794 



THE LACHRYMAL APPARATUS. 



The secretion of the conjunctiva, besides some mucus, consists of tears, which may be as 



abundant as that formed in the lachrymal glands. 



Closure of the eyelids is effected by the orbicularis palpebrarum (facial nerve, 



349), whereby the upper lid falls 

 in virtue of its own weight. This 

 muscle contracts (1) voluntarily ; 

 (2) involuntarily (single contrac- 

 tions) ; (3) reflexly by stimulation 

 of all the sensory fibres of the tri- 

 geminus distributed to the bulb and 

 its immediate neighbourhood ( 347), 

 also by intense stimulation of the 

 retina by light ; (4) continued in- 

 voluntary closure occurs during 

 sleep. 



Opening of the eyelids is brought 

 about by the passive descent of the 

 lower one, and the active elevation 

 of the upper eyelid by the levator 

 palpebral superioris ( 345). The 

 smooth muscular fibres of the eye- 

 lids also aid (p. 593). In looking 

 downwards, the lower eyelid is 

 pulled downwards by bands of con- 

 nective-tissue which run from the 

 inferior rectus to the inferior tarsal 

 cartilage (Schwalbe). 



II. The lachrymal apparatus consists 

 of the lachrymal glands, which in struc- 

 ture closely resemble the parotid, their 

 acini being lined by low cylindrical granu- 

 lar epithelium. Four to five larger, and 

 eight to ten smaller excretory ducts con- 

 duct the tears above the outer angle of 

 the lid into the fornix conjunctivae. The 

 tear ducts, beginning at the puncta 

 lachrymalis, are composed of connective 

 and elastic-tissue, and are lined by 

 stratified squamous epithelium. Striped 

 muscle accompanies the duct, and by its 

 contraction keeps the duct open. Toldt 

 found no sphincter surrounding the 

 Vertical section through the upper eyelid. A, cutis ; puncta lachrymalia, while Gerlach found 

 1, epidermis; 2, chorium ; B and 3, subcutaneous an incomplete circular musculature. The 

 connective-tissue ; C and 7, orbicularis muscle ; Z>, connective-tissue covering of the tear sac 

 loose sub-muscular connective-tissue ; E, insertion an( l cana ^ is united with the adjoining 

 ofH. Midler's muscle ; F, tarsus; ^conjunctiva; periosteum. The thin mucous membrane, 

 J, inner, K, outer edge of the lid; 4, pigment-cells ; which contains much adenoid tissue and 

 5, sweat glands; 6, hair follicles; 8 and 23, sections lymph-cells, is lined by a single layer of 

 of nerves; 9, arteries; 10, veins; 11, cilia; 12, modi- ciliated cylindrical epithelium, which 

 fied sweat glands ; 13, circular muscle of Riolan ; below passes into the stratified form. The 

 14, Meibomian gland ; 15, section of an acinus of opening of the duct is often provided 

 the same ; 16, posterior tarsal glands ; 18 and 19 w * tn a valve-like fold (Hasner's valve). 



SUS1.hv?S ^^9^1 2 n i 1 ??"" 1 - r 8 " b - mu . 8c 1 ul . ai ' The conduction of the tears oc- 



connective-tissue 21 and 22, conjunctiva, with its i . .> ,., j ,, , ,. 



epithelium; 24, fat; 25, loosely woven posterior CUrS between the llds and the bulb 

 end of the tarsus; 26, section of a palpebral artery, by means of capillarity, the closure 



of the eyelids aiding the process. 

 The Meibomian secretion prevents the overflow of the tears [just as greasing the 



