212 INTRODUCTION TO GENERAL PHYSIOLOGY 



Hearing 



Preparations of the cochlea are difficult to make. They may 

 sometimes be bought. That of the guinea-pig is the easiest to 

 prepare on account of its size. The method used is as follows : 

 The part of the skull containing the petrous bone is cut out with 

 bone forceps. A hole is filed at the top of the cochlea, which is 

 seen as a conical eminence. The preparation is then placed in 

 fixing fluid, and the bone afterwards decalcified by immersion in a 

 solution made by dissolving I gm. of phloroglucin in nitric acid 

 with the aid of heat and diluting to 100 c.c. with water. The object 

 of the phloroglucin is to counteract the effect of the acid in causing 

 the tissues to swell. Wash well and transfer to alcohol, gradually 

 increasing in strength. Then through cedar oil and paraffin as 

 usual. The transition from one liquid to the other must be 

 gradual, by using mixtures with the preceding one, since the organ of 

 Corti is very readily broken up. Sections are cut through the axis 

 of the spiral, so that it is not possible to obtain many from one 

 preparation. 



The resonance of a stretched membrane may be seen by taking 

 a triangular piece of sheet india-rubber, and attaching two of its 

 sides to strips of wood by screwing down on to each of the strips 

 a second one of the same size. One side is firmly clamped to the 

 edge of the bench, the other held in the hand and used to stretch 

 the membrane. Sand is dusted on to the membrane, and a pitch- 

 pipe or other similar source of a musical note sounded near. By 

 adjustment of the pitch of the note, and by different degrees of 

 tension on the membrane, a particular place may be found which 

 vibrates in sympathy with the note, as shown by scattering of the 

 sand. Suppose that this place is near the wider end when a low 

 note is sounded, then by raising the pitch a narrower region will 

 resonate. 



Photo- Receptors. The Eye 



Visual Purple. Keep a frog in the dark for a day. Kill it by 

 pithing in a room lit only by a sodium flame or a dim photographic 

 red lamp. Excise the eye-balls. Cut each one into a front and a 

 back half by means of a razor. Put the latter half into a dish of 

 0.7 per cent, sodium chloride. Seize the outer coat at its cut edge 

 with forceps and shake the retina loose. If it does not come free 

 from the place where the optic nerve enters, a pointed scalpel or fine 

 scissors may be used to cut it free. Taking the dish into ordinary 

 light, the beautiful crimson colour of the retina will be seen. It 

 will become bleached more or less rapidly according to the intensity 

 of the light. 



