Such subdivisions appear absurd if we imagine that each centre represents a distinct nervous mass, and we become puzzled to understand how the medulla oblongata can contain so many distinct centres in a small bulk. But if we remember that the word 'centre' simply indicates a group of cells and fibres connected with the performance of a particular act, and that two centres may be formed by the same ganglionic groups and differ from one another only by having a few ganglion cells more or less which alter the function they perform, no harm is done by the use of the term.

The act of respiration consists in the alternate enlargement and diminution of the thoracic cavity, so that the air is alternately inspired and expired.

Fig. 80.   Diagrammatic representation of various groups of ganglion cells, or 'centres,' in the medulla oblongata. The arrows indicate the directions in which the nerve currents pass.

Fig. 80. - Diagrammatic representation of various groups of ganglion cells, or 'centres,' in the medulla oblongata. The arrows indicate the directions in which the nerve-currents pass. Those pointing to the cells indicate sensory, those pointing from the cells indicate motor, nerves.

The muscles by which this is effected in ordinary respiration are the diaphragm and intercostal and scaleni muscles. The diaphragm descends, and the intercostal and scaleni muscles raise the ribs during inspiration.

Expiration is normally a passive act,1 and is not performed by muscular action, but simply by the tendency of the diaphragm and thoracic walls to return to the position of the equilibrium from which they had been removed during inspiration, and by the contraction of the elastic walls of the air-vesicles distended by inspiration.

When the supply of oxygen is deficient, other muscles are called in to aid the inspiration. Expiration appears to be a passive act, not merely in ordinary respiration, but even in dyspnoea caused by the absence of oxygen. In some experiments by Bernstein2 the inspiration and expiration were equally increased in a rabbit, when the air which it had breathed was replaced by hydrogen. But expiratory efforts are required both for the production of voice, and for the removal of irritants from the air-passages by coughing or sneezing; and forcible expira1 Bernstein, Archiv f. Anat. u. Physiol., 1882, p. 322.

2 Ibid., op. cit.

tion is produced when an irritant is applied to the mucous membrane of the nose, of the larynx, trachea, or bronchi. As every one who has drunk a bottle of soda-water knows, carbonic acid is an irritant of considerable power to these mucous membranes, and when it is breathed instead of air or hydrogen the expiration becomes much more powerful, and is no longer a passive action, but an active one, performed by active muscular exertion.

The chief respiratory centre is situated in the medulla oblongata close to the end of the calamus scriptorius, at the point designated noeud vital by Flourens, because destruction of this point arrests the respiration and causes death.

It extends equally on both sides of the middle line in the medulla, each half regulating the breathing on the same side of the body. It has been supposed to be double, and to consist of inspiratory and expiratory centres which act alternately, but it would appear that in ordinary respiration the inspiratory centre only is active.

When the centre is injured by a puncture, as in Flourens' experiment, or when one half of it is destroyed, breathing usually stops entirely, but if the respiration be kept up artificially for several hours, the normal breathing again becomes established; and the prolonged continuance of artificial respiration has been recommended by Schiff in apoplexy.

When the connection between this centre and the respiratory muscles is cut off by dividing the spinal cord just below the medulla, respiration usually ceases entirely, so that at first sight it would seem that the respiratory centre is limited to the medulla.

The effects of strychnine show that this is not the case. This drug greatly increases the excitability of the respiratory centre, and when it is injected into the blood before division of the spinal cord, the respiratory movements still continue to some extent after the cord has been divided. When it is injected after section of the cord, the respiratory movements which had ceased again recommence to a slight degree.

The reason appears to be that the respiratory centre is not limited to the medulla, but extends to the upper part of the spinal cord, though the spinal portion is of itself too weak to keep up the respiratory movements, except when stimulated by strychnine.

The amount of respiratory work which this centre excites appears to depend to a great extent, though not entirely, upon the condition of the centre itself.

The distribution of the work is chiefly determined by the irritation of one or other of the afferent nerves, and these nerves also influence the amount of work.

The centre is stimulated, and the amount of work it does increased by a venous condition of the blood circulating in it.

An arterial condition of its blood lessens or completely abolishes its activity, so that when the blood is highly aerated by forced artificial respiration, a condition of apnoea is produced, in which no spontaneous respiratory movements occur.

This condition is much more readily induced when the excitability of the respiratory centre is lessened by drugs. In an animal poisoned by chloral, for example, it is very easy to induce it, and it lasts for a long time.

When the respiratory centre is excited, as by the injection of emetine or apomorphine into the circulation, it is difficult or impossible to produce this condition.

It is uncertain whether the stimulation which the venosity of the blood produces is due chiefly to the absence of oxygen or to the presence of carbonic acid. Possibly it may also be due to the products of imperfect combustion in the venous blood. Or all these three causes may share in the stimulation, though to what extent each does so is not known.