=The nervous mechanism of respiration= has been the subject of much investigation and of many experiments, without, it must be confessed, the development of a quite complete or satisfactory theory. Respiration is a rhythmic process. About seventeen times in a minute the intercostal and diaphragmatic muscles contract. Inspiration is immediately followed by expiration, the falling movement being due, as already explained, to the elasticity of the lungs, which are stretched during inspiration. A slight pause intervenes between the end of expiration and the commencement of the next inspiratory movement. Tranquil respiration is a succession of reflex inspiratory movements, the depth of which varies according to the needs of the body—that is to say, according to the condition of the blood. If the need for aeration of the blood becomes urgent, the depth of inspiration is increased, and expiration also becomes an active movement, certain muscles, especially those of the abdomen, being called into play. In this condition two sets of reflex actions alternate. A large number of nerves are concerned even in tranquil respiration. If a man in falling “breaks his back” at the junction of the cervical and thoracic regions, costal respiration ceases. The series of intercostal nerves which arises from the dorsal spinal cord below the level at which it is injured are thrown out of action. Diaphragmatic respiration still continues, because the nerve of the diaphragm, the phrenic, arises from cervical roots. The lungs are supplied by the vagus nerve. This nerve joins the medulla oblongata as one of a group of three—glosso-pharyngeal, vagus, and spinal accessory—which by a large number of roots enter the groove between the olive and the restiform body. The vagus is the channel along which afferent impulses from the lungs enter the medulla. Such impulses call for respiratory movements. Cutting both vagi, however, does not put an end to respiration. Inspiratory movements continue, but they are much deeper and separated by much longer pauses. Such a form of respiration is inefficient. The blood is not properly aerated. The animal falls into a condition of dyspnœa, which ends in death. When the central end of the cut vagus is stimulated, the movements become more natural. Clearly, the respiratory reflex is not dependent upon the vagus, since it continues after the nerve is cut, although the impulses which pass up this nerve regulate its rhythm. They govern the length of the inspiratory movements, cut them short at the right moment, and secure their succession at proper intervals.