CHAP, in.] SIGHT. 971 



similarly perform a mechanical task in assisting to give at once 

 firmness and suppleness to the skin, yet over the body at large 

 the function of the lymph is preeminently a nutritive one, and 

 its mechanical duties are insignificant. As regards the eye the 

 case is different. The eyeball is broadly speaking a shell filled 

 with fluid, the aqueous and vitreous humours ; and for the vari- 

 ous functions of the eye it is necessary that this shell should be 

 filled to a certain extent, should be tense to a certain degree, 

 not more and not less; and this fulness, this tension, " intraocular 

 tension," which is considerable, probably much higher than the 

 ordinary pressure in the lymph-spaces of the body at large, is 

 provided by the lymphatic arrangements. If the retina were 

 not adequately supported by the vitreous humour, if it could 

 flap about or in any way alter its curvature, the dioptric arrange- 

 ments of the eye would be upset ; if the vitreous humour at one 

 time shrank, at another expanded, the movements of accommo- 

 dation could not be carried on ; if the aqueous humour were now 

 abundant, now scanty, the movements of the pupil would become 

 irregular and uncertain ; and if the whole globe were so flabby 

 as to give w^ay under the pull of each ocular muscle, the deli- 

 cate movements of the eyeball on which we lately dwelt would 

 become impossible. Hence the lymphatics of the eye have a 

 double importance, inasmuch they not only, as elsewhere, assist 

 in maintaining the due nutrition of the several tissues, but also 

 in a mechanical way help to make the eye an adequate dioptric 

 instrument. In accordance with this double duty we find a 

 special lymph apparatus added to the more general lymphatic 

 arrangements such as exist elsewhere. 



As belonging to the more general arrangements we may 

 note the following. The lymph-spaces of the cornea pass at 

 the margin of the cornea into the lymphatic vessels of the con- 

 junctiva. The scanty lymph-spaces of the sclerotic pass at the 

 extreme front into the conjunctival lymphatics, but elsewhere 

 are continuous either on the inner surface with the pericho- 

 roidal lymph-spaces, or on the outer surface and that more 

 freely, with the large lymphatic Tenonian cavity. Tenon's 

 capsule is a loose thin investment of connective tissue lying 

 between the sclerotic and the ocular muscles and forming 

 sheaths round the tendons of the latter. Between the looser 

 capsule and the denser sclerotic is a large irregular lymphatic 

 cavity bearing the above name. The perivascular and other 

 lymph-spaces of the choroid join the perichoroidal spaces, which 

 in turn communicate with the Tenonian cavity by lymph-spaces 

 or lymphatics accompanying the ciliary veins and to some 

 extent the ciliary arteries as these prerce the sclerotic. The 

 Tenonian cavity itself joins a large lymphatic cavity surround- 

 ing the optic nerve, 4 the supra vaginal ' cavity, whence the 

 lymph is carried away by the ordinary lymphatics of the orbit. 



