THE INTERNAL OR NERVOUS TUNIC OF THE EVE. 457 



pupils. The cell-bodies of these sympathetic neurones are sur- 

 rounded by pericellular plexuses, the terminations of white rami 

 fibers which leave the spinal cord through the first, second, and 

 third thoracic nerves (Langley), and which reach the superior cer- 

 vical ganglia through the cervical sympathetic. 



Melkirch and Agababow have shown that numerous sensory 

 nerves terminate in free sensory endings in the connective tissue 

 of the ciliary body and iris. The sensory nerve-supply of the iris 

 is especially rich. 



E. THE INTERNAL OR NERVOUS TUNIC OF 

 THE EYE. 



This tunic is composed of two layers : the outer, or stratum pig- 

 menti ; and the inner, or retina. 



\. THE PIGMENT LAYER. 



The pigment layer develops, as we have seen, from the outer 

 layer of the secondary optic vesicle. It consists of regular hexa- 

 gonal cells, 12 fj. to 1 8 ii in length and 9 // in breadth, which con- 

 tain black pigment in the form of granules. The inner surfaces 

 of these cells possess long, thread-like and fringe-like processes, 

 between which project the external segments of the rods and cones 

 of the retina, yet to be described. The nuclei of the pigment cells 

 lie in the outer ends of the cells, the so-called basal plates, and are 

 not pigmented. The distribution of the pigment varies according to 

 the illumination of the retina. If the latter be darkened, the pig- 

 ment collects at the outer portion of each cell ; if illuminated, the 

 pigment is evenly distributed throughout the whole cell. The pig- 

 ment granules are therefore mobile (Kiihne, 79). 



2. THE RETINA. 



The retina has not the same structure throughout. In certain 

 areas peculiarities are noticeable which must be described in detail ; 

 such areas are : (i) the macula lutea ; (2) the region of the papilla 

 (papilla nervi optici) ; (3) the ora serrata ; (4) the pars ciliaris 

 retinae ; and (5) the pars iridica retinae. 



We shall begin with the consideration of that portion of the 

 retina lying between the ora serrata and the optic papilla (exclusive 

 of the macula lutea). 



From without inward, we differentiate: (i) the layer of vis- 

 ual cells, including the outer nuclear layer ; (2) the outer molecu- 

 lar (plexiform) layer ; (3) the inner nuclear or granular layer ; (4) 

 the inner molecular (plexiform) layer ; (5) the ganglion-cell layer ; 

 (6) the nerve-fiber layer. Besides these, we must also consider the 



