Complex muscular activities require the sending out of nerve impulses from various motor centers. Such activities are usually not performed instantaneously, but require a longer or shorter time. Thus we may consider it as one action for the writer to rise from his chair, to lower the window and adjust the shade, and then to return to his seat. In this case a large number of motor centers are successively involved; at the proper instant each center discharges its impulse. To this end the motor centers must be instructed when to come into activity. This distribution of nerve impulse is effected by the power of coordination. In voluntary actions coordination is accompanied by conscious control.[9] But coordination is not a function of the higher cerebral centers, that is, of consciousness. How the connection is made between the higher cerebral centers and the lower motor centers is a complete mystery. All that can be said is that the ideas of movements are transmitted to the motor centers, and that these send out the appropriate motor impulses. [Note 9: In this connection it is advisable to point out a difference between the meanings attached to the word "control" in psychology and in Vocal Science. The psychologist classes habitual movements as either automatic or controlled. Automatic movements are purely reflex; the individual does not consciously decide whether they shall be performed or not. Psychologically considered, the _control_ of a movement is simply the conscious volitional decision whether the movement shall be performed. To adopt the language of Psychology, we should speak of _voice management_, and of _breath regulation_, instead of vocal control, breath control, etc. In the following chapters the accepted psychological usage of the word "control" will so far as possible be adopted.] Turning now to the muscular adjustments of the vocal organs, these adjustments are seen to be independent of conscious guidance. When a tone is mentally conceived the vocal organs adjust themselves, in response to some mysterious guidance, for the production of the tone. The vocal cords assume the appropriate degree of tension according to the pitch of the tone to be sung. Both the quality of the tone and the vowel are determined by the combined adjustments of the laryngeal muscles and of the muscles which fix the shape and size of the resonance cavities. The power of the tone is regulated by the force of the breath blast; for each degree of power some special adjustment of the vocal cords is required.