J 72 ANDREWS. [Vol. VII. 



dorsal part of the brain to the area of the retina opposite to the 

 pupil and thence spreading out over the retina. The nerves to 

 the anterior eyes thus pass to their inner dorsal sides, those to 

 the posterior eyes to their inner ventral sides. 



The remarkable anterior process of the head is occupied by 

 a large extension of the body cavity, the thin body wall being 

 lined by a peritonaeum that passes in over the eye to cover the 

 brain. 



A section of an eye — for all have the same structure — pre- 

 sents the general appearance seen in Fig. 8. This may be 

 described under three chief heads : retina, lens, cornea. 



Retina. — This is composed of pigmented cells with clear 

 refracting ends or rods, which may be, for convenience, consid- 

 ered separately. The retinal cells collectively make a pigmented 

 epithelium, forming the walls of a deep cup, the mouth of which 

 is the pupil. The pupil, as already stated, varies much in size, 

 but, as seen in the figure, is bounded by an inward bending of 

 the edge of the retina, an iris-like portion of the entire retina. 



When, as in the figure, the sections are made through the 

 visual axis of the eye, the retina appears as a single layer of 

 cells, the nuclei, in their peripheral ends, forming one zone. 

 The dark blue pigment (red by reflected light) is especially 

 dense along a line sharply separating the cells from the clear 

 rods, and thence extends peripherally as irregular lines, suggest- 

 ing the presence of special pigment cells and processes extend- 

 ing between the visible retinal cells. This is true of the dark 

 blue pigment only : the yellow pigment appears in the interior 

 of each cell, in its central end especially, but sometimes also 

 extending out peripherally nearly to the nucleus. These facts 

 are seen more clearly in very thin, highly magnified sections of 

 part of the retina (Fig. 22), where also there is indication of the 

 prolongation of each cell as a nerve process running parallel to 

 the surface of the retina. Moreover, there are, here and there, 

 in such sections, minute tubules or clear axial spaces passing 

 from each cell through the densest blue pigment to the rod upon 

 the central end of that cell. One is shown in Fig. 22. 



In tangential sections cutting the dense pigment zone (Fig. 

 17) these clear passages are seen as round holes equally but 

 irregularly distributed. Through these shines the yellow light 

 from the axial yellow pigment of the retinal cells, or, if the 



