CHAPTER XIII. 



THE NERVOUS SYSTEM. 



133. In the working of a nervous system in any animal, 

 there are three sets of parts involved; namely, the nervous 

 centre, the terminal organ, and the link of communication 

 between the two, namely, the nerve. The distinctive part of 

 every nervous centre consists of nucleated corpuscles, and any 

 nervous mass containing nerve-corpuscles is called a ganglion. 

 The nerve consists of uninterrupted fibres in structural con- 

 tinuity with the corpuscles, and without any branching until 

 close to their termination; and the terminal organs are like- 

 wise in structural continuity with the nerves. 



These terminal organs are, however, of very various 

 descriptions, and with as much claim, in many instances, to 

 be separately grouped as to be considered along with the 

 nervous system to which they are so intimately united. For 

 example, it would be difficult to raise a valid objection to con- 

 sidering muscular fibres in their entirety as terminal organs 

 of nerves; yet they have a development and function of their 

 own, and it would be inconvenient, as well as erroneous, to 

 look on them as mere parts of the nervous system which 

 governs them. It will be recollected that in treating of the 

 skin, several terminal nervous organs have already been 

 described (p. 68), to which the integuments owe their sensi- 

 bility; and more complex organs are devoted to the special 

 senses, which will hereafter be described. But at present 

 we shall consider only the nervous centres and the nerves. 



The nervous system, as developed in the invertebrate 

 animals, consists of a series of ganglia connected in chains 

 or other groups, and giving off nerves; but in the vertebrata 

 it is divisible into two parts. One of these is the cerebro- 

 spinal system, consisting of the brain and spinal cord ; together 



