THE ORGANS OF SPECIAL SENSE 5()1 



(2) The stroma is divisible into two layers: an anterior reticular 

 layer, containing many cells, seme of which are pigmented, and a vas- 

 cular layer, the vessels of which are peculiar in that their walls con- 

 tain almost no muscle, but have thick connective-tissue sheaths. In 

 the posterior part of the stroma are bundles of smooth muscle. Those 

 nearest the pupillary margin encircle the pupil, forming its sphincter 

 muscle, while external are scattered radiating bundles forming the 

 dilator muscle. 



(3) The vitreous membrane is continuous with, and has the same 

 structure as the membrane of Bruch. 



(4) The pigmented epithelium (Fig. 372, d) consists of several 

 layers of cells and is continuous with the pars ciharis retinae. Except 

 in albinos, both layers are pigmented. 



The Retina. — The retina is the nervous tunic of the eye. It 

 lines the entire eyeball, ending only at the pupillary margin of the 

 iris. Its nervous elements, however, extend only to the ora serrata, 

 which marks the outer limit of the ciliary body (Fig. 371). The 

 nervous part of the retina is known as the pars optica retincB, the 

 non-nervous extension over the ciliary processes as the pars ciliaris 

 retince, its further continuation over the iris as the pars iridica retina. 

 Modifications of the optic portion of the retina are found in the region 

 of the macula lutea and of the optic nerve entrance. 



TJie Pars Optica Retince. — This is the only part of the retina 

 directly concerned in the reception of impulses, and may be regarded 

 as the extremely complex sensory end-organ of the optic nerve. It 

 is divisible into ten layers, which from without inward are as follows 

 (Fig. 373): 



(i) Layer of pigmented epitheUum. 



(2) Layer of rods and cones. 



(3) Outer limiting membrane. 



(4) Outer nuclear layer. 



(5) Outer molecular layer. 



(6) Inner nuclear layer. 



(7) Inner molecular layer. 



(8) Layer of nerve cells. 



(9) Layer of nerve fibres. 

 (10) Inner limiting membrane. 



Layer of neuro-epithelium. 



Ganglionic layer. 



The layer oj pigmented epithelium (Fig. 373, B, i) consists of a 

 single layer of regular hexagonal cells (Fig. 27, p. 80 J. The nuclei 



36 



