Ill 



periphery, and terminates at tlie point where the retina 

 comes in connection with the sides of the optic vesicle. 

 The tapetum is composed of large irregular cells arranged 

 rather irregularly in two layers, and in many cases the 

 margins are difficult to define owing to the dense pigment 

 contained in them. The cells contain each a nucleus, 

 often obscured by the pigment, which is present in the 

 form of rather large granules, in shape something like 

 those found in the digestive gland cells, but of quite 

 a different coloiir — a dark brown-red. The pigment 

 granules are much larger than those found in the cells of 

 the iris. 



There only remains to' be described the inner wall of 

 the eye vesicle against which the tapetum rests. This is 

 known as the sclerotica (fig. 29, Scl.), and is a differen- 

 tiation of the connective tissue of the stalk, which becomes 

 tough and hyaline and stains rather more deeply than the 

 surrounding tissue. It passes into the septal membrane 

 at the edge of the retina, so that septal membrane and 

 sclerotica together form a closed vesicle in which is 

 situated retina, argentea and tapetum, the whole being 

 known as the ommateal sac. 



Optic Serves. — The eyes are innervated by optic 

 nerves, which arise from the circumpallial nerve and pass 

 through the centre of the eye stalk (fig. 29, Op. n.) until 

 the eye vesicle is almost reached. Here the nerve divides 

 into two branches, one of which, the inner nerve 

 (Ojy. n. i.), continues its course until immediately below 

 the sclerotica, where it breaks up into many bundles of 

 nerves, which radiate from this point and ascend on all 

 sides of the eye vesicle to reach the periphery of the 

 retina where they are continuous with the rod cells. The 

 other branch, the outer nerve (fig. 29, Op. n. o.), passes 

 distally on the shell side of the eye stalk, and, as alreadv 



