Instincts, Habits, and Adaptations 39 
ing sense-impressions and of translating them into impulses of 
motion. The nerve-cells are receivers of impressions. These 
are gathered together in nerve-masses or ganglia, the largest 
of these being known as the brain, the ganglia in general being 
known as nerve-centres. The nerves are of two classes. The 
one class, called sensory nerves, extends from the skin or other 
organ of sensation to the nerve-centre. The nerves of the other 
class, motor nerves, carry impulses to motion. 
The Brain, or Sensorium.—The brain or other nerve-centre 
sits in darkness, surrounded by a bony protecting box. To this 
main nerve-centre, or sensorium, come the nerves from all parts 
of the body that have sensation, the external skin as well as the 
special organs of sight, hearing, taste, and smell. With these 
come nerves bearing sensations of pain, temperature, muscular 
effort—all kinds of sensation which the brain can receive. These 
nerves are the sole sources of knowledge to any animal organism. 
Whatever idea its brain may contain must be built up through 
these nerve-impressions. The aggregate of these impressions 
constitute the world as the organism knows it. All sensation is 
related to action. If an organism is not to act, it cannot feel, 
and the intensity of its feeling is related to its power to act. 
Reflex Action.—These impressions brought to the brain by 
the sensory nerves represent in some degree the facts in the 
animal’s environment. They teach something as to its food 
or its safety. The power of locomotion is characteristic of 
animals. If they move, their actions must depend on the indi- 
cations carried to the nerve-centre from the outside; if they feed 
on living organisms, they must seek their food; if, as in many 
cases, other living organisms prey on them, they must bestir 
themselves to escape. The impulse of hunger on the one hand 
and of fear on the other are elemental. The sensorium receives 
an impression that food exists in a certain direction. At once 
an impulse to motion is sent out from it to the muscles necessary 
to move the body in that direction. In the higher animals 
these movements are more rapid and more exact. This is 
because organs of sense, muscles, nerve-fibres, and the nerve- 
cells are all alike highly specialized. In the fish the sensation 
is slow, the muscular response sluggish, but the method remains 
the same. This is simple reflex action, an impulse from the 
