Page:Popular Science Monthly Volume 65.djvu/553

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HYSIOLOGY studies the nervous system from three main points of view. One of these regards its processes of nutrition. Nerve cells, as all cells, lead individual lives, breathe, dispense their own stores of energy, repair their own substantial waste, are, in short, living units, each with a nutrition more or less centered in itself. The problems of nutrition of the nerve-cell and of the nervous system, though partly special to this specially differentiated form of cell life, are, on the whole, accessible to the same methods as is nutrition in other cells and in the body as a whole.

But besides the essential functions common to all living cells, the cells of the nervous system present certain which are specialized. Among properties of living matter, one by its high development in the nerve-cell may be said to characterize it. I mean the cell's transmission of excitement spatially along itself and thence to other cells. This 'conductivity' is the specific physiological property of nerve-cells wherever they exist. Its intimate nature is, therefore, a problem coextensive with the existence of nerve-cells, and enters as a factor into every question concerning the specific reactions of the nervous system.

Thirdly, physiology seeks in the nervous system how by its 'conductivity' the separate units of an animal body are welded into a single whole, and from a mere collection of organs there is constructed an individual animal.

This third line of inquiry, though greatly needing more data from the second and the first, must in the meantime go forward of itself. It is at present busied with many questions that seem special—hence its work is generally catalogued as special physiology. But it includes general problems. In the time before us I would venture to put before you one of these.

When we regard the nervous system as to this, which I would term its integrative function, we can distinguish two main types of system according to the mode of union of the conductors—(i.) the nerve-net system, such as met in medusa and in the walls of viscera, and (ii.) the synaptic system, such as the cerebro-spinal system of arthropods and vertebrates. In the integrative function of the nervous system the unit mechanism is the reflex. The chain of conduction in the reflex