CHAP, in.] SIGHT. 167 



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 rilled with 

 fluid, the aqueous and vitreous humours ; and for the various 

 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 nap about or in any 

 way alter its curvature, the dioptric arrangements of the eye would 

 be upset; if the vitreous humour at one time shrank, at another 

 expanded, the movements of accommodation could not be carried 

 on ; if the aqueous humour were now abundant, now scanty, 

 the movements of the pupil would become irregular and un- 

 certain ; and if the whole globe were so flabby as to give way 

 under the pull of each ocular muscle, the delicate 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 conjunctiva. 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 perichoroidal 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 peri- 

 vascular and other lymph-spaces of the choroid join the pericho- 

 roidal 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 pierce the sclerotic. 

 The Tenonian cavity itself joins a large lymphatic cavity surround- 

 ing the optic nerve, ' the supravaginal ' cavity, whence the lymph is 

 carried away by the ordinary lymphatics of the orbit. 



The perivascular and other lymph-spaces of the retina are in 



