762 DEYELOPMEXT OF THE ORGAXS OF SEXSE. 



connecting them with the spinal nerres ; wlnle the great sympathetic 

 itself consisted at first of a chain of rounded masses representing; the 

 ganglia, connected together, but so closely set that scarcely any inter- 

 vening nervous cord was at first perceptible. He also observed the 

 separate formation in the mesentery of birds of the large visceral nerve 

 which he discovered in these animals. The gangliated cord of the 

 sympathetic has been described and figured by Kiilliker in the human 

 foetus of eight or ten lines long. The peripheral sympathetic nerves 

 are also formed at a very early period, and are perceptible in a foetus 

 of three months. In the hinder part of the abdomen their origin 

 appears to be intimately connected in some way with the formation of 

 the suprarenal bodies. 



DEVELOPMENT OP THE EYE. 



The embryonic structures forming the eyeball and its contents may 

 be considered as proceeding from three sources, viz., 1st, by evolution 

 or expansion from the medullary wall of the first encephalic vesicle 

 (thalamencephalon), giving rise to the retina, in its nervous and pig- 

 mental structure and optic nerve ; 2nd, by involution or depression and 

 development of a part of the cuticular epiblast, forming the foundation 

 of the lens and the epithelium of the conjunctiva ; and 3rd, by the 

 intrusion of mesoblastic elements between and around the other parts, 

 so as to furnish the materials out of which are formed the general 

 coverings of the eyeball, cornea and sclerotic, the fibrous and vascular 

 choroid, the ciliary apparatus and iris, the capsule of the lens and 

 the capsulo-pupillary membrane, the vitreous humour, and all the 

 fibrous and vascular parts of the organ. 



The very early formation of the primary optic vesicles by the expan- 

 sion of the lower and anterior parts of the wall of the anterior primary 

 encephalic vesicle has already been described, and the manner in which 

 each of these vesicles forms a hollow pediculated chamber communi- 

 cating by its stalk with the general ventricular cavity of the primitive 

 brain. The first important change which the primary optic vesicles 

 undergo is connected with the depression of the rudimentary lens from 



Fir,'. 562. Fig-. 562. — Section of the Head through the Primitive 



Optic Capsule uf one side in an Embryo-Calf of 9 mji. 

 LONO, MAGNIFIED (from Julius Arnold). 



To the right is seen the optic capsule with its contracted 

 pedicle and its outer wall depressed by the thickening of the 

 corneous laj'er which forms the commencement of the forma- 

 tion of the lens. The optic stalk is in communication with the 

 thalamencephalon. Mesoblast is seen between the optic capsule 

 and the lens rudiment. 



without, and consists in a doubling back or in- 

 wards of the medullary wall of each vesicle, so as 

 to form a depression or cup at its lower and 

 anterior part, into which the commencing lens 

 in part sinks. This depression has been called the 

 secondary optic vesicle, or the opfic cup (Foster and 

 Balfour). From a very early period the outer fold of this cup under- 

 goes a much greater thickening by the rapid development of its 



