166 BLOOD-VESSELS OF THE EYE. [BOOK in. 



as by the anterior ciliary arteries, the blood-vessels as we have 

 said extending a short distance only within the circle of the 

 corneal circumference, while the scanty supply of the sclerotic 

 is furnished in the front part of the eyeball by the anterior 

 and in the hind part by the posterior ciliary arteries. 



The nutritive supply of the lens, with its capsule, and of 

 the vitreous humour is an indirect one, by means of lymph ; 

 the anterior surface of the former is bathed by the aqueous 

 humour; the lymph streams in the vitreous humour, of which 

 we shall speak immediately, furnish that substance with the scanty 

 nourishment it needs, and sweep by the posterior surface of the 

 lens. 



802. In speaking of the movements of the pupil we referred 

 to vaso-motor changes in the eye. So far as our present informa- 

 tion goes, we have evidence chiefly of vaso-constrictor fibres 

 which passing from the sympathetic to the ciliary ganglion 

 ( 725) reach the posterior ciliary arteries by the short ciliary 

 nerves; but there are facts which seem to shew that the fifth 

 nerve supplies vaso-dilator fibres through the ophthalmic branch. 



The separate distribution of the short ciliary arteries to the 

 hinder part of the choroid investment which is busy with the 

 nourishment of the retina and which takes little or no share 

 in the movements of accommodation, and of the long ciliary arteries 

 to the front part of the investment which, as iris, ciliary processes 

 and muscle, and front part of the choroid itself, is concerned in the 

 movements of the pupil and of accommodation, suggests that a 

 corresponding separate distribution of vaso-motor nerves also 

 exists; but we have no exact experimental .evidence of this. 



We saw in speaking of the brain ( 700) that clear evidence 

 of the cerebral vessels being subject to vaso-motor influences was 

 wanting; and in this respect once more the retina behaves like a 

 part of the brain. Though by help of the ophthalmoscope changes 

 of calibre in the retinal vessels can easily be observed, we have as 

 yet no decisive proof that such changes can be brought about by 

 vaso-motor nerves acting directly on the arteria centralis retinae. 

 The changes which are observed seem to be determined not 

 by the greater or less contraction of the muscular coat of the 

 retinal vessels themselves, but by the pressure to which the 

 blood in the vessels is subjected, and that may be varied by 

 many extraneous causes. 



The Lymphatics of the Eye. 



803. Though the lymph in the large serous cavities may be 

 considered to play a mechanical part inasmuch as it facilitates 

 the movements of the viscera, and though in such a tissue as the 

 skin, the lymph in the cavities and vessels of the dermis may 



