TTIE XF.RVOUS SVSTKM 



427 



less definite groups or systems of neurones having certain definite con- 

 nections may also be distinguished. The axones of such a system 

 constitute a tract or, when aggregated into a definite bundle, c\ fascicu- 

 lus. The groups of neurone bodies are termed nuclei. The group or 

 collection of bodies whose axones form a certain tract is the nucleus 

 of origin of that tract. The same nucleus may receive the termina- 

 tions of some other tract and is then the terminal nucleus of that tract. 

 A given neurone system serves as a path for the conduction of some 

 particular kind of nerve impulse. A conduction path, however, is 

 often composed of several neurone systems linked together, thus 

 forming a series of relays. 



Three -neitrone a/Yerenf sufwciseyme^iiz^/toiy/i 



j^/fie^re/t/rieri/iJi era/ neu- 



FiG. 296. — Diagram illustrating an arc transversing only the segmental, and an arc 

 transversing the suprasegmental part of the nervous system. 



All reactions performed by the nervous system must ultimately 

 take effect upon some part of the body or effector and are usually ini- 

 tiated by changes in some receptor. Such a circuit from receptor to 

 effector may be termed a neural arc (Fig. 288), and will involve affer- 

 ent peripheral neurones, efferent peripheral neurones and usually 

 imermediate neurones. The complexity of such arcs depends largely 

 upon the number and character of intermediate neurones intercalated 

 in the arc between the aff'erent and efferent peripheral neurones. 

 Such arcs may obviously traverse centrally only the cord or segmental 

 brain or may also traverse one or more of the suprasegmental parts of 

 the nervous system. Intermediate neurones which link together 

 dift'erent parts of the same segment may be termed intrasegmental 

 neurones. Intermediate neurones which link together different 

 segments of the cord and segmental brain may be termed interseg- 

 mental neurones. Other intermediate neurones form conduction 

 paths to and from suprasegmental parts {afferent and efferent supra- 



