236 



ANATOMY FOR NURSES. [Chap. XIX. 



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 accompan3dng 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 

 rods and cones is the most remark- 

 able. It is composed of specialized 

 epithelial cells which are directly 

 concerned in producing the sensa- 



by their (lendrones impulses from the tion of light. Rays of light pro- 

 rod and cone cells and transmitting d^ce nO effect Upon the Optic nerve 

 them by their axones to No, the . , •, • . , • c •^ 



Without the intervention ot tlie 

 rods and cones. This is proved 

 by the fact that at the exit of 

 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 ^V of an inch (2.12 mm.) 



Fig. 137. — Diagram showino 

 Relations of the Neurones and 

 Sensory Epithelium in the Ret- 

 ina. E, epithelial layer of nucleated 

 rod and cone cells, rods being direct- 

 ed towards choroid coat of retina; 

 Ni, neurones of first series receiving 



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 

 they unite to form the optic nerve. 



