54 G. S. WEST. 



conspicuously evident in adult animals^ and may vary in 

 number from about eight to twelve. They run in a somewhat 

 posterior direction from the periphery towards the centre, 

 each fold being- a thickening of the membrane. Alongside 

 the folds, towards the edges of the membrane, are a few dark 

 brown, much-branched pigment-cells ; these are less numerous 

 in the adult than in the young animal, and are generally con- 

 spicuous only round the borders of the pit. 



The nerve-trunks, which enter at the periphery, traverse 

 the above-mentioned folds, and by repeated divisions form 

 an immense number of fibres of extreme tenuity as they pass 

 towards the centre of the membrane ; the ultimate divisions 

 of these minute fibres form a slight anastomosis before 

 terminating in the nerve endings. 



The nerve terminations are cells of variable form, 

 many of them possessing a flat surface closely applied to the 

 cuticle of the outer face of the pit-membrane (cf. PI. 4, 

 figs. 7 — 9). Under this cuticle more than one layer of nerve- 

 cells can be distinguished, and in most cases the cell walls 

 are very indistinct, although by careful teasing and staining 

 the form of the cells can be plainly observed. They are 

 somewhat triangular or even spindle-shaped in outline, one 

 pole being connected with a minute nerve-fibre, and in many 

 cases they exhibit a considerably branched appearance (PI. 4, 

 figs. 7 and 8). The nuclei are of a round or oval form, and 

 stand out prominently. The delicacy and indistinct nature 

 of the cell walls, which the most careful treatment scarcely 

 renders visible, caused Ley dig ^ to describe and figure ag- 

 glomerations of nuclei round the small nerve branches. The 

 nerve terminations, although mostly confined to the outer 

 surface of the membrane, are also found in small numbers 

 near the edges of the inner surface. 



The nerve supply for the entire sense-organ is derived 



from the V (trigeminus) nerve. The nerve^ bundles entering 



the upper margins of the membrane are derived from the 



ophthalmic branch, and those entering the lower margins are 



> Leydig, I. c, t. iv, f. 31. 



