STRUCTURE OF THE SCLEROTIC. 591 



pupil ; 12, tlie crystallino lens, within its capsule ; 13, the vitreous humour ; a, a, a, 

 parts of a dotted line in tlie axis of tlie eye ; 6, h, 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 

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

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

 of the curvatures of the difl'erent surfaces, assuming them to be nearly spherical, viz. : 

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

 the lens ; Ip, posterior surface ; scj', 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 : — 



Tran verse diameter of the eyeball . . . . . . 1* 



Vertical diameter (Iviause) ........ 0'96 



Antero-posterior diameter ........ U"'JG 



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



Diameter of the ner^-ous part at its passage through iJie choroid 



membrane .......... ()'09 



Greatest thickness of the sclerotic, choroid, and retina together . . U-08 



Greatest thickness of the sclerotic posteriorly .... 0'05 



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



Greatest thickness of the cornea ....... O'Ooo 



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'.S5 



Greatest thickness of the ciliary muscle and ciliary processes 



together ' . . OOG 



Greatest thickness of the ciliary muscle . . . . . . O-U.'^o 



Thickness of the iris ......... O'Ulo 



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



cornea (regarding it ai^proximately as spherical) . . . . ()-3()5 



Radius of the jiosterior surface ....... 0-275 



Radius of curvatm-e of the anterior surface of the lens . . . U"3G 



Radius of the posterior surface ....... 021 



Approximate length of the radius of curvature of the outer surface 



in the posterior half of the retina . . . . . . . 0-185 



Approximate radius of curvature of the external surface of tlie 



posterior jiart of the sclerotic coat ....... O'.l 



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



middle of the retina ......... 0\"i> 



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



nerve and the middle of the fovea centralis retinte . . . O'll 



Diameter of the base of the cornea . . . . . . . 0-4S 



Diameter of the base of the iris transversely . . . . . ()•-!.") 



Diameter of the base of the iris vertically . . . . . . O'l;'. 



Diameter of the pupil O'll 



Strticture. — The sclerotic coat is foraied of connective tissue, and 

 yields gelatine on boiling. Its fibres are combined with fine elastic 

 tissue, and amongst them lie numerous connective tissue corpuscles 

 lodged in cell spaces, but not by any means so regularly arranged as in 

 the cornea. Some of the cells are pigmented. The bundles are dis- 

 posed in layers both longii^ndinally and transversely, the longitudinal 

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

 municate at intervals so as not to be separable for any distance. 



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

 work of capillaries with very wide meshes. In the neighbourhood of 

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

 noticed in the description of the sclerotic conjunctiva. 



