510 OF RESPIRATION. 



stimulus ; but that it may in some degree be restored by rest, even when the 

 lung is removed from the body. When the stimulation is long continued, how- 

 ever, as by intense irritation of the mucous membrane during life, the contractile 

 tissue passes into a state which resembles the tonic contraction of muscular fibre. 

 The contractility is greatly affected by the mode of death, and is remarkably 

 diminished by the action of vegetable narcotics, particularly stramonium and 

 belladonna : whilst it seems to be scarcely at all affected by hydrocyanic acid. 

 These facts are very important, as throwing light upon certain diseased condi- 

 tions. It has long been suspected, that the dyspnoaa of Spasmodic Asthma 

 depends upon a constricted state of the smaller bronchial tubes, excited through 

 the nervous system, frequently by a stimulating cause at some distance ; and ' 

 there din be now little doubt that such is the case. The peculiar influence of 

 stramonium and belladonna, in diminishing the contractility of these fibres, har- 

 monizes remarkably with the well-known fact of the relief frequently afforded 

 by them in this distressing malady. It has been maintained by Dr. Radcliffe 

 Hall/ that the contractility of the bronchial tubes is called into action in each 

 expiratory movement, to assist in emptying the lungs. But no evidence has 

 been adduced in support of this doctrine ; and its improbability is apparent from 

 the obvious fact, that a contraction of the air-tubes would impede, rather than 

 promote, the emptying of the air- vesicles. It seems more probable that it serves 

 to regulate the supply of air to the lobules, in accordance with the wants of the 

 system, just as the contractility of the minute arteries regulates the supply of 

 blood to the organs to which they proceed ; and it may possibly be through this 

 channel, that the remarkable variation is effected in the amount of respiration, 

 which adapts the quantity of heat produced to the depression of the external 

 temperature ( 538). It has been further suggested by Dr. W. T. Gairdner, 2 

 that the contractility of the smaller bronchi may serve to expel collections of 

 mucus which have accumulated in them, and which neither ciliary action nor 

 the ordinary expiratory efforts suffice to displace. 



543. Although there is no sufficient reason to believe that the Lungs are 

 possessed of any power of vital contractility, yet their Elasticity prevents them 

 from being altogether passive agents in the respiratory operation. The elastic 

 tension is rapidly increased by the dilatation of the lungs with air ; and the care- 

 fully -conducted experiments of Dr. Hutchinson 3 lead him to estimate it at cer- 

 tainly not less than Ib. upon each square inch of surface, when the lungs have 

 been filled by the deepest possible inspiration; so that its whole amount (reck- 

 oning an average surface of 300 sq. in. for the male, and 247 sq. in. for the 

 female), will be not less than 150 Ibs. for the male, and 123 Ibs. for the female. 

 This force is exerted in aid of the expiratory movement, and is directly anta- 

 gonistic to the i'nspiratory ; so that the inspiratory muscles must overcome it, in 

 order to produce complete distension of the pulmonary cavities. This disten- 

 sion is entirely accomplished by the action of the muscles external to the thorax, 

 or partly forming its parietes. The lung completely fills the cavity of the pleura, 

 in the healthful state at least; so that, when this is enlarged, a vacuum would 

 be produced, if it were not occupied by a corresponding enlargement of the lung ; 

 and to effect this, the air rushes down the trachea, and thence passes into the 

 entire substance of the lung, which it fills out in every dimension. This 

 distension is much more complete than any that could be occasioned by 

 simple insufflation from the trachea; for, long before the internal pressure 

 could overcome the resistance set up by the elasticity of the lungs, and 

 still more by that of the parietes of the chest ( 545), to the full dilatation 



1 "Transactions of the Provincial Med. Association," 1850. 



2 "Edinburgh Monthly Journal," May, 1851. 



3 "Cyclopaedia of Anatomy," art. "Thorax," vol. iv. p. 1058. 



