450 PRACTICE OF PHYSIC. 



thing, from a heavier to a lighter atmosphere. The same per- 

 sons are also affected by every circumstance straitening the ca- 

 pacity of the thorax, as by any ligature made, or even by a 

 plaster laid upon it ; and a like effect happens from any in- 

 creased bulk of the stomach, either by a full meal, or by air 

 collected in it. They are likewise much affected by exercise, 

 or whatever else can hurry the circulation of the blood. 



MCCCLXXXII. As asthmatic fits seem thus to depend 

 upon some fulness of the vessels of the lungs, it is probable that 

 an obstruction of perspiration, and the blood being less deter- 

 mined to the surface of the body, may favour an accumulation 

 in the lungs, and thereby be a means of exciting asthma. This 

 seems to be the case of those asthmatics who have fits most 

 frequently in the winter season, and who have commonly more 

 of a catarrhal affection accompanying the asthma ; which there- 

 fore occurs more frequently in winter, and more manifestly from 

 the application of cold. 



MCCCLXXXIII. Besides these cases of asthma excited by 

 heat or cold, there are others, in which the fits are especially 

 excited by powers applied to the nervous system ; as, by pas- 

 sions of the mind, by particular odours, and by irritations of 

 smoke and dust. 



That this disease is an affection of the nervous system, and 

 depending upon a mobility of the moving fibres of the lungs, 

 appears pretty clearly from its being frequently connected with 

 other spasmodic affections depending upon mobility ; such as 

 hysteria, hypochondriasis, dyspepsia, and atonic gout. 



MCCCLXXXIV. From the whole of the history of asthma 

 now delivered, I think it will readily appear, that the proximate 

 cause of this disease is a preternatural, and in some measure a 

 spasmodic constriction of the muscular fibres of the bronchiae ; 

 which not only prevents the dilatation of the bronchise neces- 

 sary to a free and full inpiration, but gives also a rigidity 

 which prevents a full and free expiration. This preternatural 

 constriction, like many other convulsive and spasmodic affec- 

 tions, is readily excited by a turgescence of the blood, or other 

 cause of any unusual fulness and distention of the vessels of 

 the lungs. 



