494 THE SPECIAL SENSES. 



vertical muscular curtain, formed of radiating and concentric fibres, 

 pierced at its centre with a circular opening, the pupil, through 

 which the light is admitted, and covered on its posterior surface 

 with a continuation of the choroidal pigment, which excludes the 

 passage of any other rays than those which pass through the pupil. 

 At the same time, the whole globe is inclosed and protected by a 

 thick, fibrous, laminated tunic, which in its posterior and middle 

 portions is opaque, forming the sclerotic (i), and in its anterior por- 

 tion is transparent, forming the cornea (s). The muscles of the eye- 

 ball are attached to the external surface of the sclerotic in such a 

 way that the cornea may be readily turned in various directions ; 

 while the eyelids, which may be opened and closed at will, protect 

 the eye from injury, and, with the aid of the lachrymal secretion, 

 keep its anterior surfaces moist, and preserve the transparency of 

 the cornea. 



The organ of vision is supplied with nerves of ordinary sensi- 

 bility by the ophthalmic branch of the fifth pair. The filaments 

 of this nerve which terminate about the eye are distributed mostly 

 to the conjunctiva, lachrymal gland, and skin of the eyelids; while 

 a very few of them run forward in company with the ciliary nerves 

 proper, and are distributed to the ciliary circle and iris. All these 

 parts, therefore, but more particularly the conjunctiva and skin of 

 the eyelids, possess ordinary sensibility, which appears to be totally 

 wanting in the deeper parts of the eye. The ophthalmic ganglion 

 gives off the ciliary nerves, which are distributed to the iris and 

 ciliary muscle. Finally, the muscles moving the eyeball and eye- 

 lids are supplied with motor nerves from the third, fourth, sixth 

 and seventh pairs. 



Of all the properties and functions belonging to the different 

 structures of the eyeball, the most peculiar and characteristic is the 

 special sensibility of the retina. This sensibility is such that the 

 retina appreciates both the intensity and the quality of the light 

 that is to say, its color and the different shades which this color 

 may present. On account of the form, also, in which the retina is 

 constructed, viz., that of a spheroidal membranous bag, with an 

 opening in front, it becomes capable of appreciating the direction 

 from which the rays of light have come, and, of course, the situation 

 of the luminous body and of its different parts. For the rays which 

 enter through the pupil from below can reach the retina only at its 

 upper part, while those which come in from above, can reach it 

 only at its lower part ; so that in both instances the rays strike the 



