NATURE OF THE FUNCTION. 397 



The peculiar influence of stramonium and belladonna, in diminishing the 

 contractility of these fibres, harmonizes remarkably with the well-known fact 

 of the relief frequently afforded by them in this distressing malady. 



528. Notwithstanding the high degree of contractility which the bronchial 

 tubes have been shown to possess, there is no valid reason for the belief, that 

 they contribute by any rhythmical movement to the exchange of the contained 

 air, which, in the healthy state, is continually taking place. For it can be 

 scarcely imagined that they should, by any power of their own, contract and 

 dilate uniformly with the contraction and expansion of the chest, unless their 

 muscles were equally subject with those of the thorax, to the influence of the 

 nervous system ; which all experiments concur in showing not to be the case. 

 The lungs themselves, then, are to be regarded as quite passive in the move- 

 ments of respiration ; the renewal of their contained air being accomplished 

 by the action of the muscles external to the thorax, or partly forming its pari- 

 etes. The lung completely fills the cavity of the pleura, in the healthy state 

 at least ; so that, when this is enlarged, a vacuum is produced, which can only 

 be filled by a corresponding enlargement of the lung ; and to produce this, the 

 air rushes down the trachea, and passes to the remotest air-cells. The dis-' 

 tension of the whole tissue of the lung, which is effected in this manner, is 

 much more complete than that which could be occasioned by simple insuffla- 

 tion from the trachea ; a fact of which it has been proposed to take advantage 

 in juridical inquiries in regard to suspected cases of Infanticide, where the 

 lungs are found to float, and the defence is set up that the child was still-born, 

 and that air was blown into the chest for the purpose of resuscitating it. It 

 has been ascertained by the experiments of Mr. Jennings,* that if a piece of 

 lung, which has been filled with air by insufflation, be exposed to great pres- 

 sure, the air may be expelled from it sufficiently to cause it to sink in water ; 

 but that no pressure can produce the same effect upon that which has been filled 

 by a natural inspiratory effort. It is a serious objection to the use of this test 

 in juridical investigations, however, that the early inspiratory efforts of the 

 infant are often so feeble as to produce but a very imperfect dilatation of the 

 air-cells ; so that the lung of an infant which has naturally inspired cannot, 

 by such means, be distinguished from one that has been artificially inflated. 

 The fact ascertained by Mr. J., however, is one of much physiological interest. 

 Owing to the freedom with which the air enters the lungs, when there is no 

 abnormal obstruction, the external surface is always in contact with the walls 

 of the chest, so that the pulmonary and costal pleura glide over one another 

 with every inspiration and expiration. The smooth and moistened character 

 of their surface prevents the movement from producing any sound ; but it 

 becomes evident when the friction is increased, either by the dryness that is 

 commonly one of the early changes produced by inflammation, or by the rough 

 deposit that subsequently appears. 



529. The complete dependence of the expansion of the Lungs upon the 

 production of a vacuum in the chest, is well shown by the effect of admission 

 of air into the pleural cavity. When an aperture is made on either side, so 

 that the air rushes in at each inspiratory movement, the expansion of the lung 

 on that side is diminished, or entirely prevented, in proportion to the size of 

 the aperture. If air can enter through it more readily than through the 

 trachea, an entire collapse of the lung takes place ; and by making such an 

 aperture on each side, complete asphyxia is produced. But if it be too small 

 to admit the very ready passage of air, the vacuum produced by the inspira- 

 tory movement is more easily filled by the distension of the lungs than by the 

 rush of air into the pleural cavity ; so that a sufficient amount of change takes 



* Transactions of the Provincial, Medical and Surgical Association, vol. n. 

 34 



