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, PI. 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, n. 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, ias 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 (/. c, p. 46), outside 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 tiecessary 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-fibres rather 

 than that of more deeply staining muscle-fibres. 



