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 con- 

 junctiva 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 secretion 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 conjunctiva! 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 transparent by involuntary move- 

 ments 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 ' con- 

 junctival reflex, 'and is one of the last reflexes to disappear in chloro- 

 form or ether narcosis. 



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



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