The act of breathing is carried on by ordinary skeletal muscles. This is the only act connected with bodily maintenance of which this is true. Our other “vital” organs are operated by means of smooth muscles. On account of this difference we have a certain degree of control over our movements of breathing. As we all know, we can hold the breath for a short time without difficulty, or can breathe more quickly or more deeply any time we choose. In this respect breathing differs strikingly from either the heart action or the movements of the digestive organs, over which we have no direct control at all. Our control of the muscles of breathing is, however, rather limited; we cannot hold the breath indefinitely. This means that the nervous mechanism which causes the muscles to contract will work in spite of the efforts of our will to prevent it. The actual machinery is very much like that which has already been talked about in connection with other “vital processes.” We have a “center” in the brain stem from which the nervous discharges come. This center is located immediately adjoining the vasoconstrictor center about which we learned in Chapter XIV. Because of the location of these two important centers in a single very small space, the spot where they are was named by a French physiologist more than a hundred years ago “the vital knot”; the point of this is that death can be induced more quickly and with less actual tissue destruction here than anywhere else in the body. The center which controls breathing has been named the “respiratory center.” It discharges automatically about fifteen or sixteen times a minute, causing the muscles of breathing to contract and so the bellows action of the chest to be carried on. Like the other centers in the brain stem this one can be acted upon by nervous disturbances passing into the brain stem from the sense organs. Perhaps the best example of this is the modification of breathing that comes as the result of a dash of cold water on the skin. Most of us have noticed that we give a sort of gasp upon stepping suddenly under a cold shower or plunging into cold water. It may not have occurred to us that this gasp is entirely involuntary, but we can easily prove that it is by trying to breathe with perfect regularity at the moment of stepping under a cold shower. We shall easily convince ourselves that this modification of the breathing is something over which we have no control. It is, as a matter of fact, an excellent example of a simple reflex. Coughing and sneezing are other reflexes in which sensory irritation of some sort acts upon the respiratory center modifying its discharges. In addition to these reflex changes in breathing we have also the familiar effects of muscular exercise. We know that after even moderate exercise the breathing is quickened somewhat, and after vigorous exercise it becomes very rapid and deep, and that after very severe exertion, particularly