THE CELLULAR CHANGES OF AGE 



muscle fibres a very characteristic appearance, in 

 consequence of which they are commonly desig- 

 nated in scientific treatises by the term striated. A 

 striated muscle fibre is that which is under the con- 



Pigment J 

 layer. 



Fibres 

 which pass 

 into the op- 

 tic nerve. 



Blood-vessels. 



FIG. 21. SECTION OF A HUMAN RETINA, from Stohr's Histology, sixth 

 American edition. Although the retina is very thin it comprises no less 

 than twelve distinct layers ; the outermost layer is highly vascular. The pig- 

 ment layer prevents the escape of light. The rods and cones convert the light 

 waves into a sensory impulse, which is transmitted through the remaining 

 layers of the retina to the optic nerve. The total structure is extremely 

 complicated. 



trol of our will. It should perhaps be mentioned that 

 the muscle fibres of the heart are also striated, though 

 they differ very much in other respects from the 

 true voluntary muscles. 



Last of all for this series of demonstrations, I have 



