48 



MANUAL OF PHYSIOLOGY. 



communication between two cells; they connect together the 

 numerous cells in the various parts of the brain and spinal cord, 

 or pass between the cells of these central nerve organs and spe- 

 cial cells situated throughout the body, which might be called 

 the peripheral nerve organs. 



The simplest idea, then, of a special nerve apparatus is a fibre 

 connecting two cells. The peripheral cell may be a receiving organ 

 (Fig. \ 7, s), from which, when stimulated, impulses are transmitted 

 along the fibre to the central nerve cell, where they give rise to 



FIG. 18. 



FIG. 17. 



FIG. 17. S. Sensory receiving organ with attached afferent nerve fibre. G. Central 

 organs ganglion cells. M. Peripheral organ and efferent nerve. 



FIG. 18. Three medullated nerve fibres, the medullary sheath of which is stained 

 dark with osmic acid. N. Nodes of Ranvier. Two non-medullated nerve fibres, with 

 nuclei in the primitive sheath. 



certain impressions, and so we have a sensory nerve apparatus. 

 Or the central nerve cell may be the receiving agent, getting 

 stimuli from its central neighbors, and transmitting impulses to 

 a peripheral nerve terminal, by which the energy is, as it were, 

 handed over to a muscle (M) or gland, and so we have a simple 

 motor or secretory apparatus. Where the effect of a stimulus can 

 be definitely traced from one nerve cell to another, and from thence 

 by a second fibre to a third cell, the impulse is said to be reflected 



