GENERAL ANATOMY OF THE SQUID. 359 



a central opening, the pupil (m) . The posterior surface 

 of the iris is covered with a layer of epithelial cells, filled 

 with black pigment. 



(iv. ) The lens (7) lies behind the pupil, and separates the 

 anterior chamber from the posterior. It is divided by a 

 deep equatorial groove into a small anterior portion, which 

 projects into the anterior chamber, and a much larger 

 portion, which lies in the posterior chamber. It is held 

 in place by the ciliary body (w)> which runs into the 

 groove between the two divisions of the lens. 



(v.) The ciliary body is thin near the centre of the eye, 

 but peripherally it becomes thick, and contains a ciliary 

 ganglion (71), which consists of large granular nucleated 

 ganglion cells. The posterior or internal surface of the 

 ciliary body is covered by a layer of black pigment. 



(vi.) The posterior chamber (&) is filled, in the living 

 animal, by the transparent vitreous humor, but in preserved 

 specimens the vitreous humor is somewhat opaque, finely 

 granular, and shrunken, filling only a small part of the 

 chamber. 



(vii.) The sides and back of the posterior chamber are 

 formed by the retina (Fig. 188, /*, i). This is of nearly 

 uniform thickness, and it ends abruptly around the ante- 

 rior edge, where it joins the ciliary body. It consists of 

 three layers. 



( viii. ) The inner layer (i) will be seen to be marked by fine 

 parallel striations, perpendicular to the surface of the eye- 

 ball. Examination with higher power will show that this 

 striation is produced by fine lines of black pigment, which 

 run inwards to the posterior chamber. Between the lines 

 of pigment are the transparent rods, which compose the 

 greater part of this layer. On the surface of the posterior 

 chamber the ends of the rods are covered by a delicate 

 layer of black pigment. 



