329 



cojnmon connective tissue in the form of flat fibrillated bundles 

 closely interwoven in planes which cross one another at every 

 possible angle, but which have a general direction parallel to 

 the surface of the coat. Amongst the fibrillated tissue are 

 imbedded simple fusiform and branched corpuscles, which are 

 more numerous in the young than in fully grown persons. 

 The blood-vessels of the sclerotic are not very numerous ; the 

 most important are, as before pointed out, the recurrent 

 branches of the posterior ciliary arteries, which iinite in a 

 small circle, which communicates with the vessels of the 

 nerve. Whether the sclerotic has any nerves of its own is 

 doubtful. The ciliary nerves all pass through, and do not 

 appear to furnish any branches within their canals. 



In birds, lizards, and turtles the fibrous sclerotic is 

 strengthened by the addition of bone and cartilage. The 

 bone is chiefly present in the ciliary region, where it forms 

 the well-known circle of plates. The osseous tissue contains 

 well developed lacunae, and where the plates are thick they 

 are hollowed by vascular canals and by medullary spaces 

 enclosing fat-cells. It appears to be evolved out of fibrous 

 tissue. The cartilage is always of the hyaline variety. In 

 the back of the sclerotic in many eyes the cartilaginous tissue 

 exceeds the common connective tissue. 



The Optic Nerve. 



The optic nerve pierces the sclerotic a little below and at the 

 inner side of the posterior pole of the eyeball, the nerve 

 appearing at its inner surface nearly V" to the nasal side of 

 the fovea centralis retina?, in the form of a disc usually circular, 

 sometimes elliptical, and when so the major axis is generally 

 vertical. 



The Physiological Fit. — The common aperture in the sclero- 

 tic and choroid through which the nerve passes is a canal nar- 

 rower anteriorly, where it tightly clasps the nerve, and wider 

 posteriorly, where it loosely encloses it. Around this opening 

 the choroid and sclerotic adhere very intimately, their fibrous 

 tissues intermingling here concentrically round the nerve. 

 Here, too, the minute recurrent branches of the posterior ciliary 

 arteries distributed to the outer part of the sclerotic effect a 

 slight communication with the capillaries in the n«rve sheath, 

 and indirectly with those in the nerve itself. Some of these last 

 inosculate with the choroidal blood-vessels in the level of the 

 choroidal opening. Through these collateral channels, where 

 the trunk of the arteria centralis is plugged, a small quantity 

 of blood can enter the retina. The choroidal stroma around 

 the nerve-foramen contains the same stellar pigment-cells 

 which occur in it elsewhere. In some eyes in this situation 



