532 



^STHESIOLOGT. 



Vith the base of the sphenoid and occipital bones, descending to 

 the sides of the pharynx and larynx, where they terminate in a 

 cul-de-sac ; they likewise extend from the inferior extremity of 

 the parotid gland to the anterior face of the atlas. For the 

 auricular muscles, see p. 174. 



The Eye. 



• 



The apparatus of vision comprises the essential organ, the globe 



of the eye or eyeball, and 

 its accessory parts or append- 

 ages. 



The Globe of the Eye is 

 spherical in form, having the 

 segment of a smaller sphere 

 engrafted upon its anterior 

 surface, and increasing its 

 antero-posterior diameter. It 

 consists of a membranous 

 certain trans- 



sac. 



con taming 



Fio. 205. 



' * Diagramatic horizontal section of horse's eye. 

 '1, Anterior chamber containing aqueous humour; 

 2, Posterior chamber ; 3 3, Canal of Petit ; 4. 

 Vitreous humour ; a, Sclerotic coat ; h, Choroid 

 coat ; c, Ketina ; d, Optic nen-o ; e e, Ciliary zone ; 

 ^ €', ciliary muscle ; /, JuncLlon of cornea with 

 ■derotie ; g. Iris ; g', Pupil; h. Crystalline lens. 



parent hwniouvs, of different 

 densities, which serve as re- 

 fracting media ; it is attached 

 to the orbit by the muscles 

 which move it, and reposes on 

 a cushion of fat, which not 

 only maintains it in its proper 

 position, but also assists in 

 steadying its movements. 



TUNICS OF THE EYEBALL. 



These are .three in number — 1. The sclerotic and cornea ; 

 2. The choroid and iris; 3. The retina. 



The sclerotic coat and cornea form the external tunic, and it 

 is to them that the eyeball owes its peculiar shape. About 

 four-fifths of the globe are invested by the sclerotic, the 

 remaining fifth b}^ the cornea. 



The ScLi:p.O'riC coat is a dense white fibrous membrane, which 

 extends from the insertion of the optic nerve, with the sheath of 

 which it is continuous, to the cornea. Its external surface is in 



