420 THE SKIN AND THE SENSE-ORGANS. 



Morano twelve to fifteen rods and cones may be encased or sur- 

 rounded by the processes of a single cell. 



The thickness of this layer varies from 60-70 /* ; the nuclei of 

 the cells have a diameter of 10-12 /ix, the width of a single cell is 

 from 20-25 p, (Hoffmann). 



(10) The connective-tissue elements of the retina and the ex- 

 ternal and internal limiting membranes. The elements of the 

 retina are supported by connective-tissue elements or sustentacular 

 cells, which have a radial arrangement, and which form the two 

 limiting membranes (Miiller). 



Each sustentacular cell (Fig. 259 10) has two segments, an inner 

 and an outer, the boundary between these lying in the inner nuclear 

 layer, and being marked by the presence of a large oval nucleus. 

 The inner segment of each cell terminates internally in a wide ' foot ' 

 or base, or may form several such after having undergone division 

 (Schultze) : these bases together form a transparent, thin mem- 

 brane, the internal limiting membrane (memlrana Hmitans interna). 



Within the ganglion -layer these cells possess peculiar appen- 

 dages, which fit round the ganglion-cells and support them 

 (Schwalbe). 



The outer segments of the sustentacular cells extend into the 

 outer molecular layer, and then break up into irregular processes 

 which extend radially to the external limiting membrane, and 

 which they probably form. The external limiting membrane 

 (membrana Hmitans externa] is therefore a membrane corresponding 

 to the internal limiting membrane, and formed by the flattened 

 ends of the processes belonging to the sustentacular cells. 



The sustentacular cells have a distinct, resistant cell-wall 

 (Schwalbe) ; the cell-contents are a finely granular protoplasm, and 

 a large oval, nucleated nucleus placed in the inner nuclear layer. 



f. The ciliary processes have the same structure as the rest of 

 the choroid coat : the vessels form more or less longitudinal meshes 

 and are more irregular than in the rest of the choroid. 



g. The posterior chamber and vitreous body (Fig. 260). The 

 vitreous humour occupies the greater portion of the cavity of 

 the eyeball, i. <?. the posterior chamber. The humour consists of a 

 mass of cells enclosed in a transparent hyaloid membrane (mem- 

 lrana hi/aloidea), which is in contact with the internal limiting 

 membrane of the retina. 



The cells forming this structure are small, flattened, transparent, 



