236 



ANATOMY FOE NUESES. [CHAP. XIX. 



E 



perceptive, layer of the eyeball. It forms a nearly transparent 

 membrane situated between the inner surface of the choroid 

 and the outer surface of the vitreous humour, and extending 

 from the exit of the optic nerve to the commencement of the 

 ciliary processes. The structure of the retina is interesting 

 in that it consists not only of a sensory epithelium and a single 

 group of neurones, but contains also a second series of neurones. 



A study of the development of 

 the retina explains this remark- 

 able fact, for it shows that the 

 retina is in part really an out- 

 lying portion of the brain. 



The accompanjdng figure shows 

 the relation of the neurones and 

 epithelial cells. Here it will be 

 observed that it is the axones of 

 the second series of neurones which 

 collect together to form the optic 

 nerve, and after penetrating the 

 choroid and sclerotic coats, pass 

 back to terminate in the brain 

 (Fig. 137). 



The retina is usually described 

 as consisting of eight layers and 

 two limiting membranes ; of these 

 layers, that called the layer of 



FIG. 137. DIAGRAM SHOWING rork and POTIPS is thp rnost rprrmrk 

 RELATIONS OF THE NEURONES AND 



SENSORY EPITHELIUM IN THE RET- able. It is Composed of specialized 

 INA. E, epithelial layer of nucleated -,1 T i n 1-1 T j_i 



rod and cone cells, rods being direct- epithelial cells which are directly 

 ed towards choroid coat of retina; concerned in producing the sensa- 



NI, neurones of first series receiving . ., ,. . ^ .. 



by their dendrones impulses from the tion OI light. Kays Ol light pro- 

 ^uce no effect upon the optic nerve 

 without the intervention of the 



rods and cones. This is proved 



rod and cone cells and transmitting 



them by their axones to N 2 , the 



neurones of the second series. The 



axones of the neurones of the second 



series pass along the inner surface 



of the retina to the blind spot, where by the fact that at the exit bf 



they unite to form the optic nerve. , i , , , j 



the optic nerve there are no rods 



and cones, and this spot is quite blind, rays of light falling upon 

 it producing no sensation. There is one point of the retina 

 which is of great importance, and that is the macula lutea, or 

 yellow spot. It is situated about -fa of an inch (2.12 mm.) 



