76 INTERNAL ORGANS. 



is only connected with the three parts just mentioned 

 by very small twigs, and is therefore considered as 

 being partly independent of them. This partially in- 

 dependent system of nerves in man is termed the gang- 

 lionic system, and it is sometimes also called the great 

 sympathetic nerve, or the intercostal nerve. 



Now it is generally maintained by naturalists, that 

 insects possess only this ganglionic system of nerves, 

 and have no brain or spinal cord like that of the 

 larger animals. 



It may be well, before proceeding farther, to describe 

 a ganglion, which is a knot or mass of nervous sub- 

 stance, at a point where two or more nerves meet, and 

 appears to consist of two substances similar to 

 those of the brain, while it differs from a nerve in 

 being firmer in texture, redder in colour from a greater 

 supply of blood, and covered with a membrane of 

 closer texture. The fibres or threads of the nerves 

 which join such a nerve-knot or ganglion, become 

 twisted within it, as Scarpa says, into a bundle, 

 and threads from the several joined nerves unite to 

 form a new nerve, which is always larger than any of 

 those whence it has been formed. This constitutes a 

 ganglion in man. 



Instead of a brain, then, insects have generally in 

 the head a double nerve-knot or ganglion, contained 

 in a horny cavity larger than itself. This, for the 

 sake of distinction, may be called the ganglionic 

 brain (*) ; inasmuch as it differs from the ganglionic 

 system in man by sending nerves to the eyes, the ears, 

 and the mouth, while the former appears (so far as is 

 yet understood) to supply nerves only to the heart, 

 stomach, intestines, and other organs, whose motions 

 are involuntary. The ganglionic brain also differs 

 from the brain in man by being surrounded with 



(l) In Latin, Cerebrum ganglionicum. 



