494 EYE. SCLEROTIC COAT. 



the latter from Schneider, who was the first to show that the secre- 

 tion of the nose proceeded from the mucous membrane, and not from 

 the brain, as was formerly imagined. It is continuous with the 

 general gastro-pulmonary mucous membrane, and may be traced 

 through the openings in the meatuses, into the sphenoidal and eth- 

 moidal cells ; into the frontal sinuses ; into the antrum maxillare ; 

 through the nasal duct to the surface of the eye, where it is con- 

 tinuous with the conjunctiva ; along the Eustachian tubes into the 

 tympanum and mastoid cells, to which it forms the lining membrane ; 

 and through the posterior nares into the pharynx and mouth, and 

 thence through the lungs and alimentary canal. 



The surface of this membrane is furnished with a columnar epithe- 

 lium supporting innumerable vibratile cilia. 



Vessels and Nerves. The Arteries of the nasal fossae are the an- 

 terior and posterior ethmoidal, from the ophthalmic artery ; and the 

 spheno-palatine and pterygo-palatine from the internal maxillary. 



The Nerves are, the olfactory, the spheno-palatine branches from 

 Meckel's ganglion, and the nasal branch of the ophthalmic. The 

 ultimate filaments of the olfactory nerve terminate in minute papillae. 



THE EYE, WITH ITS APPENDAGES. 



The form of the eyeball is that of a sphere, of about one inch in 

 diameter, having the segment of a smaller sphere ingrafted upon its 

 anterior surface, which increases its antero-posterior diameter. The 

 axes of the two eyeballs are parallel with each other, but do not 

 correspond with the axes of the orbits, which are directed outwards. 

 The optic nerves follow the direction of the orbits, and therefore enter 

 the eyeballs to their nasal side. 



The Globe of the Eye is composed of tunics and of refracting media 

 called humours. The tunics are three in number, the 



1 . Sclerotic and Cornea, 



2. Choroid, Iris, and Ciliary processes, 



3. Retina and Zonula ciliaris. 



The humours are also three 



Aqueous, 

 Crystalline (lens). 

 Vitreous. 



FIRST TUNIC. The Sclerotic and Cornea form the external tunic 

 of the eyeball, and give it its peculiar form. Four-fifths of the globe 

 are invested by the sclerotic, the remaining fifth by the cornea. 



The Sclerotic (erxX^oj, hard) is a dense fibrous membrane, thicker 

 behind than in front. It is continuous, posteriorly, with the sheath 

 of the optic nerve, which is derived from the dura mater, and is 

 pierced by that nerve as well as by the ciliary nerves and arteries. 



