386 THE SENSORIAL FUNCTIONS. 



a longitudinal series of ganglia, usually twelve or thirteen in 

 number, connected in their whole length by a double fila- 

 ment. By degrees the different parts of which it consists 

 approach each other, the thoracic ganglia, in particular, coa- 

 lescing into larger masses, and becoming less numerous, some 

 being apparently obliterated; the whole cord becomes in con- 



443 



^;^ 



sequence shorter, and the abdominal ganglia are carried for- 

 wards. The optic nerves are greatly enlarged during the 

 latter stages of transformation, and are often each of greater 

 magnitude than the brain itself. A set of nerves has also 

 been discovered, the course of which is peculiar, and appears 

 to correspond with the sympathetic or ganglionic system of 

 nerves in vertebrated animals, while another nerve resem- 

 bles in its mode of distribution, the pneumo-gastric nerve, 

 or par vagum. Very recently Mr. Newport has distinctly 

 traced a separate nervous tract, which he conceives gives 



