182 MR. TOYNBEE ON THE ORGANIZATION AND NUTRITION 



The superficial or conjunctival arteries, upon arising from the sclerotic, take a 

 course parallel with the circumference of the cornea, and are sometimes so long as 

 to receive the name of its circular vessels ; from these, branches are given off which 

 pass in a direction towards the centre of the cornea. These, after division and sub- 

 division, form a minute plexus on the border of the latter, and they terminate on its 

 surface from one-eighth to half of a line from its circumference, by becoming con- 

 tinuous with the venous system, Plate XVI. fig. 1 . 



The modes in which the arteries are continued into veins, are various ; sometimes 

 they form loops with the venous radicles ; sometimes a single arterial branch divides, 

 and both of its divisions take a course retrograde to that of the artery, and they 

 empty themselves into the venous plexuses. This species of vascular arrange- 

 ment is seen in Plate XVI. fig. 1. Sometimes the artery divides into two or 

 three branches which form loops with a venous radicle. These vessels appear to 

 differ much in their sizes ; this difference perhaps depends upon whether, as in 

 the specimen from which the fig. 1. was taken, they only contain a sufficient 

 quantity of colouring matter to enable their course to be traced with distinctness. 

 The small veins, which, are continuous with the arteries just described, take an 

 opposite course to the latter, and upon reaching the margin of the sclerotic, they 

 empty themselves into the large tortuous veins, which have been noticed above, just 

 as the latter are emerging from the substance of the sclerotic, where it is attached 

 to the cornea. 



The deep or sclerotic arteries of the cornea are those from which this structure 

 derives its chief nutrition. They are large enough to be considered the continuation 

 of the trunks of the sclerotic arteries ; they pass without much diminution of their size 

 towards the point where the sclerotic membrane is continuous with the cornea. At 

 the margin of the latter they suddenly stop and turn back, forming loops ; sometimes 

 with, and at other times without dilatations, they become continuous with the veins, 

 Plate XVI. fig. 2. These veins emerge from the substance of the sclerotic, close to 

 the margin of the cornea, at which point they receive the conjunctival veins ; and 

 they take a backward course, and form those tortuous veins which have already 

 been noticed, and which are seen by the naked eye. 



Such are the investigations which have induced me to state, in the introduction 

 to this second class of tissues, that the cornea in a healthy state does not contain 

 any blood-vessels. The vessels which have the function of supplying this tissue 

 with a nutritive fluid, are those belonging to the two classes which I have described. 



In a diseased state, the vessels which, in the healthy condition terminate abruptly 

 at the margin of the cornea, are prolonged through its entire substance ; while those 

 which, when healthy, extend over the surface of the cornea from the one-eighth to 

 half of a line, in disease form a band of considerable extent. I shall defer entering 

 upon these pathological conditions to a future opportunity. 



