STRUCTURE OF THE SCLEROTIC. 461 



At the anterior margin of the sclerotic, and sometimes around 

 the entrance of the optic nerve, there is in Birds a bony ring, 

 composed of a series of plates. 



Hyaline cartilage is also present in the sclerotic of Amphibia 

 and Fishes. Helfreich* took the opportunity, in his paper on 

 the nerves of the sclerotic, to make a few observations upon 

 the structure of the sclerotic of the Frog, which I shall here 

 reproduce. 



The connective-tissue layer closely adhering by its inner sur- 

 face to a slightly rose-coloured layer of cartilage, with extraordi- 

 narily distinct and beautifully marked cells, appeared of a dark 

 olive tint, composed of compact parallel and vertically arranged 

 fibrous bands, to the outer side of which was applied, though 

 separated by a sharp line of demarcation, the investing looser 

 sheath of connective tissue. Antero -posterior sections of the 

 sclerotic carried through its whole extent showed that the 

 connective-tissue and cartilaginous layers vary in thickness 

 at different points. The cartilaginous layer was thickest at 

 the posterior pole of the globe of the eye, and diminished 

 rapidly at the anterior part, terminating by a rounded edge 

 just in front of the plane of insertion of the recti muscles. 

 The connective-tissue layer exhibited the converse relation in 

 regard to its thickness at various points. The cartilaginous layer 

 was completely homogeneous, and never presented any interrup- 

 tions or openings for the passage of vessels and nerves ; whilst in 

 the connective-tissue layer these last presented the same clean 

 aspect and elegance as in surface views. In the posterior 

 parts of longitudinal sections the coarser trunks and doubly 

 contoured fibres were visible, and towards the anterior por- 

 tions, as far as to the border of the cartilaginous layer, and 

 even beyond this, the fine light-blue violet-tinted axis cylinders 

 were seen running for considerable distances either straight or 

 with a slightly sinuous course. Here and there they exhibit 

 slight varicose enlargements, and in regard to their course it 

 was noticeable that they gradually approximated the line of 

 demarcation between the connective-tissue and the cartilaginous 

 layers. 



* Loc. cit. 



