THE NERVOUS SYSTEM 119 



tenuated processes, the nerve fibers, reach out to the most distant parts 

 of the animal. These processes are the most characteristic parts of the 

 neurone. Extending as they do in the largest animals for some meters 

 from their cell bodies, they afford an example of a cell process such as is 

 seen in no other histological unit. Not only are the nerve cells or 

 neurones thus highly specialized in their structure, but they also exhibit 

 profound physiological differentiation. Thus aihong the primary sen- 

 sory neurones each one is connected, as a rule, with a particular portion 

 of the animal for which no other neurone is responsible, and among the 

 motor neurones each one controls a group of muscle fibers not called into 

 action by any other neurone. Hence functional specialization among 

 these elements has # come to be so extreme that the nervous system may 

 be described as one in which differentiation has reached to its very cells, 

 a condition that is shown in no other elements of the body except pos- 

 sibly in the reproductive cells. 



Notwithstanding the high degree of differentiation exhibited by the 

 neurones of the higher animals, these elements may be easily grouped 

 into relatively few classes distinguishable through their connections. 

 These classes are three in number: first, the afferent, or as they are 

 commonly called, the sensory neurones extending in general from the 

 surface of the animal to the central organs and transmitting sensory 

 impulses; secondly, the efferent neurones connecting the central organs 

 with the muscles, glands, etc., and transmitting efferent impulses; and 

 finally, what may be called the association neurones, to extend to the 

 whole nervous system, a term used by Flechsig for elements in a limited 

 part of the brain, or those neurones which lie entirely within the central 

 organ and connect one part of this organ with another. Although the 

 nervous organs of the higher animals are composed of an abundance of 

 all three classes of neurones, the association neurones in all probability 

 far outnumber those of the other two classes and constitute the chief 

 mass of these organs. 



Almost all nervous operations in the higher animals involve all three 

 classes of neurones. The typical nervous reaction of these animals con- 

 sists of a. sensory stimulation followed by a motor response. This oper- 

 ation has been called a reflex, to use that term in its widest sense, that 

 is, irrespective of the association of the action with voluntary or con- 

 scious operations. Such a reflex takes place over an arc of neurones, the 

 sensory members transmitting to the association elements, and these in 

 turn to the motor elements, but in describing the reflex its parts are not 

 conveniently dealt with from the standpoint of the neurone. The 

 reflex, as ordinarily understood, begins with the activity of a sense 

 organ or receptor, from which a sensory impulse passes to the central 

 nervous system or adjustor, whence the nervous disturbance makes its 

 way to the third element or effector, usually a muscle. The sense 



