186 ANATOMY OF THE HEAD. 



seventh muscle ; and is peculiar to quadrupeds ; being a 

 large and powerful bundle of fibres, which arising from the 

 bottom of the orbit, envelop the optic nerve ; and insert 

 themselves around the posterior middle of the sphere. This 

 muscle acts on the globe of the eye by very forcibly drawing 

 it within the bottom of the orbit ; by which means it 

 partially protects it from injury. The cartilago nictitans is 

 situated at the inner or inferior corner of the eye ; having 

 a distinct fasciculi of muscular fibres running to it from 

 the depressor oculi. It is familiarly called the haw ; and 

 has been excised under the idea of thereby curing spe- 

 cific ophthalmia. It is protruded by the pressure forward 

 of the fat behind the eye, whenever the action of the 

 retractor muscle draws the organ within the orbit ; and 

 is replaced partly by the pressure of the eye ; partly by 

 the fat ceasing to act as a propeller ; and partly by the con- 

 traction of the bundle of fibres before mentioned. From 

 this description, it will be readily seen that this organ 

 acts as a third eyelid. When any foreign substance gets 

 into the eye, the irritation thereby occasioned causes a 

 copious flow of tears ; which wash the body to within the 

 reach of the cartilago nictitans, and at the same time loosens 

 its hold upon the conjunctival membrane, covering the cornea. 

 Then pain occasions the retractor muscle to act repeatedly 

 and forcibly ; which propels the little shovel of a cartilago 

 nictitans suddenly and frequently over the eye ; by which 

 motion it effectually takes up and removes any particle that 

 may be within the reach of its action. 



The eyelids. — The lids are a species of curtain placed 

 before the eye, and exist as an upper and a lower, formed so 

 as to adapt themselves to the globe. The superior is the 

 most considerable ; the lower, in the horse, has but little 

 motion : the corners where they unite are termed canthi ; 

 one of which is the external ; and the larger the internal is 

 also less angular, and furnishes an attachment to the muscle 

 of the orbicularis oculi. The thin cartilaginous rims which 

 form the edges of the eyelids, are each pierced by some little 

 holes ; which pour out a sebaceous matter from the meibo- 

 mian glands : the upper lid is most furnished with hairs, 

 which are not placed in one, but in several small rows ; 

 most abundant on the outer angle, in which direction the 



