08 THE NP:RVOUS SYSTB:M 



impulse in motor nerves. In sensory nerves the rate is said to be somewhat 

 higher; in human nerve from 30 to 42 meters per second. 



Physiological Types of Nerve Cells. Many classifications could 

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

 but at this place attention will be called to only one. Nerve cells may be 

 classified as afferent or sensory, efferent or motor, and connecting or trans- 

 mitting cells. 



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, ultimately producing 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 may be included those units which 

 act as connecting links within the central organ, especially within coordinate 

 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 aggre- 

 gations 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 by 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 greater or less complexity. 



It is by means of the nerve centers that the activities of the differentiated 

 parts of the human body are brought into intimate correlation. The nerve 

 centers exercise their influence through the power of inhibiting or decreasing ac- 

 tivity; or, on the other hand, of augmenting or increasing the activity in the 

 peripheral tissues or in other parts of the nervous system. For example, the 

 vagus center regulates the activity of the heart muscle by its power to decrease 

 or inhibit cardiac contractions. This center, we have already found, is incon- 

 stant tonic activity; that is to say, in constant regulative control of the heart. 

 The cardiac augmentory center, on the other hand, produces just the opposite 

 effect, increasing the activity of the cardiac muscle. What is true for the heart 

 is likewise true in general for other tissues of the body. The numerous nerve 

 centers in the central nervous system are brought into correlation through an 



