438 EYE SCLEROTIC COAT. 



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

 moidal cells ; into the antrum maxillare ; through the nasal duct to 

 the surface of the eye, where it is continuous 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 pharnyx and mouth, and thence through the lungs and ali- 

 mentary canal. 



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

 thelium supporting innumerable vibratile cilia. 



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

 anterior and posterior ethmoidal, from the ophthalmic artery ; and 

 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 cor- 

 respond 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. 



1. The Sclerotic and Cornea form the external tunic of the eye- 

 ball, and give to it its peculiar form. Four-fifths of the globe are 

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



The Sclerotic (rfxX^o?, 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 it is 

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

 Anteriorly it presents a bevelled edge which receives the cornea in 

 the same way that a watch-glass is received by the groove in its 



