Cough consists in a deep inspiration followed by a forcible expiration with closed glottis, so that the air is driven rapidly through the larynx, carrying with it foreign substances, liquid or solid, which may be present in the air-passages. As it is a modified respiratory act, the nerve-centre by which the muscles employed in it are co-ordinated is situated in the medulla oblongata.

The afferent fibres by which cough may be excited are chiefly branches of the vagus. One of the most powerful is the superior laryngeal nerve distributed to the glosso-epiglottidean folds and to the whole of the interior of the larynx, and this being a special expiratory nerve we find that irritation of the larynx and also of the trachea is usually characterised by a cough with very violent expulsive efforts. Irritation of the mucous membrane of the trachea especially at the bifurcation of the bronchi, and irritation of the substance of the lung, also give rise to cough; and irritation of the costal pleura and of the oesophagus does so also.1 Irritation of the auditory meatus at the point to which the auricular branch of the vagus is distributed will also cause coughing; and cough appears to be also induced by irritation of certain parts of the interior of the nose. These are the surfaces of the inferior and middle turbinated bones, the most sensitive part being the posterior end of the inferior turbinated bone and the portion of the septum immediately opposite.1 The sudden application of cold to the skin on various parts of the body will sometimes cause coughing. Probably the cough in this case is not due to the stimulus being conveyed directly to the respiratory centre by the cutaneous nerves, but to its causing congestion of the air-passages, as in Rossbach's experiments (p. 252). The congestion then causes irritation of the sensory nerves of the bronchi, and occasions cough.

Pathology Of Cough 119

Fig. 82. - Diagram of the afferent nerves by which cough may be excited. These nerves are shown passing to the respiratory centre in the following order from above downward - from the auditory meatus, pharynx, upper part of oesophagus, larynx and trachea, bronchi, lung, costal pleura, liver and spleen.

1 Kohts, Virchow's Archiv, 66, 191.

I have seen irritation of the liver and spleen, induced by percussion over them, in a man suffering from chronic enlargement due to malaria, likewise cause coughing.2 In addition to those nerves, however, it appears that irritation of the glossopharyngeal branches distributed to the pharynx, where the digestive and respiratory tracts coincide as they cross one another, may not only excite coughing, but may also act as an auxiliary to irritation of the branches of the vagus. The combined action of the two may thus induce cough, when irritation of the vagus alone would not do so. Thus we find that many persons begin to cough as soon as they lie down, but that sometimes by lying round partially on the face, the cough ceases. In these persons the uvula is often found to be long and much congested, and the tickling which it produces as it rests upon the pharynx or pillars of the fauces seems to aid the irritation in the respiratory passages, and produce cough.

Cough due to irritation of those parts of the respiratory tract where the nerves are chiefly expiratory, as the pharynx, larynx, trachea, and large bronchi, is usually, as might be expected, loud, explosive, and prolonged; while cough due to irritation of those parts where the nerves are chiefly inspiratory is short and hacking (Fig. 82).

Cough produced by irritation of the pharynx where the respiratory and digestive passages cross one another, is not only violent, noisy, and barking, but, as we would naturally expect, is not unfrequently accompanied by retching or vomiting.

Pharyngeal irritation may accompany dyspepsia, and it is probably the origin of the so-called stomach-cough. Irritation of the stomach itself, or of its nerves, causes vomiting, but does not produce cough.

Nevertheless there is a rationale for the common expression 'stomach-cough.' In some experiments on the reflex origin of cough, R. Meyer 3 has noticed that when some part, from which cough can be reflexly induced, is already in a.state of irritation, cough can be brought on with great ease by irritation of a neighbouring part which would not by itself cause cough. Something of this kind appears to occur with the stomach, for although irritation of the stomach alone will not cause coughing, yet it will do so if irritation of the larynx and trachea are already present. Thus I have observed violent spasms of coughing occur, along with acidity and heartburn, some time after a meal, in a person suffering from congestion of the pharynx, larynx, or trachea. The connection between the cough and the acidity was shown by the cough ceasing as soon as the acidity was relieved by a dose of alkali and the consequent removal of the irritation to the stomach, which the acidity had produced.

1 On Nasal Cough, by John N. Mackenzie, M.D., reprint from The American Journal of the Medical Sciences, July 1883.

2 These observations were made in January and April 1879, but not published. Naunyn, in a paper published in the Deutsch. Archiv f. klin. Med. in March 1879 recorded similar observations.

3 R. Meyer, Correspondenzblatt d. Schweizcr Aerzte, No. 1, 1876.