THE FIBROUS TUNIC. 



1449 



(microphthalmos) and situated deep in the orbit near the optic foramen. The patient 

 may otherwise be entirely normal ; or other developmental errors, as hare-lip or 

 cleft-palate may be present. In some instances where no eyeball was found, the 

 optic nerve had not entered the orbit, and in others the chiasm had not formed, the 

 primary optic vesicle having failed to develop. 



Multiple eyes occur in some monsters. As digits sometimes bifurcate to form 

 supernumerary digits, so the cephalic end of the embryo may divide, giving rise to 

 two heads. These may fuse, when, according to the extent of fusion, there will be 

 four, three, or two eyes ; or if both the orbits and the eyes fuse there may be 

 only one eye (cyclopia). 



The actual size of the eye in man varies little, the apparent size depending 

 chiefly upon the projection from the* orbit and the part exposed between the lids. 

 The variation in different animals depends rather upon the necessity for acuteness of 

 vision than upon the size of the animal. The larger the globe the farther the cornea 

 and lens from the retina, and 

 therefore the larger and 

 more distinct the image on 

 the retina of the object seen. 

 The more active the animal 

 the greater is the necessity 

 for acuteness of vision, and 

 therefore the larger the eye. 

 The eyes of birds are pro- 

 portionally larger than those 

 of other animals. Nocturnal 

 animals, such as the owl, 

 have large eyes. The large 

 retinal image probably com- 

 pensates for the scarcity of 

 light, to which they are 

 accustomed. 



Fibre layer 

 Ganglion cells 

 Bipolar cells 

 Visual cells 



Pigment layer 

 Strom a 



Lamina fusca 



Fibrous tissue 

 of sclera 



Episcleral 

 endothelium 

 Space of Tenon 

 between sclera 

 and capsule 

 of Tenon 



Section of three coats of eyeball, about five millimeters from optic papilla; 

 capsule of Tenon seen below sclera. X 4. 



THE FIBROUS TUNIC. 



The Sclera. The 

 sclera, or sclerotic coat, is 

 a firm, dense fibrous coat 

 which forms the posterior 

 four-fifths of the outer coat 

 of the eye, being closely con- 

 nected with the sheaths of 

 the optic nerve posteriorly, 

 and joining in front with 

 the cornea. In the neigh- 

 borhood of the optic nerve it measures i mm. in thickness, and gradually becomes 

 thinner toward the equator, until, just posterior to the attachment of the tendons 

 of the ocular muscles, it measures only .4 mm. After receiving the expansions 

 of these tendons it again becomes thicker and reaches a thickness of .6 mm. In 

 children and in individuals who have thin sclerae and deeply pigmented eyes, 

 the sclera possesses a bluish white color, while in old age it assumes a yellowish 

 tinge. The optic nerve passes through this tunic at a position i mm. below and 

 from 2. 53 mm. to the inner side of the posterior pole of the eye ; the canal is 

 partially bridged over by interlacing fibrous bundles, the lamina cribrosa, which 

 are intimately associated with the supporting tissue of the nerve. Grouped around 

 the nerve entrance are small openings for the ciliary nerves and posterior ciliary 

 arteries, and toward the equator four or five for the vence vorticose which emerge 

 from the choroid. 



Structure of the Sclera. The sclera is composed of interlacing bundles of 

 white fibrous tissue, which on the outer and inner surface have chiefly a meridional 

 direction, while the central bundles form a fairly regular alternation of circular and 



