1534 A MANUAL OF ANATOMY 



developed from the thickened anterior part of the choroid. The outer layer 

 of the ciliary portion, as elsewhere, forms the pigmentary layer of the retina. 

 The inner layer of the ciliary portion, which is very thin, forms (i) the pars 

 ciliaris retinse, on the posterior surfaces of the ciliary processes, and (2) the 

 pigmented pars iridica retinae {uvea) on the posterior surface of the iris. 



The fundus of the optic cup is the proper retinal region. The outer layer 

 forms, as in the ciUary region, the pigmentary layer ol the retina. The inner 

 or retinal, layer becomes differentiated into all the layers of the retina except 

 the pigmentary layer. The changes which it undergoes are very complicated. 

 Its thickness is considerably increased, and it subdivides into two layers — 

 outer and inner — from which the various retinal strata (except the outer 

 pigmentary layer) are specialized. 



The optic stalk is transformed into the optic nerve. The stalk is at first 

 hollow, its cavity communicating with that of the optic vesicle on the one 

 hand, and with the third ventricle of the brain on the other. As stated, the 

 choroidal fissure involves the under surface of the optic stalk near the optic 

 vesicle, as well as the under surface of the optic vesicle itself. When the 

 choroidal fissure undergoes closure, the hyaloid artery, which passed through 

 that fissure, becomes enclosed within the optic stalk, and forms the arteria 

 centralis retina of adult life. By the closure of the choroidal fissure, and 

 the consequent enclosure of the hyaloid artery, the cavity of the distal portion 

 of the optic stalk becomes obliterated. Inasmuch as the ventral or lower 

 wall of this part of the stalk has been previously invaginated, the wall of the 

 stalk is now composed of two layers — outer and inner — the inner being 

 formed by the invaginated ventral or lower wall. The outer layer of the 

 optic stalk is now continuous with the outer layer of the optic cup, whilst 

 the inner layer of the optic stalk is continuous with the inner layer of the 

 optic cup. As regards the proximal part of the optic stalk, its cavity becomes 

 gradually closed. The wall of the optic stalk becomes thickened, its cells 

 proliferate, and they give rise to the neuroglial or sustentacular tissue of the 

 future nerve. The nerve-flbres, which build up the optic nerve, are regarded 

 as having two sources. The majority of them represent the a^ons of the 

 ganglion-cells of the retina, which pass in the optic stalk to the diencephalon 

 and mesencephalon. These are therefore centripetal fibres. Other fibres are 

 regarded as being centrifugal, these arising in connection with the diencephalon 

 and mesencepalon. 



Vitreous Body. — ^This body is formed within the optic cup, for the most 

 part posterior to the lens-vesicle. It is principally developed from the ecto- 

 derm, but the mesoderm also takes part in its formation. The ectodermic 

 fibres are derived from those cells which pertain to the sustentacular fibres 

 of the retina, or fibres of Miiller. These ectodermic fibres form a very delicate 

 reticulum, which constitutes the primary vitreous body. In all probability 

 the vitreous body is reinforced by fibres from the mesodermic tissue which 

 enters the optic cup, along with the hyaloid artery, through the choroidal 

 fissure on the ventral or under surface of the optic cup. The peripherad part 

 of the vitreous body gives rise to the hyaloid membrane. 



The vitreous body soon becomes very vascular. The chief artery is the 

 hyaloid artery, which enters the optic cup, along with mesoderm, through the 

 choroidal fissure. The anterior ciliary arteries also contribute to its vascu- 

 larity, these vessels gaining entrance between the margin of the optic cup 

 and the lens-vesicle. The hyaloid artery occupies the centre of the vitreous 

 body, lying in a passage, called the hyaloid canal, or canal of Stilling. On 

 reaching the posterior part of the lens-capsule it breaks up into a number of 

 branches. The arteries of the vitreous body at this stage confer great vas- 

 cularity upon the lens-capsule, hence the name tunica vasculosa, and through 

 it they contribute to the nourishment of the lens-vesicle during the period 

 of its active development. The anterior part of this tunica vasculosa, as 

 stated, gives rise to the membrana pupillaris. 



Towards the latter part of intra-uterine life the bloodvessels disappear, 

 the hyaloid canal, or canal of Stilling, remaining permanent. The part of the 

 hyaloid artery which becomes enclosed within the optic stalk forms the 

 arteria centralis retina. 



