SECTION XI 

 THE NUTRITION OF THE EYEBALL 



THE eyeball is protected in front by the eyelids. These are lined internally 

 with a delicate mucous membrane continuous with the conjunctiva covering 

 the anterior surface of the eyeball. This membrane is kept constantly moist 

 by the secretion of the lacrymal gland, a small acino- tubular gland built up on 

 the type of a serous gland, situated at the upper and outer angle of the orbit. 

 The secretion, ' the tears,' has a slightly alkaline reaction and contains about 

 98-2 per cent, water and 1 -8 per cent, total solids, viz. 0-5 per cent, coagulable 

 protein and 1 -3 per cent, inorganic salts, of which sodium chloride is the most 

 important constituent. Since the tears are constantly flowing over the 

 eyes they serve not only to moisten the surface but to wash away any irritant 

 material or bacteria which may be deposited from the air. The tears have a 

 bactericidal power which is lost if the fluid be boiled for two or three minutes. 

 Our knowledge as to the nervous mechanism of the secretion of the tears is 

 still incomplete. Stimulation of the conjunctiva evokes an increased secre- 

 tion of lacrymal fluid which can also be induced by emotional conditions. 

 It is stated that lacrymal secretion can be produced by the stimulation 

 of the cervical sympathetic as well as by stimulation of certain cranial 

 nerves, e.g. facial and fifth nerve. Structural changes analogous to those 

 to be described in the salivary glands have also been found in the lacrymal 

 gland as a result of secretion. The excess of fluid is drawn off from the 

 conjunctival sac by the nasal duct, which leads to the nasal cavity on the 

 same side. If the eyes be kept open for some minutes, the conjunctiva 

 covering the eyeball becomes dry and irritation is set up. Normally the 

 membrane, and especially that over the cornea, is kept moist and trans- 

 parent by involuntary movements of the eyelids, which close or blink 

 about twice a minute, so distributing the lacrymal secretion over the 

 whole conjunctival surface. This blinking is a reflex act, the afferent 

 channels being fibres of the fifth nerve, and the efferent the fibres of the 

 facial nerve supplying the orbicularis palpebrarum. It is spoken of as 

 the ' conjunctival reflex,' and is one of the last reflexes to disappear in 

 chloroform or ether narcosis. 



Just below the mucous membrane of the lids we find a series of specialised 

 sebaceous glands, the ' Meibomian glands.' The fatty secretion of these 

 glands is poured out at the edge of the lids, keeping these and the eyelashes 

 greasy, and so preventing their being wetted by the tears. Any overflow of 

 tears from the conjunctival sac is thereby prevented, unless the secretion 



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