THE INTERNAL OR NERVOUS TUNIC OF THE EYE. 



465 



mater. Between these two sheaths is a fissure, divided into two 

 smaller clefts by a continuation of the arachnoid. Both these clefts 

 are traversed by connective -tissue trabeculae. The inner cleft com- 

 municates with the subarachnoid space ; and the outer narrower 

 cleft, with the subdural space. 



The fibers of the optic nerve are medullated, but they possess no 

 neurilemma. They are grouped into small bundles by septa and 

 bands of fibrous tissue penetrating the optic nerve from the inner or 

 pial sheath. Within these bundles the nerves are separated by neu- 

 roglia tissue, neuroglia cells and fibers, which further forms a 

 thin sheath about each bundle. In the region of the sclera and cho- 

 roid the optic nerve-fibers lose their myelin, and the septa of the inner 

 or pial sheath become better developed and relatively more numer- 

 ous. Connective-tissue fibers from the sclera and choroid also trav- 

 erse this region of the optic nerve, giving rise to what is known as 

 the lamina cribrosa. At from I y 2 to 2 cm. from the eyeball there 

 enter into the optic nerve laterally and ventrally (according to J. Deyl, 

 mesially) the central artery and vein of the retina, which very soon 

 come to lie within the axis of the nerve. Here they are surrounded 

 by a common connective-tissue sheath which is in direct connection 

 with the pial sheath. The optic nerve-fibers extend through the 

 lamina cribrosa into the retina, where they spread out as the nerve- 

 fiber layer in the manner previously described. 



9. BLOOD-VESSELS OF THE OPTIC NERVE AND RETINA, 

 The blood-vessels of the optic nerve are principally derived from 



the vessels of the pial sheath. In that portion of the nerve con- 

 taining the central vessels of the 



retina the latter anastomose with 



the pial vessels, so that this por- 

 tion of the optic nerve is also 



supplied by the central vessels. 



At their entrance through the 



sclera the short posterior ciliary 



arteries form a plexus around the 



optic nerve, the arterial circle of 



Zinn, which communicates, on 



the one hand, with the vessels of 



the pial sheath, and, on the other, 



with those of the optic nerve. 



At the level of the choroid the 



vessels of the latter communicate 



by means of capillaries with the 



central vessels of the optic nerve. 

 The central artery and vein 



of the retina enter and leave 



the retina at the optic papilla, dividing here, or even 

 30 



Artery. 



Zone sur- 

 rounding 

 artery free 

 from capil- 

 laries. 



Fig. 362. Injected blood-vessels of 

 the human retina ; surface preparation ; 



within 



