The cerebellum is the great centre for co-ordinating the muscles of locomotion. Each step we take implies the action of many muscles and many thousands of muscular fibres; the actions of all must be very precisely graded as to amount, and very accurately arranged as to proper sequence. We do not, however, consciously think about the muscles to be used in every movement of each step; if we do think at all about our walking the cerebral hemispheres simply send a message to the cerebellum, and leave it (with the aid of the spinal cord) to regulate all the details. When we walk without thinking about it, the contact of the foot with the ground stimulates sensory nerves of the sole, which then stimulate the locomotor centres; these centres excite in proper order the nerves which control the muscles; and the co-ordinated action of the muscles produces the next step.

How does such a pigeon differ from an uninjured pigeon? What is the main function of the cerebellum? What is implied in each step that we take?

A pigeon with its cerebellum destroyed and all the rest of its nervous system intact would stand unsteadily, would stagger when it attempted to walk, and flutter uselessly when thrown into the air. But, having its cerebrum, it could will and feel; it would not stand quiet until touched; it would initiate movements when left to itself, though it could not perform them properly. It would will, and feel, and think, but could not co-ordinate the action of its muscles except for some simple movements, regulated by the medulla oblongata or the spinal cord.

Automatic Nerve Centres

Automatic Nerve Centres send out nervous impulses through efferent nerves without waiting to be excited by afferent nerves or by the Will. The most conspicuous are the small nerve-centres buried in the heart, which excite its beat even when it is separated from all the rest of the body. Another automatic centre is that which lies in the medulla oblongata and stimulates the nerves which control the muscles of respiration. When this centre is cut off from all sensory nerves it still acts, and its activity goes on even against the Will. We can voluntarily hold the breath for a short time, but not long enough to kill ourselves by suffocation. Although automatic nerve-centres act independently of impulses carried to them by nerve-trunks, they are nevertheless usually more or less subject to control by them. For example, stimulating the branch of the pneumogastric nerve (p. 323) which goes to the automatic heart nerve-centres slows the beat of the organ; and a dash of cold water on the skin makes us draw a deep breath.

Do we have to think about using each muscle concerned in walking? What happens when we think about our walking? What when we walk without thinking about it ?

What do we see in a pigeon whose cerebellum has been destroyed? Does it initiate movements? Does it execute them well? What is its condition as regards willing, feeling, and thinking? What can it not do?

What is done by automatic nerve-centres? Give examples.