1482 



HUMAN ANATOMY. 



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Fig. 1240. 







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Outer layer 

 Inner layer 

 Posterior wall 

 of lens-sac 



Mesoblast 

 Lip of optic cup 



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Development of the Retina. — As already pointed out, the retina develops from the walls of 

 the optic vesicle, the pigmented layer being derived from the uninvaginated outer wall, the pig- 

 ment appearing early and first near the anterior margin of the optic cup; the remainder of the 

 retina comes from the rapidly growing cells of the inner wall. The first cells to be differentiated 

 in the nervous portion of the retina are the spongioblasts which develop into the supporting 



neurogliar fibres, the fibres of Mi'tller. These are 

 strengthened by the addition of mesoblastic elements, 

 which enter the inner layers along with the blood- 

 vessels. The neuroblasts develop from cells which 

 correspond in position to those of the external nu- 

 clear layer. As they divide, the cells are displaced 

 inward, so that the ganglion-cells represent the oldest 

 descendents. When seven or eight layers have been 

 differentiated, the ganglion-cells send out axones, 

 which form the fibre-layer and converge toward the 

 optic nerve. The visual cells are the last to appear, 

 the layer of rods and cones developing as cuticular 

 outgrowths from the cells of the external nuclear layer. 

 Anteriorly the walls of the secondary optic vesicle 

 are reduced to a double layer of cells. For a certain 

 distance, corresponding to the position of the future 

 ciliary body (pars ciliaris retinae), the outer cells are pigmented, whilst the inner ones are trans- 

 parent. Still farther forward, the rudimentary portion of the nervous tunic is continued over 

 the posterior surface of the iris (pars iridica retinas) as a double layer of deeply pigmented 

 cells which extends as far as the pupillary margin which thus corresponds to the anterior lip of 

 the secondary optic vesicle. 



The optic nerve is developed secondarily and in close association with the early optic stalk, 

 which is at first hollow, and later becomes grooved along its inferior surface. The walls of 

 this fcetal cleft become approximated and, after the entrance of the blood-v-essels, the lips 

 of the cleft fuse, the vessels being thus enclosed. Since the fibres of the optic nerve are for 

 the most part axones of the ganglion-cells of the retina, it is evident that they are not developed 

 within the nerve, but invade the latter as outgrowths of fibres from the retina, pushing along 

 the optic nerve and tract to reach their cerebral connections. In addition to these centripetal 

 fibres, a certain number of centrifugal ones appear later as outgrowths from cells within the 

 brain. The supporting tissue is developed by 



Foetal cleft 



Sagittal section of developing eye at same 

 stage as preceding specimen, showing invagi- 

 nation of optic vesicle along fcetal cleft. X 3°- 



proliferation of the cells of the optic stalks 

 and their differentiation into neurogliar ele- 

 ments, assisted by the mesoblastic elements 

 from the surrounding pia and the portion 

 which enters the cleft with the blood-vessels. 

 The nerve-fibres are at first naked axis-cylin- 

 ders, which later acquire medullary sheaths. 

 Development of the Fibrous and Vascular 

 Tunics. — With the separation of the lens- 

 vesicle from the overlying ectoblast, the meso- 

 blast insinuates itself between these structures, 

 in addition to surrounding the entire ecto- 

 blastic optic vesicle. The portion surrounding 

 the optic vesicle posteriorly thickens rapidly 

 and becomes differentiated into the vascular 

 tunic, or choroid, whilst the outer layer be- 

 comes the fibrous tunic, or sclera. The choroid 

 appears first, the pigmentation of its cells being 

 evident by the seventh month. The meso- 

 blastic process between the lens and the ecto- 

 blast is very thin at first, but subsequently 

 splits into two layers. The anterior of these 



Fig. 1241. 



Upper eyelid 

 Outer pigmented 

 retinal layer 

 Inner retinal 

 layer 

 Mesoblast 



Lens, now solid 

 Optic nerve 



Vascular vitreous 



tissue 



Ectoblast 



Lip of retinal coat 



Mesoblast 

 Lower eyelid 



Much later stage, showing lens now solid; layers of 

 optic vesicle converted into retuial coat; vascular vitreous 

 tissue; condensation and ituasioii of mesoblast. X 20. 



becomes the substantia propria of the cornea and its lining endothelium. The latter produces 

 the membrane of Descemet. The posterior mesoblastic layer carries blood-vessels which help 

 to form the capillary net-work surrounding the lens. The space between the' two mesoblastic 

 layers represents the future anterior chamber of the eye. About the fourth fcetal month the an- 

 terior lip of the optic vesicle pushes, forward in advance of the lens and carries with it additional 

 mesoblastic tissue. From this the iris is developed, the stroma being formed by the mesoblast, 

 whilst the posterior pigmented portion represents the anterior part of the optic vesicle, from 

 which the dilatator muscle (and, according to some authorities, also the sphincter pupillae) is 

 derived. The ciliary processes are produced by the rapid lateral expansion of the walls of the 



