NERVOUS SYSTEM. 221 



and the ink-gland to the bottom of that cavity, where they 

 form a ganglion which sends nerves to the digestive, the 

 circulating, and the respiratory organs. So that, although 

 the brain of the cephalopods is still perforated by the oeso- 

 phagus, as in all the inferior classes, we find all the principal 

 parts of the nervous system of the vertebrata already deve- 

 loped in this class, and after undergoing a series of changes 

 of form and position in the inferior tribes of animals, regu- 

 lated by the general development and form of the body, they 

 have here acquired the form and situation which they pre- 

 serve throughout all the higher classes to man. This system 

 begins the development of its ganglionic axis in the lowest 

 acephalous mollusca, as in the lowest helminthoid articulata, 

 below the oesophagus, and extends along the ventral surface of 

 the abdomen ; but in the higher gasteropods, as in the highest 

 insects and Crustacea, we find it advanced in its position, ac- 

 cumulated around the entrance of the digestive canal, and 

 mounting to a dorsal position, which nearly the whole of the 

 lengthened spino-cerebral axis has attained in the cepha- 

 lopods. 



FIFTH SECTION. 



Nervous System of the Spini-cerebrated or Vertebrated 

 Classes. 



The great axis of the nervous system occupies en- 

 tirely a dorsal position in the vertebrated classes: it is 

 enclosed in an osseous sheath, which is continued over 

 its posterior prolongation, and it is no where perforated 

 by the alimentary canal. The fibrous structure of the 

 encephalic portion which is perceptible in the cephalo- 

 pods, becomes more distinct and obvious as we ascend 

 through the vertebrated classes; and that anterior part 

 of the nervous axis becomes likewise proportionally larger, 

 leaving only slight traces, in the fourth ventricle, of its 

 original opening for the passage of the alimentary canal. 

 The spinal chord, the medulla oblongata, the optic lobes, 



