46 NERVOUS SYSTEM AND GENERAL SENSATION. 



bral nerves, and are designed chiefly for the organs of 

 sense located in the head. Those which join the spinal 

 marrow are also in pairs, one pair for each vertebra or 

 joint of the back. The number of pairs varies, therefore, in 

 different classes and families, according to the number of 

 vertebrae. Each nerve is double, in fact, being composed 

 of two threads, which at their junction with the spinal mar- 

 row are separate, and afterwards accompany each other 

 throughout their whole course. The anterior thread trans 

 mits the commands of the will which induce motion ; the 

 other receives and conveys impressions to the brain, to pro- 

 duce sensations. 



63. In the Articulated animals, comprising the crabs, 



barnacles, worms, spi- 

 ders, insects, and oth- 

 er animals formed of 

 rings, the nervous sys- 



Fig. 10. tern consists of a se- 



ries of small centres or swellings, called ganglions, (Fig. 10,) 

 placed beneath the alimentary canal, on the floor of the gen- 

 eral cavity of the body, and connected by threads ; and of a 

 more considerable mass placed above the oesophagus or 

 throat, connected with the lower ganglions by threads which 

 form a collar around the alimentary canal. The number of 

 ganglions generally corresponds to the number of rings. 



64. In the Mollusks, (Fig. 11,) the nervous system con- 



sists of a single gangl ionic 

 circle, the principal swell- 

 ings of which are placed 

 symmetrically above and 

 below the oesophagus, and 

 from whence the filaments, 

 Fig. 11. which supply the organs 



in different directions, take their origin. 



