DIAGRAM ILLUSTRATING REFLEX ACTION IN MAN OR 

 ANY BACKBONED ANIMAL 



From the sensory nerve-ending (S E) in the skin, a stimulus 

 passes up a sensory nerve-fibre (S F) to a sensory nerve-cell (S C) 

 in the spinal ganglion of a dorsal or afferent root (D R) of a spinal 

 nerve. The fibre, continued from the sensory nerve-cell, divides 

 in the spinal cord (SP C), and the message passes on to an asso- 

 ciative, intermediate, or internuncial nerve-cell (A). Thence it 

 is shunted to a motor nerve-cell (MO), from which a command 

 passes down a motor nerve-fibre (M F), issuing by a ventral or 

 efferent root (V R) of a spinal nerve. The motor nerve-fibre ends 

 in a nerve-plate (M E) on a muscle-fibre (MU), which is stimu- 

 lated to contract. 



10. 



DIAGRAMMATIC CROSS-SECTION THROUGH THE RETINA OR PERCIPIENT 

 LAYER AT THE BACK OF THE EYE. (After Hesse.) 



The figure gives some idea of the intricacy of this layer, which is not thicker than the 

 paper of this book. 



i. Inner or anterior limiting membrane, next the vitreous humour in the cavity of the eye. 

 2. A branch of the optic nerve dividing up. 3. A layer of ganglion cells. 4. An inner layer of 

 nerve-fibres. 5. A layer of bipolar cells (so-called "inner granular layer"). 6. An outer 

 layer of nerve-fibres. 7. Layer of visual cells (so-called "outer granular layer"). 8. Outer 

 or posterior limiting membrane. 9. The rods (longer and thinner) and the cones (shorter and 

 broader). 10. Pigment layer of the retina, n. Tangential cells. 12. Bipolar cells. 13. 

 An amacrine cell. 14. Centripetal fibres of the optic nerve. 15. Centrifugal fibres of the 

 optic nerve. 16. Mutter's supporting cells. I, II, III, the three areas of nerve-cells in the 

 retina. 



It is not in the least within the scope of this work to explain the minute structure of the 

 retina; the figure has been introduced to give some impression of the complexity of the vital 

 architecture. The essential fact is that the rods and cones somehow convert the pulses of the 

 luminiferous ether into stimulations of the fibres of the optic nerve. 



