OKGAN OF SIGHT IN MAMMALIA. 



253 



meter. 1 The sclerotic, or ' tunica albuginea,' is of a fibrous 

 structure, and so much as is visible at the fore-part of the globe 

 forms the ( white of the eye : ' being thinner here than behind, 

 the dark choroid appearing through it sometimes gives it a bluish 

 tint; it resumes thickness near the cornea. This, fig. 193, b, 

 forms the segment of a smaller sphere than the rest of the eye- 

 ball ; it is perfectly transparent in the living eye, and consists of 

 a proper tunic, a most delicate continuation of conjunctive mem- 

 brane, fig. 207, <?, over the outer surface, and an elastic layer on 

 the inner surface with which the membrane of the aqueous 

 humour is blended: the proper tunic is laminated. It is inti- 

 mately connected with the sclerotic ; the elastic layer is con- 

 tinued beneath the sclerotic, ' as if slipped between it and the 

 ciliary ligament,' fig. 193, d. The choroid is the vascular tunic of 

 the eye and is stained, in Man, within and without with a deep 

 brown or black pigment : the outer surface is flocculent, through 

 the attachment to the cellulosity uniting it with the sclerotic : the 

 inner surface is smooth, highly and minutely vascular: this 

 surface, artificially separated from the outer surface supporting, 

 as in fig. 196, the trunks and larger branches of the vessels and 

 nerves, was termed the 4 tunica Ruyschiana.' The arteries supplying 

 the choroid are the ( short ciliary:' the 196 



( long ciliary ' arteries are chiefly distri- 

 buted to the iris, and also give anterior 

 branches to the sclerotic. The veins 

 of the choroid converge in arches to 

 four or five trunks which pierce the 

 sclerotic at equal distance from each 

 other behind the middle of the eyeball : 

 from this disposition, shown in fig. 196, 

 they are termed ' venae vorticosae.' The 

 choroid receives minute branches from 



Outer surface of cuoroUJ ;m<i iris ; .M;ni, 



the ciliary nerves in their passage to magnified, c\-n". 



the iris. On the outer part and anterior border of the choroid 

 is a circle of grey softish substance, applied, like a band, round 

 the margin of the aperture into which the iris is fitted : it 

 adheres closely to the sclerotic at the line of the attachment 

 of the cornea. The ciliary nerves penetrate and subdivide in 

 this zone, which is termed ' ciliary ligament,' fig. 197, a. On the 

 inner surface of the anterior border of the choroid is a circle of 

 longitudinal folds of that membrane, called ( ciliarv processes,' 



1 According to the careful admeasurements in cvi". 



