PHYSIOLOGICAL TYPES OF NEURONES 539 



Similarly impressions made upon an efferent nerve may cause, a, contraction 

 of muscle (motor nerve); b, it may influence secretion (secretory nerve); 

 c, it may influence nutrition (trophic nerve) ; or d, it may inhibit, augment, 

 or stop any other efferent action. 



By artificial stimulation nerve impulses can be made to pass in both di- 

 rections in all classes of nerve processes. That is to say, if a motor axone is 

 artificially stimulated in the middle of its course it will not only convey a nerve 

 impulse to its distribution, but also a nerve impulse will pass back over the fiber 

 to the cell body and out over the dendrites. Normally, in the complex of the 

 body, it is probable that such a neurone will be stimulated only at its points 

 of contact with other neurones chiefly through its dendrites, and especially 

 by means of the sensory cells. The dendrites therefore will receive the nerve 

 stimulus, carry it through the cell body to the axone and its distribution. 

 In such cells there is isolated, or uninterrupted, conduction throughout the 

 extent of the neurone. The nerve impulse is able to pass from a given 

 neurone to adjacent ones only at the termination of the axone or its branches, 

 and such terminations may be considered as special organs for the trans- 

 ference of the nerve impulses. This activity involves isolated conduction in 

 nerve fibers bound in a common nerve trunk. It has been supposed that the 

 myelin sheath of a medullated nerve acts as an insulator of the axis-cylinder, 

 but this can be only relatively true, for the reason that non-medullated nerves 

 do not possess the myelin sheath. In non-medullated nerves we must sup- 

 pose that the primitive sheath is sufficient to give insulated conduction, or 

 that it is an inherent property of the axis-cylinder itself to carry the nerve 

 impulse without transmission to adjacent fibers. 



We have already, page 500, discussed the rate of transmission of the ne-rve 

 impulse in motor nerves which was given as from 27 to 30 meters per second. 

 In sensory nerves the rate is said to be somewhat higher; in human nerve 

 from 30 to 42 meters per second. 



Physiological Types of Neurones. Many classifications could be 

 made of nerve cells, based on the differences in their functional relations, 

 but attention will be called to only one. Neurones may be classified as 

 afferent or sensory, efferent or motor, and connecting or transmitting. 



Under afferent neurones are classed all those neurones which transmit 

 the effects of external stimuli received through the sense organs, both general 

 and special sense organs. These neurones carry nerve impulses toward the 

 central nervous system, which may ultimately produce those changes in the 

 cerebral cortex which are associated with states of consciousness. 



Under efferent neurones are included all those which transmit nerve im- 

 pulses from any part of the central nervous system to the muscles, that is, 

 motor nerves; or transmit nerve impulses to the glands, secretory nerves; or 

 that transmit nerve impulse, which inhibit peripheral action, inhibitory 

 nerves. 



