THE CORNEA, AND CHOROID COAT. 423 



single vision with the two eyes, and exact vision. The muscles in this 

 case, as in most others, are governed, not directly but indirectly, by 

 our endeavoring to accomplish a certain end ; and as we cannot see 

 an object singly by directing one eye upwards and the other down- 

 wards, or both eyes outwards, we cannot accomplish those acts. If, 

 however, the position of the image in one eye be slowly displaced 

 sideways, upwards, or downwards, by means of a prism, held and 

 turned slowly before the eye, then the eyeball in question is moved 

 within certain limits to one side, or up or down, so as to maintain the 

 singleness of vision ; when the prism is removed the object is, though 

 for a short time only, seen double. All the movements of 'the eyeball, 

 excepting that of rotation around the antero posterior axis, which is 

 performed by the oblique muscles, may, by practice, be executed, 

 without the exercise of vision, when the eyelids are closed, but with 

 more or less difficulty or restraint; convergence of the eyes, as in 

 squinting, is the most difficult to imitate. The inability to rotate the 

 eyes by a direct volitional act, is due to the fact that we have not 

 learned how to accomplish it, and not to any special structural limita- 

 tion. (Helmholz.) This rotation is shown by Helmholz to be of great 

 importance under certain circumstances ; for example, in maintaining 

 the meridian plane of the eye in a uniform position, as to vertically, 

 in its various secondary movements, and also in accomplishing certain 

 special adjustments necessary for stereoscopic vision. 



The eyeball is supplied with motor, sensory, and sympathetic 

 nerves, derived from the third and fifth cranial nerves, and from the 

 lenticular or ophthalmic ganglion. The so-called ciliary nerves, from 

 twelve to fifteen in number, perforate the sclerotic, and are distributed 

 to the choroid coat, to the iris, and to a muscular structure known as 

 the ciliary muscle. The ciliary arteries derived from the ophthalmic 

 artery, are fine, of considerable length, and pursue a somewhat tor- 

 tuous course before they enter the eyeball. The veins are fewer, 

 but large. 



The coats of the eyeball are the cornea and sclerotic, the choroid and 

 the retina. 



The sclerotic coat, Fig. 83, s, so named from its comparative firm- 

 ness, forms the outermost tunic of the larger spheroidal portion or 

 posterior f ths of the eyeball, leaving an opening in front, into which 

 is fitted the transparent structure called the cornea, corresponding 

 with the smaller spheroidal portion, or remaining th of the eyeball. 

 The sclerotic is a strong, opaque, fibrous structure, composed essen- 

 tially of white fibrous tissue, arranged in interlaced bundles mixed 

 with elastic tissue, and in its deeper layers with pigment fibres ; its 

 vascularity is not great. It is perforated behind by the optic nerve, 

 h, and presents, at the place of perforation, or lamina cribrosa, a num- 

 ber of minute orifices for the passage of the nervous funiculi ; in the 

 centre of this lamina is a larger opening called the optic pore, for the 

 transmission of the small artery which supplies the retina, the central 

 artery of the retina. 



The cornea, c, or the transparent convex structure, which occupies 

 the opening in the anterior part of the sclerotic, like a sort of watch- 



