428 PHYSIOLOGY CHAP. 



nictitating membrane takes place only on the stimulated side 

 (Langendorff). 



Keflex winking occurs when the optic nerve is stimulated by 

 a sudden illumination or by the rapid approach of some foreign 

 body to the eye : or by mechanical or chemical stimulation of 

 the end of the trigeminal nerve, by contact with the lashes, 

 conjunctiva, or cornea, or by the action of irritating gases on 

 the latter. 



The reflex effect increases with the intensity of stimulus. 

 Gentle stimulation produces contraction only of the pars palpe- 

 bralis of the orbital muscle, which is innervated by the facial 

 nerve ; when the stimulation is stronger, other muscles, as the 

 corrugator supraciliaris, are involved. 



Apart from accidental stimuli there is a periodic blinking of 

 the lids, which may be frequent when these nerves are irritated 

 by cooling or drying of the corneal and conjunctival surfaces of 

 the eyeball. The closure of the eyes is followed by their rapid 

 opening, effected by the levator palpebrae superior. 



Nothing is definitely known of the localisation of the centre 

 of the lid reflex. According to Nickell it lies in the upper part 

 of the medulla oblongata. The cortical centre of the orbicularis 

 probably comes into action during voluntary uni- or bi-lateral 

 closure of the lids ; but as a matter of fact the lid reflex is not 

 abolished or affected by its extirpation in the dog (Eckhard). 



The action of the striated muscles is reinforced by that of the 

 smooth muscles of the eyelid, described by H. Miiller, which are 

 kept in tonic contraction by the influence of the sympathetic. 

 Section of the cervical sympathetic produces narrowing of the 

 palpebral fissure and slight retraction of the eyeball ; its electrical 

 excitation produces, besides dilatation of the pupils, widening 

 of the fissure (lagoplithalmus) and prominence of the eyeball 

 (exoplitlialmus). 



In order that the cornea may retain its perfect transparency 

 it is essential that it should be continually irrigated by a thin 

 layer of lachrymal fluid, which keeps it clean and prevents it 

 from drying. Tears are the secretory product of the lachrymal 

 gland, which is an elongated and flattened mass, the size of a 

 small almond, situated in the upper and outer part of the orbit, 

 in contact with the superior and external recti muscles. The 

 anterior and inferior portion of the gland is separated from the 

 rest by an aponeurotic layer, and may be regarded as a gland in 

 itself (glandula lacrimalis inferior). Other smaller glands Lie 

 along the conjunctival fornix, 30 to 40 in the upper, 6 to 8 in the 

 lower sac. 



The lachrymal are similar to the salivary glands, i.e. they are 

 ramified tubular glands. The secreting cells that line the alveoli 

 present during rest and after activity, e.g. in crying, suffer changes 



