CH. IX] THE ORGANS OF SPECIAI^ SENSE 107 



or eye socket, by their action control and facilitate the movement 

 of the eyeball in any desired direction. Together they jslay an 

 extremel3' important j^art in the case of man in ensuring that the 

 images .received by the two eyes give rise to a single visual 

 impression. 



The accessory structures, the eyelids, eyelashes, and the 

 lachr3'mal glands, together with the conjunctiva, a delicate mem- 

 brane linmg the imier surface of the e3"elids and continued over 

 the front surface of the eyeball, protect the eye from the liability 

 to injury from dust and other foreign bodies. The watery secretion 

 from the lachrymal glands contmually floods across the surface 

 of the conjunctiva to the inner angle of the eyes, whence it drains 

 away through the lachrymal duct to the nasal cavity, carrying 

 Mdth it any dust particles which may have lodged on the surface 

 of the conjunctiva. 



The internal structure of the eye. A longitudinal section through 

 the eyeball demonstrates that the eye is divided into two compart- 

 ments by a delicate, elastic, transparent, lenticular-shaped body, 

 the crj'stalline lens, which is doubly convex, and is held in place by 

 a strong membranous frame called the suspensor}' ligament, which 

 in turn is itself inserted into the inner or choroid coat of the eye- 

 ball. The front comj^artment, bounded externally by the cornea 

 and internally by the crystalline lens, is filled with a semi-fluid, 

 transparent substance called the aqueous humour. The posterior 

 compartment is also filled with a clear jellj'-like substance called 

 the vitreous humour, and these humoiu's by their action cause the 

 eye to preserve its globular shape. Lpng in front of the crystalline 

 lens is a pigmented, curtain-like structure called the his which is 

 pierced in the centre by an aperture, the pujiil, through which the 

 light rays penetrate into the mterior of the e3^e. The shape of the 

 pupil varies in the domestic animals, in man and the dog it is 

 circular, in the horse, sheep and ox it is elliptical, the long axis of 

 the ellipse being horizontal. The iris itself is pigmented, is opaque 

 to light, contains circular and longitudmal unstriated muscular 

 fibres, which by their action cause the pupil to dilate or contract 

 and thus regulate the amount of light which is allowed to pass 

 through the crj-staUine lens. 



Action of the crystalline lens. Light rays passing from a near 

 or distant object and falling on the surface of the eye are refracted 

 or bent in passing through the crystalline lens and are brought to 



