EYES OF TYPHLOPS. 55 



The sensory epithelium consists of the outer nuclear layer and the cone layer 

 which is made up of single and twin cones. There are no rods. A single cone 

 consists of two sections, an outer extremely small section, 5 to 6 fi in length and an 

 inner much larger section, almost completely filled with a larger, pear-shaped, 

 strongly refractive body, the ellipsoid, 14 to 16 fj. in length and 8 to /x across its 

 widest part, which is turned toward the limiting membrane. The twin cone con- 

 sists of two parts, one similar to a simple cone, the other cylindrical and very slender, 

 its structure being otherwise like that of a simple cone. It is probable that the two 

 parts of the twin cone are connected with but one nucleus. The nuclei of the cones 

 vary greatly in form, and leading from these into the inner layers of the retina are 

 relatively very large fibers or processes. Passing between the limiting membranes 

 are the radial supporting Miillerian fibers. 



THE EYES OF TYPHLOPS VERMICULARIS. 



The work thus far on blind snakes has been done by Kohl on Typhlops vermi- 

 cularis, a species found in Greece and the southwestern part of Asia, and on Typh- 

 lops braminus, a species found in the islands of the Indian Ocean and in Africa 

 south of the equator, accounts of which are given in his "Rudimentare Wirbelthier- 

 augen." ' He found that in depth the eye of Typhlops vermicularis is equal to 

 about one-sixth that of Tropidonotus. The brille is thicker in Typhlops than in 

 Tropidonotus and compared with the axial diameter of the respective eyes it is seven 

 times thicker. In Typhlops the brille is equal in thickness to about half that of 

 the ordinary skin of the head. In Tropidonotus it is equal to one-fourth. 



The cornea of Typhlops measures 0.0052 mm., and compared with the relative 

 sizes of the eyes is equal to about half that of Tropidonotus, which measures 0.064 

 mm. The conjunctiva is thickened at the edge of the disk-shaped sac and consists 

 here of gland cells, the fornix conjunctiva. The supporting membranes of the 

 eyeball, choroid, and sclera are relatively equal to about half those of Tropi- 

 donotus. 



Harder's gland in Typhlops is many times larger than the eyeball. The six 

 muscles are present. The lens is elliptical, while that of Tropidonotus is almost 

 globular. The ratio of the lens volume of Typhlops to the eye volume is 1 to 14.04, 

 while in Tropidonotus it is 1 to 3.6. The lens epithelium of the former is relatively 

 6 times greater than that of Tropidonotus. 



The retina at the back of the eye of Typhlops, and the retina of Tropidonotus 

 bear the actual ratio of 8 to 13, while compared with the eye axis in each case the 

 Typhlops retina is 4 times greater. The fovea centralis and area are absent. 



The fiber layer has its greatest thickness near the exit of the nerve and gradually 

 becomes thinner until, near the iris, scarcely a fiber is found. The globular gan- 

 glion cells are arranged in a single layer except occasionally for short distances, 

 when they lie in a double row. The inner nuclear layer seems to be subdivided 

 into four layers. 



There are no twin cones. Each cone consists of a cone cell, stalk, middle and 

 end members. The cone nuclei lie in two series, but the stalks vary in length so 

 that the distal ends of the cone members reach nearly the same level. 



1 Kohl, Dr. C, Rudimentare Wirbelthieraugen, Erster Theil, Heft 13, Bibliotheca Zoologica. Verlag von 

 Theodor Fischer, 1892, Cassel. 



