The Life of the Fish 19 
what is being done. With a boy, many familiar actions may be 
equally reflex. The boy can also do many other things “of his 
own accord,” that is, by conscious effort. He can choose among 
a great many possible actions. But a fish cannot. If he is 
scared, he must swim away, and he has no way to stop himself. 
If he is hungry, and most fishes are so all the time, he will spring 
at the bait. If he is thirsty, he will gasp, and there is nothing 
else for him to do. In other words, the activities of a fish are 
nearly all reflex, most of them being suggested and immediately 
directed by the influence of external things. Because its 
actions are all reflex the brain is very small, very primitive, and 
very simple, nothing more being needed for automatic move- 
ment. Small as the fish’s skull-cavity is, the brain does not 
half fill it. 
The vacant space about the little brain is filled with a fatty 
fluid mass looking like white of egg, intended for its protection. 
Taking the dead sunfish (for the live one we shall look after 
carefully, giving him every day fresh water and a fresh worm 
or snail or bit of beef), if we cut off the upper part of the skull 
we shall see the separate parts of the brain, most of them lying 
in pairs, side by side, in the bottom of the brain-cavity. The 
largest pair is near the middle of the length of the brain, two 
nerve-masses (or ganglia), each one round and hollow. If we 
turn these over, we shall see that the nerves of the eye run into 
them. We know then that these nerve-masses receive the 
impressions of sight, and so they are called optic lobes. In 
front of the optic lobes are two smaller and more oblong nerve- 
masses. These constitute the cerebrum. This is the thinking 
part of the brain, and in man and in the higher animals it makes 
up the greater part of it, overlapping and hiding the other ganglia. 
But the fish has not much need for thinking and its fore-brain 
or cerebrum is very small. In front of these are two small, 
slim projections, one going to each nostril. These are the olfac- 
tory lobes which receive the sensation of smell. Behind the 
optic lobes is a single small lobe, not divided into two. This 
is the cerebellum and it has charge of certain powers of motion. 
Under the cerebellum is the medulla, below which the spinal 
cord begins. The rest of the spinal cord is threaded through 
the different vertebrae back to the tail, and at each joint it sends 
