nerve-tissue has been 

 devised which makes it worth while to mount preparations for the 

 sake of displaying its minute characters," but we will mention a 

 few particulars to be observed in its treatment. The nerve should 

 be taken from the animal as soon as possible after death, and laid 

 upon a glass slide, with a drop or two of serum if possible. The 

 needles may be used to clean it, but extreme delicacy is necessary. 

 It will be found that the nerve is tubular and filled with a substance 

 which is readily ejected by very slight pressure. When the nerve 

 is submitted to the action of acetic acid, the outer covering, which 

 is very thin, is considerably contracted, whilst the inner tube is 

 left projecting; and thus is most distinctly shown the nature of 

 the arrangement. Mr. Lockhart Clarke, who has made great 

 researches into the structure of the spinal cord, gives a part of his 

 experience as follows : He takes a perfectly fresh spinal cord and 

 submits it to the action of strong spirits of wine. This gives the 

 substance such a degree of hardness that thin sections may be 

 readily cut from it, which should be placed upon a glass in a 

 liquid consisting of three parts of spirit and one of acetic acid, 

 which renders them very distinct. To mount these sections, they 

 must now be steeped in pure spirit for two hours and afterwards 

 in oil of turpentine, and lastly must be mounted in Canada balsam. 



TRACHEA OF INSECTS. The nature of these was described in 

 Chapter III., but the method of procuring them was not explained, 

 as this clearly belongs to " dissection." The larger tubes are 

 readily separated by placing the insect in water, and fixing as 

 firmly as possible, when the body must be opened and the viscera 

 removed. The tracheae may then be cleaned by the aid of a camel- 

 hair pencil, and floated upon a glass, where they must first be 

 allowed to dry, and then be mounted in balsam. Mr. Quekett 

 gives the following method of removing the trachea from the 

 larva of an insect ; " Make a small opening in its body, and then 

 place it in strong acetic acid. This will soften or decompose all 

 the viscera, and the tracheae may then be well washed with the 



