502 LENS. CANAL OF PETIT. 



soft, the next firmer, and the central form a hardened nucleus. These 

 layers are best demonstrated by boiling, or by immersion in alcohol, 

 when they separate easily from each other. Another division of the 

 lens takes place at the same time: it splits into three triangular 

 segments, which have the sharp edge directed towards the centre, and 

 the base towards the circumference. The concentric lamellae are com- 

 posed of minute parallel fibres, which are united with each other by 

 means of scalloped borders ; the convexity 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 flirtings of the zonula ciliaris ; behind by the 

 hyaloid membrane ; and within by the border of the lens. 



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

 anterior ciliary arteries, and the arteria centralis retinas. The long 

 ciliary arteries, two in number, pierce the posterior part of the scle- 

 rotic, and pass forward on each side, between that membrane and the 

 choroid, to the ciliary ligament, 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 

 layer 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 jthese latter arteries in iritis that gives rise to the peculiar 

 red zone around the circumference of the cornea. 



The arteria centralis retinas, enters the optic nerve at about half an 

 inch from the globe of the eye, and passing through the porus opticus 

 is distributed upon the inner surface of the retina, forming its vascular 

 layer ; one branch pierces the centre of the vitreous humour, and sup- 

 plies 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 pig- 

 mentum nigrum absorbs all loose and scattered rays that might confuse 

 the image impressed upon the retina. The iris, by 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 pig- 

 mentum nigmm, the rays of light traverse the iris and even the scle- 

 rotic, and so overwhelm the eye with light, that sight is destroyed, 



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

 surprical and anatomical Essays, in the early part of the 18th century. He 

 died in 1/50. 



