LABORATORY WORK 195 



slide under them, lifting out and draining off the water. Press into 

 contact with the glass by means of filter paper, and lay aside in a 

 warm place until completely dry. Subsequent treatment will not 

 then wash them away, except in rare cases. 



Next dissolve away the paraffin by warming until it melts and 

 pouring over some solvent ; xylol is generally used. Wash away 

 the xylol with acetone this by alcohol, first strong and then dilute, 

 and finally water. Various stains can then be applied. We may 

 use : i per cent, eosin for ten minutes, rinse with water ; then I per 

 cent, toluidine blue for twenty minutes. Remove excess of stain 

 with absolute alcohol. Drop on solution of dried Canada balsam in 

 acetone and apply cover-glass. The balsam will harden in a few 

 hours. The use of other stains will be found in histological text- 

 books, such as Schafer's " Essentials of Histology." 



To see the globules of fat in the intestinal epithelium, the follow- 

 ing method is adopted : Feed a rat with butter and kill it with 

 chloroform four hours later. Place a piece of the upper part of the 

 small intestine into a mixture of equal parts of I per cent, osmic 

 acid and 3 per cent, potassium bichromate and leave for ten days. 

 Unsaturated fats reduce osmic acid and become stained black. 

 Solvents of fat cannot be used, so that the tissue must be soaked in 

 strong gum, and sections cut by freezing with ether spray on a 

 simple microtome arranged for the purpose. The sections are then 

 mounted in glycerine. 



The synthesised fat can also be seen by placing a small bit of 

 the mucous membrane in 0.5 per cent, osmic acid for forty-eight 

 hours and then in water for a few days. A shred is placed in 

 glycerine on the slide, a cover-glass over it, and broken up into cells 

 by tapping the cover-glass. 



Voluntary and Involuntary Muscle 



The microscopic appearance of the former is best seen in the 

 muscles of an insect, such as a wasp or beetle. Cut off the head, 

 and divide the trunk with scissors lengthwise. Notice muscles 

 attached both to the legs and to the wings. Take a shred of the 

 former, and tease out with needles on a slide into separate fibres, if 

 possible. Add a drop of the insect's blood, cover and examine 

 with a high power. The cross striation will be seen, and, by care- 

 ful focussing, the longitudinal fibrils (" sarcostyles ") embedded in 

 the " sarcoplasm." 



The cells of the involuntary muscle may be seen thus : Allow a 

 small piece of intestine to macerate in per cent potassium 

 bichromate for two days. Hold it in water on a microscope slide 

 with forceps and fray out with a needle. The cells separated in 



