THE LAMPREY EYE 



557 



cartilaginous one, as have those of the placental mammals and the 

 snakes. Here in the lampreys however it may be regarded as primitive — 

 particularly if one adheres to the dural theory of the evolutionary origin 

 of the sclera (see p. 119), rather than to the older idea of a cartilag- 

 inous 'optic capsule' accompanying the hypothetical original chondro- 

 cranium. The cornea is very thin in all lampreys, consisting of little more 

 than a Descemet's mesothelium and a thick Descemet's membrane. 

 Since the cornea and the skin have not fused, there is of course no 

 corneal epithelium — contributed in higher forms by the epidermis of 

 the skin. 



Fig. 161 — The eye and surrounding structures in a lamprey, Lampetra fluriatilis, in 

 horizontal section; the anterior end of the animal is to the left. Modified from Franz. 



av- anterior surface of vitreous; er- external rectus; to- inferior oblique; ir- internal rectus; 

 n- optic nerve; s- speaacle; sk- skin; sp- subsjiectacular space (virtual, and occupied by a 

 mucoid continuation of the orbital capsule); sr- superior rectus; /- tendon of corneal muscle, 

 inserted in spectacle; v, v- venous sinuses. 



In the European river lamprey, Lampetra fluviatilis, the inner surface 

 of the cornea, near the iris angle, bears a conspicuous thickening com- 

 posed of epithelioid cells, much like the annular ligament of a teleost 

 (see Fig. 169, p. "y??) . The cells may represent a piling-up of Descemet's 

 mesothelium, though it has been claimed that the mesothelium passes 

 over them and reflects onto the anterior face of the iris. The writer can 

 see nothing of such an arrangement. No function has been suggested for 

 the thickening. Delicate strands, perhaps coated with mesothelium, cross 

 from it to the periphery of the iris, like a pectinate ligament. These 



