REFLEX ACTION 107 



main parts, the peripheral nervous system and the central nervous 

 system. These regions are, oi course, continuous, since the 

 peripheral nervous system, which consists of the cranial, spinal, 

 and autonomic nerves, serves to connect the central nervous 

 system, or the brain and spinal cord, with other systems and organs 

 of the body. The structure and relationships of the divisions of 

 the nervous system have been described in Chapter 3 and should be 

 reviewed in this connection. As our consideration of this phase of 

 physiology progresses, the student must not lose sight of the fact 

 that we are discussing the activities of the nervous system both 

 with respect to the irritability of individual cells and with respect 

 to the coordination of the animal as a whole. 



Reflex Action 



Simple Reflexes. — Every nervous coordination is the result 

 of a reaction by some part of the body to a stimulus received by 

 some other region. The simplest type of response is that known 

 as reflex action. This term is derived from the analogy that can 

 be drawn between the most primitive reflexes and the reflection 

 of light from a mirror. When one touches a hot stove with one's 

 finger the muscles of the arm react to withdraw the hand. Some- 

 thing passes from the point stimulated to the central nervous sys- 

 tem, and is passed back to produce the contraction of the arm 

 muscles. Another case is that of the knee-jerk reflex in which the 

 leg is extended as the result of a sharp tap below the knee cap. 

 In both of these examples the response to the stimulus is apparent 

 in the same general region that received the stimulus. This is 

 somewhat comparable with the reflection of hght by a mirror, in 

 that the central nervous system seems to reflect the effect of the 

 stimulus. Complete analysis of these simple reflexes shows that 

 they are but the expression of a nervous mechanism which some 

 investigators beUeve explains all nervous coordination. The origi- 

 nal meaning of the term reflex has, therefore, been extended. 



The cellular structure within the nervous system that fur- 

 nishes the mechanism for reflex action is known as the reflex arc. 

 A nerve cell, or neurone, has a nucleus surrounded by cytoplasm as 

 have most cells, but this cytoplasm is extended as two or more 

 processes (Fig. 82, p. 148). The processes from two or more cells 

 meet one another at their ends, but do not fuse. The places of con- 



