OF THE RESPIRATORY MOVEMENTS. 519 



but to be always either expressions of the emotions, or simple results of sensa- 

 tions as when crying arises from the sense of pain and laughter from that 

 of tickling. The origin of the act of Hiccup does not seem very clear ; but the 

 movement is probably of a purely reflex nature. 



555. The purposes of the acts of Coughing and Sneezing are, in both instances, 

 to expel substances from the air-passages, which are sources of irritation there; 

 and this is accomplished in both, by a violent expiratory effort, which sends 

 forth a blast of air from the lungs Coughing occurs, when the source of irrita- 

 tion is situated at the back of the mouth, in the trachea, or bronchial tubes. 

 The irritation may be produced by acrid vapours, or by liquids or solids, that 

 have found their way into these passages ; or by secretions which have been 

 poured into them in unusual quantity, as the result of disease ; or by the simple 

 entrance of air (especially if cold), when the membrane is in a peculiarly irri- 

 table state. Any of these causes may produce an impression upon the excitor 

 fibres of the Pneumogastrics, which, being conveyed to the Medulla Oblongata, 

 gives rise to the transmission of motor impulses to the several muscles, that 

 combines them in the act of coughing. This act consists 1st, in a long in- 

 spiration, which fills the lungs ; 2d, in the closure of the glottis at the moment 

 when expiration commences ; and 3d, in the bursting open (as it were) of the 

 glottis, by the violence of the expiratory movement; so that a sudden blast of 

 air is forced up the air-passages, carrying before it anything that may offer an 

 obstruction. The difference between Coughing and Sneezing consists in this 

 that in the latter, the communication between the larynx and the mouth is 

 partly or entirely closed by the drawing together of the sides of the velum 

 palati over the back of the tongue ; so that the blast of air is directed, more or 

 less completely, through the nose, in such a way as to carry off any source of 

 irritation that may be present there. It is difficult to say how far these actions 

 are simply reflex; or how far they may require the stimulus of sensation for their 

 performance. 



556. Various alterations are produced in the lungs, by section of the Pneu- 

 mogastric nerves ; and it has been supposed that these exert some more imme- 

 diate and direct influence over the condition of those organs, than their connec- 

 tion with the respiratory movements will serve to account for. The inquiry 

 into the nature and succession of these changes has been most carefully prose- 

 cuted by Dr. J. Reid (Op. cit.) ; and as his results have a very important 

 bearing on several physiological and pathological questions of great interest, a 

 summary of them will be here given. In the first place, it has been fully estab- 

 lished by Dr. Reid, that section of the Yagus on one side only does not neces- 

 sarily, or even generally, induce disease of that lung ; and hence the important 

 inference may be drawn, that the nerve does not exercise any immediate influ- 

 ence on its functions. When both Vagi are divided, however, the animal rarely 

 survives long ; but its death frequently results from the disorder of the digestive 

 functions. Nevertheless, the power of digestion is sometimes restored suffi- 

 ciently to re-invigorate the animals ; and their lives may then be prolonged for 

 a considerable time ( 446). In fifteen out of seventeen animals experimented 

 on by Dr. Reid, the lungs were found more or less unfit for the healthy per- 

 formance of their functions. The most common morbid changes were a con- 

 gested state of the bloodvessels, and an effusion of frothy serum into the air-cells 

 and bronchial tubes. In eight out of the fifteen, these changes were strongly 

 marked. In some portions of the lungs, the quantity of blood was so great as 

 to render them dense. The degree of congestion varied in different parts of the 

 same lung ; but it was generally greatest at the most depending portions. The 

 condensation was generally greater than could be accounted for by the mere 

 congestion of blood in the vessels, and probably arose from the escape of the 

 solid parts of the blood into the tissue of the lung. In some instances the con- 



