970 BLOOD-VESSELS OF THE EYE. [BOOK m. 



anterior ciliary arteries, the blood-vessels as we have said extend- 

 ing a short distance only within the circle of the corneal cir- 

 cumference, 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 nour- 

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

 lens. 



604. In speaking of the movements of the pupil we re- 

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

 information goes, we have evidence chiefly of vaso-constrictor 

 fibres which passing from the sympathetic to the ciliary ganglion 

 ( 536) 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 ( 522) 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 b}* 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 mus- 

 cular 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. 



605. 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 



