22 NERVOUS SYSTEM AND GENERAL SENSATION. 



which arise from the brain are twelve pairs, called the cere- 

 bral nerves, and are chiefly destined for the organs of sense 

 located in the head. Those which arise from 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 

 vertebra?. Each nerve is double, in fact, being composed 

 of two threads, which spring from the spinal marrow by 

 separate roots, and accompany each other throughout their 

 whole course. One of these transmits the commands of the 

 will, which produce motion ; the other receives and conveys 

 impressions to the brain, and produces sensations. 



63. In the Articulated animals, comprising the crabs, 

 barnacles, worms, spi- 

 ders, insects, and oth- 

 er animals formed of 



rings, the nervous sys- 

 tem consists of a se- Fig. 10. 

 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 ganglionic 

 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. 



