452 ARTERIA CENTRALIS RETINAE. 



?exity on the one border fitting accurately the concave scallop upon the 

 other. 



Immediately around the circumference of the lens is a triangular canal, 

 the canal of Petit * about a line and a half in breadth. It is bounded, in 

 front, by the flutings of the zonula ciliaris ; behind, by the hyaloid mem- 

 brane ; and, within, by the border of the Jens. 



The Vessels of the globe of the eye are the long and short, and anterior 

 ciliary arteries, and the arteria centralis retinae. The long ciliary arteries, 

 two in number, pierce the posterior part of the sclerotic, and pass forward 

 on each side, between that membrane and the choroid, to the ciliary liga- 

 ment, where they divide into two branches, which are distributed to the 

 iris. The short ciliary arteries pierce the posterior part of the sclerotic 

 coat, and are distributed to the middle fayer of the choroid membrane. 

 The anterior ciliary are branches of the muscular arteries. They enter 

 the eye through the anterior part of the sclerotic, and are distributed to 

 the iris. It is the increased number of these latter arteries, in iritis, that 

 gives rise to the peculiar red zone around the circumference of the cornea 

 which accompanies that disease. 



The arteria centralis retince enters the optic nerve at about half an inch 

 from the globe of the eye, and passing through the porus opticus is distri- 

 buted upon the inner surface of the retina, forming its vascular layer ; one 

 branch pierces the centre of the vitreous humour, and supplies the capsule 

 of the lens. 



The Nerves of the eyeball are, the optic, two ciliary nerves from the 

 nasal branch of the ophthalmic, and the ciliary nerves from the ciliary 

 ganglion. 



Observations. The sclerotic is a tunic of protection, and the cornea a 

 medium for the transmission of light. The choroid supports the vessels 

 destined for the nutrition of the eye, and by its pigmentum nigrum absorbs 

 all loose and scattered rays that might confuse the image impressed upon 

 the retina. The iris, oy means of its powers of expansion and contraction, 

 regulates the quantity of light admitted through the pupil. If the iris be 

 thin, and the rays of light pass through its substance, they are immediately 

 absorbed by the uvea ; and if that layer be insufficient, they are taken up 

 by the black pigment of the ciliary processes. In Albinoes, where there 

 is an absence of pigmentum nigrum, the rays of light traverse the iris, and 

 even the sclerotic, and so overwhelm the eye with light, that sight is de- 

 stroyed, except in the dimness of evening or at night. In the manufacture 

 of optical instruments care is taken to colour their interior black with the 

 same object, the absorption of scattered rays. 



The transparent lamellated cornea and the humours of the eye have for 

 Iheir office the refraction of the rays in such proportion as to direct the 

 \mage in the most favourable manner upon the retina. Where the refract- 

 ing medium is too great, as in over convexity of the cornea and lens, the 

 image falls short of the retina (myopia, near-sightedness) ; and where it is 

 too little, the image is thrown beyond the nervous membrane (presbyopia, 

 far-sightedness). These conditions are rectified by the use of spectacles, 

 which provide a differently refracting medium externally to the eye, and 

 thereby correct the transmission of light. 



* John Louis Petit, a celebrated French surgeon: he published several surgical and 

 anatomical Essays, in the early part of the 18th century. He died in 1750. 



