Special Senses. 93 



HUMAN SPERMATOZOA. 



A small drop of semen is mixed with glycerine, to 

 which a little absolute alcohol has been added ; it is 

 then covered and sealed up. It is best in this case to 

 use thin covers that have been measured, and to note 

 their thickness on the label, as the human spermato- 

 zoon is so very minute it requires the highest powers 

 to make out the filament. 



SPECIAL SENSES. 

 INTERNAL EAR. COCHLEA. 



The guinea-pig is the best animal, as the large tym- 

 panic bulla is easily exposed. Eemove the periosteum 

 from the bulla, and open it carefully with the point of a 

 pair of straight scissors ; as soon as a small opening 

 has been made it can be enlarged, and the cochlea will 

 be at once seen : as much of the surrounding bone 

 must be removed as can be done without injury to the 

 parts required, and it will be ready for preparation. 



This may be done in several ways, but the two fol- 

 lowing are the best. 



1. Place it at once in absolute alcohol, and let it 

 remain 24-48 hours, then place it in ^ per cent, solu- 

 tion of osmic acid for 24 hours. After this place it 

 in a half per cent, solution of chromic acid, to which 

 1-2 drops of hydrochloric acid have been added. Let 

 it remain in this until the bone is softened throughout. 



2. Place the piece of bone in the ordinary chromic 

 acid mixture for a week, and then remove it to per 

 cent, chromic acid, to which 1-2 drops of hydrochloric 



