258 Sir Everard Home on the influence of 
In the garden snail the nervous system resembles that of 
the muscle, but has also numerous nervous branches going 
to different parts of the body. The temperature of this spe- 
cies of snail, when its operculum is closed, does not exceed 
that of the surrounding air : this is proved by making a 
hole in the shell and introducing a small thermometer, in 
which the mercury undergoes no change. 
It therefore appears that the existence of brain and nerves 
does not necessarily endow the animal with a power of pro- 
ducing heat. 
In the leech, the earth worm, and all the insect tribe, the 
brain and spinal marrow very closely resemble that of the 
garden snail ; but in all these tribes there is a pair of nerves 
running down from the spinal marrow the whole length of 
the body of the animal, which are united together at regular 
intervals by what are called ganglions, composed of nervous 
fibres, apparently entangled and agglutinated together ; and 
in all such animals it was proved by Mr. Hunter, in his 
paper on heat, that their temperature exceeds that of the 
atmosphere when below 56°, although in very different 
proportions ; the excess in the leech being only one degree, 
while in a hive of bees it is 26°. 
As the only difference between the nervous systems of 
those animals that have no power of producing heat, and 
those that have, consists in there being ganglions, I was 
led to suspect that this power was derived from the gang- 
lions with which the nerves are furnished. Their structure 
is shown in the splanchnic ganglion. 
To ascertain how far there were sufficient grounds for 
this suspicion, I began to consider, whether any parts of 
