ORGANS OF SENSATION. 77 



powerful muscles which move different portions of it, 

 whereas, there are no muscles to move the human 

 brain. Besides the nerves to the eyes, the ears, the 

 mouth, from the fore part of the ganglionic brain in 

 insects, and two nervous films behind, difficult to 

 detect, and probably running to the heart, two thick 

 nerves go off from the base, and after forming by 

 their separation a sort of ring or collar, which, dipping 

 down, embraces the gullet, they re-unite at what may 

 be called the second nerve-knot or ganglion ( l ), below 

 the gullet, whereas the brain is above the gullet. 



In the same way, two nerves go off from the lower 

 partof this second nerve-knot or ganglion, and re-unite 

 at a third ( 2 ). 



Two nerves go off from the third in a similar way, 

 and thus, at intervals, a chain of nerve-knots or gan- 

 glia, united by double nervous cords, is formed along 

 the belly of the insect to the vent. 



The number of nerve-knots or ganglia varies in 

 different species. Sometimes there is one for each 

 ring of the body, and in other cases, not so many as 

 this. The louse has only three, while the mole-cricket 

 has nine, and the green field cricket has ten. 



When one of these nerve-knots is carefully observed, 

 it is found to be usually spherical or pear shaped, 

 sometimes flat, (in the gipsy-moth, the third is heart- 

 shaped) consisting, like the ganglionic brain, of two 

 lobes, not always of the same form, even in the same 

 insect. 



Besides the two main nerves, or double nervous 

 chord which unites the several nerve-knots into a chain, 

 there goes off from each, on both sides, small nervous 

 branches, and these again divide into branchlets 

 smaller and smaller, which are distributed to the gullet, 



(1) In Latin, Ganglion secundum. 

 (2) In Latin, Ganglion tertium. 



