536 



.(ESTHESIOLOGY. 



is prolonged forwards to the ciliary processes. The retina is the 

 terminal expansion of the optic nerve, over the internal face of 

 the choroid coat, from which it is easily separated. It is com- 

 monly described as consisting of nine layers, arranged as follows 

 from within outwards : — 1. Internal limiting layer; 2, Optic 

 nerve fibres ; 3. Ganglionic cells ; 4. Internal molecular layer ; 

 5. Internal granular layer ; 6. External molecular layer ; 

 7. External granular layer ; 8. External limiting membrane ; 

 9. Basilary layer, or rods and cones. The vascular portion 

 consists of the ramifications of the arteria centralis retinae and 

 its satellite vein, their branches being continued into the zonula 

 of Zinn. 



The Ciliary zone, or Zonula of Zinn, is a thin vascular 

 layer, which connects the anterior margin of the retina "with 

 the anterior surface of the lens, and presents a number of folds 

 which are received between the corre- 

 sponding folds of the ciliary processes. 

 The under surface of this zone is 

 in contact with the hyaloid mem- 

 brane, and is separated from the 

 circumference of the lens by the 

 small circular canal termed the canal 

 of Petit 



Fig. 210. 

 Olobe of the eye seen from before 

 — the cornea and aqueous humonr 

 removed. 1, Sclerotic ; 2, Anterior 

 part of choroid ; a a, Ciliarj- muscle ; 

 b b, Iris; c, Pupil; d d, Corpora 

 nigra. 



HUMOURS OF THE EYE. 



These, the refracting media, are 

 three in number; the aqueous and 

 vitreous humours, and the crystalline 

 lens. 

 The aqiieous humour consists chiefly of water, possessing an 

 alkaline reaction, and completely fills the anterior and posterior 

 chambers of the eye. The anterior chamber is the space inter- 

 vening between the internal face of the cornea and the iris, pupil, 

 and ciliary muscle. The posterior chamber, ignored by some 

 authorities, is the very narrow space bounded in front by the iris, 

 and behind by the capsule of the lens and zonula ciliaris. Both 

 chambers are lined by a thin membrane, which secretes the 

 aqueous humour. 



The vitreous hv/mour occupies about four-fifths of the whole 

 interior of the globe. It is a highly albuminous and perfectly 



