THE SCLEROTIC COAT. 713 



retina ; 8", fovea centralis retinae ; r, the nervous layer of the retina ; /, the bacillar 

 layer; 9, ora serrata at the commencement of the ciliary part of the retina ; 10, canal 

 of Petit; 11, anterior division of the aqueous chamber in front of the pupil; 12, the 

 crystalline lens, within its capsule ; 13, the vitreous humour ; a, a, a, parts of a 

 dotted line in the axis of the eye; b, b, b, b, a line in the transverse diameter. It will 

 be observed that from the pupil being placed nearer the inner side the axis of the eye-ball 

 a, a, does not pass exactly through the centre of the pupil, and that this line falls a 

 little to the inner side of the fovea centralis. The following letters indicate the centres 

 of the curvatures of the different surfaces ; assuming them to be nearly spherical, viz. : 

 c a, anterior surface of the cornea ; c p, posterior surface ; I a, anterior surface of the 

 lens ; I p, posterior surface ; s c p, posterior surface of the sclerotic ; r a, anterior surface 

 of the retina. 



In connection with this figure the following average dimensions of the parts of the 

 adult eye in fractions of an English inch may be stated : 



Transverse diameter of the eyeball . . . . . . . 1* 



Vertical diameter (Krause) ......... 0*96 



Antero-posterior diameter . . . . . . . . . 0'96 



Diameter of the optic nerve with its sheath . . . . . . 0'16 



Diameter of the nervous part at its passage through the choroid membrane 0*09 



Greatest thickness of the sclerotic, choroid, and retina together . . 0'08 



Greatest thickness of the sclerotic posteriorly 0'05 



Smallest thickness at the sides and in front ...... 0'025 



Greatest thickness of the cornea 0'055 



Distance from the middle of the posterior surface of the cornea to the 



front of the lens 0'07 



Antero-posterior diameter of the lens . . . . . . . 0'19 



Transverse ditto 0'35 



Greatest thickness of the ciliary muscle and ciliary processes together . . 0'06 



Greatest thickness of the ciliary muscle ...... 0'035 



Thickness of the iris 0'015 



Length of the radius of curvature of the anterior surface of the cornea 



(regarding it approximately as spherical) .0 '305 



Radius of the posterior surface 0'275 



Radius of curvature of the anterior surface of the lens . . . . 0'36 



Radius of the posterior surface . . . . . . . . 0'21 



Approximate length of the radius of curvature of the outer surface in the 



posterior half of the retina . . (H85 



Approximate radius of curvature of the external surface of the posterior 



part of the sclerotic coat ......... 0*5 



Distance of the middle of the posterior surface of the lens from the 



middle of the retina 0'575 



Distance between the centre of the spot of entrance of the optic nerve and 



the middle of the fovea centralis retinae 0*14 



Diameter of the base of the cornea ....... 0'48 



Diameter of the base of the iris transversely . . . . . . 0'45 



Diameter of the base of the iris vertically ...... 0*43 



Diameter of the pupil . . . . 0'14 



STRUCTURE. The sclerotic coat is formed of connective tissue, and yields 

 gelatine on boiling. Its fibres are combined with fine elastic tissue, and 

 with fusiform and stellate nucleated cells, and are aggregated into bundles, 

 which are disposed in layers both longitudinally and transversely, the longi- 

 tudinal arrangement being most marked at the surfaces. These layers com- 

 municate at intervals, and the sclerotic presents a ramified and laminar 

 appearance on a vertical section. 



A few blood-vessels permeate the fibrous texture in the form of a net- work 

 of the smallest capillaries with very wide meshes ; and in the neighbourhood 

 of the cornea a ring of greater vascularity exists, which has been already 

 noticed in the description of the sclerotic conjunctiva. The existence of 

 nerves in the sclerotic has not yet been allowed by all anatomists. 



