Chap, iv.i THE ORBIT AND EYE. 55 



conjunctiva, and as easily emptied of their blood by 

 pressure. The differences presented by these two 

 sets of vessels serve in one way to distinguish in- 

 flammation of the conjunctiva from that involving 

 deeper parts. The conjunctival vessels around the 

 margin of the cornea form a closer plexus of anasto- 

 mosing capillary loops, which become congested in 

 severe superficial inflammation of the cornea, and 

 may then form a zone around the margin of the 

 cornea, which can, however, be distinguished from 

 the " ciliary zone " by the general characters just 

 named. The retina has a vascular system of its own, 

 supplied through the arteria centralis retince, which is 

 nowhere in direct communication with the choroidal 

 vessels, except just at the entrance of the optic nerve. 

 Indeed, the outer layers of the retina which are in 

 relation with the choroid coat are entirely destitute of 

 vessels. Thus, when the central artery of the retina 

 becomes plugged, sudden blindness follows, and as the 

 meagre collateral circulation that is established by the 

 minute anastomoses about the entrance of the nerve 

 is quite insufficient, the retina soon becomes cedema- 

 tous and inflamed. A permanent plugging of the 

 central artery means, thei-efore, a practical extinction 

 of the vascular system of the retina. In some cases 

 of embolism only a branch of the retinal artery is 

 plugged, the patient retaining vision except in that 

 part of the retina supplied by the branch. 



In cases of haemorrhage between the choroid and 

 retina the blood must come from the choroidal vessels; 

 and in haemorrhage into the vitreous, which often 

 follows injury, the blood may be derived from the 

 retinal vessels, since they run in the inner layers of 

 that membrane, or from the vessels in the ciliary 

 region. 



Nerve supply of the eye-ball. 1. The ciliary 

 nerves derived from the lenticular ganglion and the 



