THE EYE. 185 



three coats of the choroid merely consists of a number of 

 veins, which take a very tortuous course ; and are therefore 

 imagined to represent an aqueous vortex. 



The retina {Fig 19 . 9). — The last and most important 

 expansion of the eye is the retina ; so called from the net- 

 like structure of its nervous layer : it consists of three coats. 

 The first is membraneous, being designed to keep the pigment 

 from falling upon the nerve (Fig 19 . e). The second 

 tunic is composed of a net- like expansion of the optic nerve ; 

 which enters the eye at its most backward part ; and upon 

 the healthy condition of this nerve that vision depends 

 {Fig 19 ./). The optic nerve does not enter the sclerotic 

 coat directly in its centre ; but rather to one side ; and its 

 expansion extends almost to the attachment of the iris. 

 The root of the optic nerve can be well seen with the naked 

 eye when the horse stands in a good reflected light ; and it 

 has been mistaken for a cataract. The third is arterial ; 

 the small vessels, which form it, being supplied by the 

 central artery of the retina {Fig 19 . I). 



The muscles of the globe of the eye are seven : four of these 

 are termed recti, or straight, and perform the offices of 

 elevation, depression, abduction, and adduction; and are in- 

 dividually named elevators, depressors, abductors, and ad- 

 ductors accordingly. The four arise from the bottom of the 

 orbit, and are inserted into the anterior part of the sclero- 

 tica ; the elevator being on the top ; the depressor at the 

 bottom ; the abductor to the outside ; and the adductor on 

 the inside of the eye. They are all inserted by tendons, and 

 these tendons run to the edge of the cornea, forming the 

 white of the horse's eye, and constituting the tunica albu- 

 genia. Two others of the seven muscles are termed the 

 obliquus major and minor. The first of these arises from 

 the inner and posterior portion of the orbit ; and passes ob- 

 liquely through a cartilaginous ring, which is lubricated with 

 synovia, at the anterior and outer side of the supra-orbital 

 foramen : then returning, its tendon passes under the ab- 

 ductor tendon, to insert itself into the side of the globe ; 

 this, therefore, can partially rotate the eye. The obliquus 

 minor arises from a little posterior to the nasal duct, and 

 inserts itself beneath the abductor muscle. Use. To rotate 

 the eye partially downward. The retractor oculi forms the 



