532 



^ESTHESIOLOGY. 



with 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 

 sac, containing certain trans- 

 parent humours, 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. 



FIG. 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 ; b, Choroid 

 coat ; c, Retina ; d, Optic nerve ; e e, Ciliary zone ; 

 e' e. Ciliary muscle ; /, Junction of cornea with 

 sclerotic ; g, Iris ; g', Pupil ; h. Crystalline lens. 



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 by the cornea. 



The SCLEROTIC 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 



