VESSELS OF THE SCLEROTIC AND CHOROID. 323 



A. THE SCLEROTIC. 



This coat receives small branches from all the vessels just 

 described. They are not, however, very numerous, and form 

 a wide-meshed plexus chiefly upon the surface, in which, as a 

 rule, two veins, one on either side of it, accompany each artery. 

 The relations of the episcleral vessels in the anterior segment 

 of the sclerotic, adjoining the margin of the cornea, are differ- 

 ent from this, and will receive subsequent consideration in con- 

 nection with the vessels of the margin of the cornea and of the 

 conjunctiva. 



B. THE CHOROID. 



The choroid is supplied by a very large number of vessels, 

 which very freely branch and interweave. 



This rich plexus of vessels, which attains its fullest development in 

 the ciliary processes, appears to be destined to secrete the fluid that pre- 

 serves the intraocular pressure which would otherwise rapidly undergo 

 diminution, owing to filtration through the fibrous capsule. The 

 vessels of the choroid may also, perhaps, be destined for the nutrition 

 of the external non-vascular layers of the retina ; and this is rendered 

 so much the more probable by the circumstance mentioned above > 

 that in many animals the whole retina is destitute of vessels, in which 

 case its nutrition must necessarily be maintained by the cboroid. 



Prom the preceding enumeration of the vessels of the ciliary 

 vascular system, it is obvious that there is by no means a com- 

 plete correspondence between the arteries and veins of the 

 choroid. The choroidal tract in relation to its arterial supply 

 is divisible into two tolerably separate districts ; one of which, 

 formed by the choroid proper, receives its supply from the short 

 posterior ciliary arteries, whilst the other, consisting of the 

 ciliary body and iris, is supplied by the long posterior and the 

 anterior ciliary arteries. The most anterior part of the choroid, 

 however, receives in addition a number of recurrent branches 

 from the anterior district, whereby a communication is effected 

 between the latter and the district supplied by the posterior 

 arteries. The efflux of venous blood is differently provided for- 

 The greater part of the venous blood of the entire choroid 

 (including the iris and ciliary processes), has a common outlet 



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