66 BULLETIN OF THE 



which lies immediately over it. No trace of a suspensory ligament seems 

 to be present, nor have I been able to find a processus falciformis. 



In a single instance, viz. in the eye portions of which are shown in 

 Figures 13 and 21, and which has already been spoken of as showing 

 in several respects a higher state of development than any other speci- 

 men studied, I find close behind the ii'is, within the cavity of the 

 eye, a few strands of tissue containing a few small nuclei, which may 

 possibly be regarded as the hyaloid membrane, though I do not feel at 

 all certain that this is their nature. In the same specimen a very few 

 blood corpuscles are also found close behind the lens within the eye 

 chamber. With this single exception I have been unable to find a trace 

 of tissue within the chamber. 



The optic nerve, although exceedingly slender, is always present, so 

 far as ray observations have gone. A very striking fact in connection 

 with it is the thick sheath of pigment that surrounds it in its passage 

 through the retina (Figs 6, 13, and 15, pig.'"). This sheath invests the 

 nerve very closely, no space existing between it and tlie latter ; however, 

 in its course through the retina, a considerable non-pigmented space is 

 sometimes seen between its outer sui'face and the pigment of the ret- 

 ina; indeed, in a majority of cases the outer boundary of the sheath 

 can be traced entirely through the retinal pigment. 



These facts make me incline to the opinion that this sheath really 

 belongs to the nerve, and has arisen by the pigmentation of the outer 

 portions of it. The fact that in many cases it continues on without 

 interruption through the cellular portion of the retina, nearly to its 

 inner surface (Plate II. Figs. 6 and 13), gives considerable confirmation 

 to this view. It would seem, however, were this the right interpreta- 

 tion, that we should find a rather more gradual disappearance of the 

 pigment in passing, on a section, from the main mass of the wall of 

 the sheath to the unpigmented portion of the nerve, than we do ; but 

 the inner surface of the sheath is not quite as sharply defined as its 

 outer surface is. 



On account of the position of the eyes, far anterior to the brain, and 

 near the anterior extremity of the head, the optic nerves are very long. 



The muscles of the eyeballs (Fig. 8) are also very long and slender, 

 and are probably always present, though I have not been able to detect 

 them in the sections in all cases ; but in eyes dissected out and cleared 

 in glycerine or clove oil, or slightly stained in Schneider's acetic acid 

 carmine, I have always found them. Figure 8 is a camera drawing 

 from a glycerine preparation, showing all the muscles excepting the 

 internal oblique, and also the nerve. 



