112 THE COMMON FROG. [chap. 



CHAPTER VIII. 



THE NERVOUS SYSTEM OF THE FROG. 



The nervous system consists of the brain, spinal 

 marrow, and nerves. 



The whole consists of a soft, white substance, ulti- 

 mately composed of minute threads, termed nerve- 

 fibres, and minute round bodies called ** ganglionic 

 corpuscles." 



The brain is contained in the cavity of the skull, 

 and consists of a rounded mass made up of cor- 

 puscles and fibres, and itself contains a cavity which 

 is a remnant of the original canal formed by the 

 upgrowth and overclosure of the walls of the primi- 

 tive ^roove of the embr\'o. 



The spinal marrow (as has been said earlier) 

 traverses the canal formed by the successive neural 

 arches of the vertebrae, being directly continuous 

 with the brain, which it, as it were, continues on 

 down the back. Like the brain, it is largely com- 

 posed of corpuscles, as well as fibres, and itself 

 contains a longitudinal cavity (continuous with that 

 in the brain), which is also the ultimate condition 



