3/0 PRACTICAL HISTOLOGY. [XXXII. 



(b.) Methylene-Blue Method. Dogiel 1 by means of this method finds 

 that in man there are more nervous elements present than one has been in 

 the habit of supposing. He arranges the nervous elements (in three layers) 

 and other layers as follows : 



Membrana limitans Pigment epithelium (I). 

 externa. > Neuro-epithelial layer (II). 



External reticular) g! 1 ij 6 ? 1 ie ^ cells ("fe^ > ^" ^ u ^ er ganglionio 



Bipolar nerve-cells (c). 



Inner reticular 

 layer. 



. Layer of spon gio- 

 Middle ganglionic layer. blasts of W. 



Miiller. 



Inner ganglionic layer. C. 



Layer of nerve-fibres. D. 



Membrana limitans 

 intern a. > 



The neuro-epithelial layer embraces the layer of rods and cones and subjacent 

 external nuclear layer. What he calls the outer ganglionic layer corresponds 

 to the inner nuclear layer, and contains three varieties of nerve-cells. The 

 middle ganglionic layer corresponds to a layer of cells of which there are 

 several varieties lying on the confines between the internal granular (reticular) 

 layer and the internal nuclear layer. These cells were called spongioblasts 

 by W. Miiller, but according to Dogiel, they are nerve-cells. The inner 

 ganglionic layer corresponds to the cellular layer. 



20. Epithelium of the Eetina. Harden the eye of a frog in picro-sulphuric 

 acid (24 hours), stain in bulk in borax -carmine, and make vertical sections 

 after embedding in paraffin. 



21. Isolated Elements of the Eetina. (a. ) Macerate the retina in Schieffer- 

 decker's fluid as described for the isolation of the cells of the cord ( Lesson XXX. ), 

 using the sulphuric acid method to remove the water. 



(b.) For M tiller's fibres 10 per cent, chloral hydrate is good. 



22. Retina of Frog in Light and Darkness. (a.) Keep one frog in abso- 

 lute darkness for thirty-six hours. Kill it in the dark, and harden the eye in 

 alcohol, (b.) Place another frog in direct sunlight for a few hours ; kill it, 

 and harden the retina in alcohol. Sections are made and stained with picro- 

 carmine. The pigment-cells covering the rods of the retina in (a) jire 

 retracted, while those in (b) are pushed out between the segments of the rods. 



23. V.S. Macula Lutea. Secure a human eye as fresh as possible. Harden 

 it, and make sections through the macula lutea, and observe the peculiarities 

 of its structure. There are no rods in the fovea centralis, while the cones are 

 long and narrow. The other layers are reduced to a minimum at the fovea 

 centralis, but at the margins of the fovea they are thicker. There are a large 

 number of ganglionic bipolar cells. 



24. V.S. Entrance of Optic Nerve. Make from an eyeball stained in 

 bulk in borax-carmine a section longitudinally through the optic nerve to 

 include its entrance into the eyeball. Observe the lamina cribrosa, i.e., the 

 felt- work of fibrous tissue of the sclerotic, perforated by the fibres of the optic 

 nerve. 



25. Eye of Triton Cristatus. Excise one eyeball, pin it to the under surface 

 of a cork, and fix the cork in a glass thimble containing a little osmic acid. 

 In ten minutes or so the retina is "fixed," owing to the tenuity of the 

 sclerotic. Divide the eyeball into two by a cut at the equator, place the 

 posterior half in dilute alcohol (6 to 10 hours), and then in picro-carmine for 



1 Archivf. inik. Anat., xxxviii. p. 317, 1891. 



