154 ORGAN OF HEARING. 



ner, the general arrangement of the rods and cones, and their 

 relation to the elements of the outer granular layer, that of 

 the intermediate layer to the granules of the inner granular 

 layer; the finely granular layer, and the relation of its fine 

 fibrillae to the fibrils of the inner granular layer on the one 

 side and the processes of the ganglion cells on the other ; and 

 finally, the layer of nerve fibres. The general arrangement of 

 the radial fibres, or, rather, bundles of radial fibres, may be 

 also made out : each bundle, attached to the limitans internet 

 by a broad basis, enters the finely granular layer, thence pass- 

 ing through the inner granular layer (where the bundles be- 

 come ramified, and inclose nuclei), then on through the inter- 

 mediate layer and outer granular layer (where again ramifica- 

 tions and junctions are met with) to become attached, finally, 

 to the limitans externa. (See description of Figs. 139 and 

 140). 



Organ of Hearing. The outer part of this organ, including 

 the external ear, meatus, and Eustachian tube, should be 

 studied in portions taken from a 3'oung human subject. To 

 prepare the membrana tympani (human, or from a cat or dog), 

 it must be exposed by the aid of saw and bone-forceps a 

 manipulation requiring an accurate knowledge of the topo- 

 graphical details of the temporal bone. This done, the mem- 

 brane is excised, and either stained with silver at once, to 

 show the epithelium of the two surfaces, or pencilled on its 

 outer surface with a brush moistened with serum, to show the 

 lymphatics. If the gold method is used, the epithelium is also 

 pencilled on the outer surface, and the membrane immersed in 

 the solution from half an hour to an hour. It must then be 

 treated in the usual way. 



The study of the membranous labyrinth, especially the canal 

 of the cochlea and the semicircular canals is a matter re- 

 quiring an immense deal of care and practice. It should be 

 undertaken both in foetal and adult organs. For the examina- 

 tion of it in the embyro, a foetal calf or pig from ten to fifteen 

 centimeters long may be used. The whole cartilaginous lab} T - 

 rinth may be readily separated from the rest of the skull after 

 the maceration of the latter in solution of bichromate of potash 

 (half to one per cent.) for a week or two. After separation it 

 is placed in spirit for a few days. A second opening (besides 

 the already existing fenestra rotunda) should then be made 

 on the side opposite to it, or, better, at a point corresponding 

 to the top of the cochlea. The whole organ is now stuck on 

 a needle and immersed in a warm but of course, not hot 

 mixture of wax and oil, so as to fill up, at least in part, the 

 canals which exist in the organ ; this is then embedded in the 

 ordinary way, marks being made on the mass for the purpose 

 of indicating the exact position of the preparation. Sections 



