In examining an organism we are forced to consider the structure and functioning of its various parts separately. But it is also essential to bear in mind that although these parts have different functions the organism works as a whole and that all its parts work together for the benefit of the organism, and, moreover, these parts are dependent upon each other for their proper and efficient working. Now the parts of an organism do not function unless they are stimulated to do so. A possible exception to this is the beating of the vertebrate heart, for the heart can be removed from an animal such as a frog or chick and will go on beating if a nutrient fluid containing oxygen dissolved in it be allowed to pass through its chambers. Noor nervous impulses are required to keep it working. The action is termed automatic.
Now since the activity of the parts of an organism is dependent upon stimulation, it follows that the parts must be effectively controlled so that they will work harmoniously together. Such control by the central nervous system is altogether of a higher type than the conveyance of impulses from one part to another—it involves sorting out the impulses so that the important ones get precedence over the others, otherwise several stimuli acting on and in the organism at the same time may produce conflicting results. The process of sorting out impulses so that only suitable ones reach their destination is called co-ordination. This becomes easier every time such sorting out is repeated. Our movements, for example, when learning to ride a bicycle are clumsy and jerky at first, become smooth later, and finally are carried out seemingly of their own accord, so that we can converse or admire the scenery when riding, until something unforeseen forces us to concentrate our attention on controlling the machine. Actually what happens is that the conscious part of the brain is involved at first, but once co-ordination has been effected the control is gradually taken over by the cerebellum. Thus we ’learn by experience ’and become ’creatures of habit. ’
The part played byas chemical messengers must not be neglected in this connection, but it must also be remembered that either directly or indirectly the glands producing them are under the control of the central nervous system, so that it is this system which has the fundamental control of the whole organism.