MUSEUM OF COMPARATIVE ZOOLOGY. 85 
in question (Epeira), the entrance of the optic nerve being slightly dor- 
sal ; but the significance of this fact was not perceived by him. 
The same peculiarity is also noticeable in the figure of Epeira diadema 
given by Schimkewitsch (’84, Pl. II, Fig. 4), where, besides, the radiating 
fibres of the two bundles described by Grenacher are also figured. They 
are, however, erroneously assumed by the author to be muscle fibres.* 
In these cases (and doubtless similar conditions prevail in many others) 
the optic nerve leaves the bulb of the eye not directly opposite the lens, 
and not always at the point which corresponds to the shortest distance 
between the eye and the brain. It is noticeable that the place of emer- 
gence is in some instances (Figs. 10, 11, 20, 23, 24, 2. opt.) very near to 
the superficial border of the retina. If the opinion held by Grenacher 
were to be substantiated in these cases, we should expect to find the major 
part of the optic-nerve ramifications bending abruptly backward as soon 
as they had entered the cuticula of the bulb, and forming behind the 
bulb a kind of nerve-fibre sheath, which would gradually become thinner 
* Schimkewitsch (p. 14) finds in these nerve-fibres the sphincter described by 
Leydig. ‘‘ But,” he adds, ‘‘I have never seen that this sphincter takes its origin 
from the integument, as claimed by Grenacher. . .. The action of the muscle as a 
constrictor has been observed by Leydig; but I am not able to understand how the 
muscle would be able to change the visual axis, [even] if it were attached to the in- 
tegument, as Grenacher supposes, since the cornea-lens is quite immovable.” 
Leaving aside the question as to the accuracy of Grenacher’s conclusions about a 
change in the direction of the visual axis, it must be sufficiently evident upon com- 
paring the figures given by the two authors (Grenacher, ’79, Fig. 18, M, M’) that the 
structures in question have nothing in common. Whatever may be the effect of its 
contraction, the muscle figured by Grenacher encircles the eye, lying, as he expressly 
states (J. c., p. 46), owtside the cuticula which invests the eye ; whereas that to which 
Schimkewitsch attributes the function of a sphincter lies wholly within the cuticular 
envelope. ; 
Leydig (’58, p. 441) observed powerful, jerking contractions of the pigmented 
layer in the eyes of several living spiders. It is a long step that Schimkewitsch 
has to take when he says Leydig has observed the action of his supposed sphincter 
muscle. It is the more surprising that he should have adopted such an interpreta- 
tion of the fibres, when a much more natural one had already been given, as above 
quoted, by Grenacher. He adduces no argument to prove the contractile nature of 
the fibres, and, it would seem, must have arrived at his conclusion rather hastily, 
and without the remembrance of Grenacher’s description of the optic nerve. 
If it were necessary to strengthen with special arguments the natural interpreta- 
tion given by Grenacher, one might insist —in addition to the observed direct 
continuation of the fibres with the optic nerve—upon the absence of transverse 
striations, and a susceptibility to staining reagents like that of nerve-tibres rather 
than that of more deeply staining muscle-fibres. 
