Although the respiratory centre is in the common sense automatic, yet it is constantly affected by many influences coming from other parts, which reflexly modify the respiratory movements. Thus mental emotions variously influence both the rate and depth of breathing, sometimes causing more rapid and sometimes slower respiratory action. The application of stimulus to almost any part of the air passages completely changes the respiratory rhythm, as may be seen by irritating the nasal mucous membrane. The ordinary sensory nerves passing from the skin are also capable of exciting respiratory movements. This is well shown by the gasping that follows the sudden application of cold to the body. It is along these sensory, nerves that one tries to transmit impulses by applying mechanical, thermal or other stimulus to the skin of a new-born infant, whose respiratory centre having been kept long in the condition of apncea, is slow to respond to an exciting influence caused by a deficiency of oxygen.

Fig. 154. Diagram of the Nervous Mechanisms of Respiration. {After Fick).

Sc. Centre for inspiratory movements, from which pass efferent channels, represented by the continuous white line (o) to the inspiratory muscles represented by the diaphragm (D).

Ec. Centre for expiratory movements, from which efferent channels (/) pass down the cord to the muscles of expiration, represented by the abdominal muscles (A).

To both these centres afferent impulses of two kinds come from the cerebral centres {a, b, c, d) to check or excite activity. These voluntary impulses may be called afferent as far as the respiratory centres are concerned. From the cutaneous surface {f,g) and the nose (e), impulses arrive, which modify the action of the inspiratory centre. From the larynx (G) come checking impulses (h) to the inspiratory, and exciting impulses (i) to the expiratory centre. And, finally, from the lungs come both exciting and inhibiting impulses (k, I,m, n) to both the expiratory and inspiratory centres, and by these channels the rhythm of ordinary breathing is regulated.

Experiment proves that most, if not all, afferent nerves can affect the respiratory centre, either by increasing or reducing its activity; but there is one special nerve, namely, the pneumogas-tric or vagus and its branches, which have both these capabilities developed to such a degree that they must be regarded as the regulating nerves of respiration.

Though section of one vagus has little or no effect on respiration, if the two vagi be cut a marked change takes place in the respiratory rhythm. The rate of the inspiration is reduced to less than half the normal rate, while each breath becomes deep and prolonged, so that the respiratory function of the lungs goes on for some time unimpaired, and the haemoglobin of the blood receives the due amount of oxygen. Although the character of the breathing is completely changed, from the rapid gentle motion of natural respiration to a series of slow deep gasps, the air volumes per minute and the chemical changes remain the same. If the central end of the cut vagus be now stimulated gently, the rate of the respiratory movements may again be quickened to the normal. If the stimulus be very strong, respiratory spasm can be produced. On the other hand, if the central end of the cut superior laryngeal branch of the vagus be stimulated, breathing becomes slow, and can be made to cease in the position of ordinary expiration, while a violent spasm of the laryngeal and expiratory muscles is caused.

So that in the pneumogastric nerve, fibres exist which convey impulses of two kinds to the inspiratory centre; the one increases its excitability and hastens the discharges of inspiratory impulses, the other decreases its irritability and checks the inspiratory movement. The marked change just described as occurring when the two pneumogastrics are cut proves that these afferent influences are constantly at work, quickening the respiratory rhythm. We may assume that the slow, deep respirations which follow section of the vagi are caused by the unregulated automatic action of the inspiratory centre. No impulse is discharged until the venosity of the blood in the centre arrives at a certain point, and then the accumulated energy is sent to the respiratory muscles, and a deep gasping inspiration occurs, and thus each respiratory act is called forth by the blood becoming so venous as to act as a powerful stimulus.

So long as the centre is stimulated by the regulating influence of the vagi this venous condition is not allowed to occur, the intense excitation of the centre is thereby prevented, and the necessary movements performed with a minimum of muscle energy.

The exact mode of stimulation of the pulmonary terminals of the afferent fibres of the pneumogastric is not certain. It has been suggested that distention or retraction of the lungs may act as a mechanical stimulus to fibres inhibiting and exciting respectively the inspiratory centre. Each expansion of the lungs calls forth the ensuing relaxation, and the relaxed state, in its turn, induces a new inspiration, and thus the lungs themselves are able to guide the thoracic movements by means of the pneumogas-trics.

The expiratory part of the centre probably takes no part in ordinary breathing, but is called into play in dyspnoea, vocal use of the expiratory blast of air, and in various modified respiratory movements.