596 William Patten 



lines; tlie tliick, oiiter oiie consìsts of faint longitudinal fibres, the super- 

 ficial ones of which may contaiu a few nuclei (PI. 30, fig. 39}. It has 

 only been in three or four instances that the cross markings bave been 

 observed, but they were theu seen so distinctly that there can be no 

 doubt that such a structure was present. These lines produce the effect 

 of plates of alternating density, rather than of fibres. This structure of 

 the sclerotica seems to be best preserved by treatment with chromic acid, 

 Ir, o/o for 24 hours. The sections must be examined in water, or some 

 other medium with a low refractive index. The sclerotica has been con- 

 sidered to be continuous with the pseudo-cornea, by means of the con- 

 nective tissue layer beneath the iris. The two latter structures are, 

 however, merely continuations of the contractile tissue of the stalk into 

 the anterior pole of the eye , while the sclerotica is directly continued 

 into the septal membrane. 



The opti e nerve arises from the circumpallial nerve, and, after 

 extending through the centre of the stalk, divides into two, nearly equa! 

 branches , of which the basai one abuts against the sclerotica, a short 

 distance on the shell side of the optic axis, and then, losing its sheath, 

 divides into many bundles of free nerve fibres , which , cliugiug closely 

 to the sclerotica, ascend radiatinglytowards the peripheryof the retina, 

 where they penetrate, in quite distinct groups, the ommateal membrane, 

 and become continuous with the attenuated ends of the retinophorae, 

 through the centre of which they are extended as axial nerve fibres. 

 The basai, or axial brauch of the optic nerve, therefore, consists 

 entirely of the axial nerve fibres of the retinophorae. The division of 

 the optic nerve takes place in a piane at right angles to that of the 

 mantle. The lateral, or gauglionic brauch, produced by this division, 

 ascends toward the shell side of the retina , over which it is bent at 

 nearly right angles , and is continued over the surface of the septum, 

 the thick outer layer of which it penetrates just below the inner surface 

 of the lens. After passing both layers of the septum, in the manner 

 already described, its fibres either unite with the gauglionic layers, or 

 pass between their cells to the surface of the rods. In the fibrous layer, 

 two kinds of fibres may be distinguished , the most numerous are ex- 

 tremely fine , and arise from the outer ends of the oblong gauglionic 

 cells : the larger and less numerous oncs are the single prolougations 

 of the more deeply situated gauglionic cells of the outer layer. Even 

 in the disc-like expansion of the gauglionic nerve brauch, one may see 

 both kinds of fibres, and follow them a short distance into the mòre 

 compact part of the nerve. That the sheath of the gauglionic brauch is 



