But while in such unicellular creatures all functions are properties of the individual cell, among higher forms systems take the place of the protoplasm of the single cell. There is a circulatory system, a respiratory system, etc.; but we must once more point out that such systems are made up of cells, so that every function of the highest animal may be finally reduced to what takes place in the unicellular animal. A circulatory system consists of a heart and blood-vessels, all filled with blood, which latter is "the life," as was known from the earliest times; yet this same blood is of no more use for the nourishment of the body while it is contained in those tubes which constitute the blood-vessels than is bread locked up in a pantry to a hungry boy. That which really provides the nutriment for the body is a fluid derived from the blood, a something like the liquid part of blood and known as lymph. This latter is to the cells of any tissue, as a muscle, as is the water filled with the food on which an amœba lives. In like manner, in spite of the complicated apparatus which supplies oxygen and removes carbon dioxide, the respiratory system, respiration is finally the work of the cell, as in amœba; a muscle-cell respires exactly as does the one-celled animal.
When we consider the marvellous complexity of structure of one of the higher animals, and the amazing variety of its functions, the question naturally arises as to how all this is brought about without any sort of clashing of the interests of one part with those of another. Why is it that the stomach has enough and not too much blood? By what means has Nature solved the problem of supplying more oxygen to parts in action than to those at rest? How is it that one set of muscles acts with instead of antagonizing another set, as in any complicated series of movements, such as walking?
To bring about this harmonization, or co-ordination, the nervous system has been provided. As the nervous and muscular systems are of preëminent importance in voice-production, they will now be considered with more detail than it is necessary to give to other systems.
Fig. 4. Body of a nerve-cell of the spinal cord, specially stained so as to show the minute structure. (Schäfer's Histology.)
Fig. 5. A large nerve-cell from the spinal cord of the ox, magnified 175 diameters. (Schäfer.)
Fig. 6. A cell of another form, from the superficial or outer part of the greater brain (cortex cerebri). The great amount of branching is suggestive of the power to receive and to transmit nervous influences (impulses) from various other cells; in other words, complexity of structure suggests a corresponding complexity of function.
