THE SENSES. 



701 



space of the anterior chamber. Above the angle at the corneo-scleral 

 junction is a canal, which is called the canal of Schlemm. It is a lym- 

 phatic channel, but appears to be in communication with blood-vessels, 

 as it may be under certain circumstances filled with blood. 



Structure of the Retina. The retina (fig. 416) is a delicate membrane, 

 concave with the concavity directed forward and apparently ending in 

 front, near the outer part of the ciliary processes, in a finely notched 

 edge, the or a serrata, but really represented to the very margin of the 

 pupil. Semitransparent when fresh, it soon becomes clouded and opaque, 

 with a pinkish tint from the blood in its minute vessels. It results from 

 the sudden spreading out or expansion of the optic nerve, of whose ter- 



Fig. 416. A section of the retina, choroid, and part of the sclerotic, moderately magnified 

 , Membrana limitans interim; ft, nerve-fibre layer traversed by Miiller's sustentacular fibres; 

 c, ganglion -cell layer; d, molecular layer; e, internal nuclear layer; /, internuclear layer; gr, ex- 

 ternal nuclear layer; 7t, membrana limitans externa, running along the lower part of i, the layer 

 of rods and cones; k, pigment-cell layer; Im, internal and external vascular portions of the chor- 

 oid, the first containing capillaries, the second larger blood-vessels, cut in transverse section; n, 

 sclerotic. (W. Pye.) 



minal fibres, apparently deprived of their external white substance, to- 

 gether with nerve cells, it is essentially composed. 



Exactly in the centre of the retina is a round yellowish elevated spot, 

 about fa of an inch (1 mm.) in diameter, having a minute depression in 

 the centre, called after its discoverer the macula lutea, or yellow spot of 

 Simmering. The minute depression in its centre is called the fovea 

 centralis. About ^ of an inch (2.5 mm.) to the inner side of the yel- 



