NERVE 47 



Stimulation of either branch of the nerve will cause con- 

 traction of both muscle masses. In one case the impulse must 

 travel to the right at the junction of the two nerves and in the 

 other to the left. Therefore an impulse can pass in either 

 direction in a nerve. 



Nerve fibres are classified into two groups, depending upon the 

 direction in which the impulses usually travel. Those which carry 

 toward the brain and cord are called afferent, while those carrying 

 impulses in the opposite direction are designated as efferent. The 

 latter activate muscles and glands. Each type of fibre can conduct 

 in both directions, but in their normal location, conditions are so 

 arranged that impulses generally start only at one end in 

 afferent nerves at the peripheral end, in efferent nerves at the 

 central end. Even though an efferent nerve be stimulated 

 at its peripheral end, the impulse travels only as far as the synapse 

 or connection with the next nerve cell or neurone. In other words 

 the synapse allows the passage of impulses in one direction only 

 in sensory nerves, inward, i.e., towards the central nervous system; 

 in motor nerves, outward; i.e., towards the periphery. 



When we consider the necessity of careful insulation to prevent 

 the spreading of an electric current in contiguous wires, it is ex- 

 tremely interesting to know that there is complete isolation of 

 impulses travelling along contiguous nerve fibres. If this were not 

 the case, endless confusion would result where great numbers of 

 both afferent and efferent fibres are closely bound up in the same 

 nerve trunk. 



Experiment 19. The object of this experiment is to determine 

 whether nerve impulses travelling over certain fibres in a nerve 

 trunk spread to all the fibres in that trunk or remian insulated 

 in fibres with which they are travelling on entering the trunk. 

 Pith a frog, remove the viscera and the skin of the legs. Lay 

 it on its back on a glass plate. The various nerves which make 

 up the sacral plexus may each be stimulated by unipolar induc- 

 tion. To do this connect the tissues of the back, by means of the 

 plate electrode and a wire, to the gas or water pipes leading to 

 the ground. 



A. Attach to another wire a pithing needle and fasten the 

 wire to one post of the secondary coil of the inductorium. With 



