LACRYMAL ORGANS. 



85 



fold, is towards the cornea. The rolling of the 

 eyeball outwards has a tendency to undo the 

 fold, which on the contrary is rendered more 

 distinct when the cornea is turned towards the 

 nose. In quadrupeds the semilunar fold is 

 much more developed, and contains within it 

 a more distinct cartilaginous plate. It consti- 

 tutes what in them is called inembrana nic titans. 

 The third eyelid in birds is the same structure 

 carried to its highest pitch of development. In 

 man, in whom it is very small, its component 

 structures are readily developed to a consider- 

 able size by inflammation. According toSoem- 

 merring the semilunar fold is larger in the 

 negroes. We shall have occasion to recur to 

 the membrana nictitans of quadrupeds and the 

 third eyelid of birds. 



The intimate nature of the connexion between 

 the tarsal cartilages and the conjunctiva which 

 lines them has been already noticed. Beyond 

 the tarsal cartilages the adhesion of the palpe- 

 bral conjunctita becomes looser and looser 

 until its transition into the ocular conjunctiva. 



The ocular conjunctiva is smoothly spread 

 over the front of the sclerotica, where it first 

 passes on the latter. The interposed cellular 

 tissue is loose enough to allow it to slide upon 

 the sclerotica, or even to be raised up in wrin- 

 kles according to the motions of the eyeball, 

 which are thus facilitated. But as the con- 

 junctiva approaches the cornea it is more and 

 more closely applied to the sclerotica and con- 

 sequently less readily falls into wrinkles. The 

 debated question of a conjunctival covering of 

 the cornea will be considered when speaking 

 of the intimate structure of the conjunctiva. 



The cellular tissue between the conjunctiva 

 and sclerotica is sometimes the seat of extrava- 

 sations of blood, subconjunctival eccfiymosis, 

 sometimes the seat of an accumulation of se- 

 rous fluid, as in the oedema attending erysipela- 

 tous ophthalmia. It is sometimes the seat of a 

 more serious form of oedema, that known by 

 the name of chemosis, and common in the 

 purulent inflammation of the conjunctiva. It 

 may also be the seat of emphysema, and is 

 occasionally so of phlegmon. 



Nature of the conjunctiva. The conjunctiva 

 forms part of that membraneous system, conti- 

 nuous with the skin at all the natural apertures 

 of the body, which lines the interior of the 

 respiratory and digestive canals, and to which, 

 as to that lining the genito-urinary passages, 

 the generic name of mucous membrane is given. 

 Of course different parts of this system present 

 specific peculiarities in structure and function, 

 and this is the case even in regard to the palpe- 

 bral and ocular parts of the conjunctiva, though 

 so near each other. Some of the Germans have 

 'unnecessarily involved this subject. Thus 

 W'alther viewed the conjunctiva as mucous in 

 the eyelids, tegumentary over the sclerotica, 

 and serous over the cornea. Whence we some- 

 times meet in their ophthalmological works such 

 expressions as " the conjunctiva considered as 

 a mucous membrane," and " the conjunctiva 

 considered as a serous membrane." In refe- 

 rence to these opinions of his countrymen, 



Miiller* has thought it necessary to remark 

 that the conjunctiva is as certainly a mucous 

 membrane as any other of which the character 

 has not been doubted. * * * On the other 

 hand it has nothing in common with the serous 

 membranes either in secretion, for the limpid 

 secretion of the eyes is derived from the lacry- 

 mal gland, or in its form, which is not that of a 

 shut sac. 



Within the upper eyelid towards the outer 

 canthus (Jig. 12y'), the conjunctiva presents the 

 minute mouths, nine or twelve in number, of 

 the ducts of the lacrymal gland. At the inner 

 canthus the conjunctiva is continuous through 

 the lacrymal points with the membrane lining 

 the canalicules, and so through them, the la- 

 crymal sac and nasal duct, with the mucous 

 membrane of the nose. At the margin of the 

 eyelids its continuity with the skin is seen. 



The oculo-palpebral space of the conjunc- 

 tiva receives the tears much in the same way 

 that the mouth receives the salivary secretions. 

 Like other mucous membranes the conjunctiva 

 secretes a mucous fluid which lubricates its 

 surface and serves to protect it from the irri- 

 tating action of external agents, and even from 

 that of the lacrymal secretion which is naturally 

 poured out on it. 



Intimate structure of the conjunctiva. 



Palpebral conjunctiva, conjunctiva palpe- 

 brarum. The conjunctiva lining the eyelids is 

 thicker and more vascular than that which 

 invests the sclerotica. On the posterior surface 

 of the eyelids, about one-twelfth of an inch 

 from and parallel with the posterior acute edge 

 of the margin, there is a very slight groove. 

 Between this and the edge of the eyelid the 

 conjunctiva is sufficiently distinct by its moist 

 shining; surface and its vascularity, from the 

 more integument-like though delicate invest- 

 ment of the margin of the eyelids with which 

 it is continuous. But it is immediately in the 

 groove and especially beyond it that the con- 

 junctiva, as pointed out by Eble,f first shows 

 itself truly as a mucous membrane, that is, pre- 

 sents all the characters commonly ascribed to 

 mucous membranes. 



The palpebral conjunctiva consists of a 

 chorion, the free surface of which presents 

 papillae, constituting what is called the papillary 

 bodu, and the whole is covered by an epithelium. 



The chorion of the palpebral conjunctiva is 

 intimately incorporated with the tarsal fibro- 

 cartilages, so that the latter and their investing 

 conjunctiva might be considered together as 

 constituting a compound or nbro-mucous struc- 

 ture. Beyond the cartilages the chorion ap- 

 pears in its independent and separable form as 

 a felt-work composed of an interlacement of 

 filamentous cellular tissue, and is the nidus for 

 the ramification of the vessels and nerves. 



Pupillary body. If the upper eyelid be 



* Handbuch der Physiologie des Menschen, 

 Bd. i, S. 429, Coblenz, 1838, or Translation by 

 Baly, p. 436. 



f Ueber den Bau and die Krankhciten der Binde- 

 haut dcs Anges,p. 9. Wien, 1828. 



