578 THE NERVOUS SYSTEM 



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 nerve 

 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. 



Under central or transmitting neurones are included those units which 

 act as connecting links within the central organ, especially within coordi- 

 nate parts of the central nervous system, between the afferent and efferent 

 neurones. 



Nerve Centers. Whenever a number of neurones are gathered in 

 one group to accomplish some specific function it is called a nerve center. 

 The term usually applies to the aggregation of cell bodies and their dendritic 

 processes in contradistinction to nerve trunks. There are aggregations 

 of nerve cells into different specific groups, to which we cannot in every 

 case ascribe a specific function. These groups are not called nerve centers, 

 but are described by the general anatomical term, ganglia. Such ganglia 

 are represented in the sympathetic chain, the spinal-root ganglia, the ganglia 

 of certain cranial nerves, etc. The nerve centers are found in the spinal cord, 

 the medulla, and the higher cranial groups. The medulla is particularly 

 rich in nerve centers. The cerebro-spinal axis is in fact an aggregation of 

 nerve centers of varying complexity in different parts. 



