3^8 PRACTICAL HISTOLOGY. f XXXII. 



The outer segments are stained black by osmic acid, and the 

 inner segments red by carmine. 



Study successively the other layers to make out some of the 

 details already described on p. 366. 



12. Ketina of Frog. (i.) Place the posterior segment of the 

 eyeball, with its vitreous humour removed, in i per cent, osmic 

 acid (24 hours) ; wash well, and tease a fragment of the now 

 blackened retina in glycerine. Observe 



(a.) The rods, each with an outer and inner segment; the outer 

 segment, however, is blackened by the osmic acid, and usually 

 shows a tendency to split transversely. 



(b.) In the cones there is a small refractive oil globule between 

 the outer and inner segments. It first becomes brown, and then 

 black, in osmic acid. In some animals, e.g., some birds and 

 reptiles, the oil globule is pigmented, the pigment being held in 

 solution by the fat of the globule. 



(c.) Numerous pigment cells, each consisting of a rather thick 

 body, part of which contains the nucleus and is non-pigmented, 

 while the other half is pigmented, and sends numerous fine pro- 

 cesses between the outer segments of the rods. 



(d.) Other parts of the retina are also isolated, e.g., the nuclei of 

 the nuclear layer and the limiting membranes. 



(ii.) Fix the whole eyeball in osmic acid, and make V.S. of all 

 the coats after embedding in paraffin or celloidin. 



13. Cones. To see large cones, place the fresh eye of a cod in 2 

 per cent, potassic bichromate (2-3 days) or osmic acid (24 hours), 

 or isolate them by dilute alcohol. 



14. T.S. Optic Nerve (L and H). Place the optic nerve in 2 per 

 cent, ammonium bichromate (2-3 weeks) or in Flemming's mixture 

 (10-20 hours). Make T.S. Stain in hsematoxylin, picro-carmine, 

 or safranin, or by Weigert's method. Mount in balsam. Double- 

 stain some in picro-carmine and haematoxylin. 



(a.) Observe the sheaths, thick and well-marked, sending septa 

 into the nerve, and thus breaking it up into small bundles of nerve- 

 fibres, each surrounded by a fibrous sheath. In the larger septa 

 are blood-vessels. The anastomosing septa form a kind of alveolar 

 system, so that a section of this nerve is readily distinguished. 



(b.) The nerve-fibres are all medullated, but they possess certain 

 peculiarities. They are without sheath of Schwann, and the 

 fibrous tissue runs parallel with the nerve-fibres. 



(c.) As to the sheaths, one is continuous with the dura mater; 

 under it is a space, the subdural space ; inside is a prolongation of 

 the arachnoid with subarachnoid space, and a thin continuation of 

 the pia mater. 



If desired, make L.S. This enables one to note the structure of 



