580 OF RESPIRATION. 



movements of respiration ; the renewal of their contained air being accom- 

 plished 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 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 pro- 

 duce this, the air rushes down the trachea, and passes to the remotest air-cells. 



a. The distension 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 insufflation 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 pressure, 

 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 of 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 moans, be distinguished from one that has been artificially inflated. The fact ascer- 

 tained 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 

 pleurae 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. 



b. 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 tjie 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 inspiratory 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 place for the maintenance of life. This is frequently observed in 

 the case of penetrating wounds of the thorax, in the surgical treatment of which, it is of 

 jjreat importance to close the aperture as completely as possible ; when this has been accom- 

 plished, the air that had found its way into the cavity is soon absorbed, and the lung re- 

 sumes its fall play. When one lung is obstructed by tubercular deposit, or is prevented in 

 any other way from rightly discharging its function, an opening that freely admits air into 

 the pleural cavity of the other side, is necessarily attended with an immediately fatal result; 

 and in this manner it not unfrequently happens, that chronic pulmonary diseases suddenly 

 terminate in Asphyxia, a communication being opened by ulceration between a bronchial 

 tube and the cavity of the thorax. 



761. The dilatation of the chest during Inspiration, is chiefly accomplished 

 by the contraction of the Diaphragm, which, from the high arch that it pre- 

 viously formed, becomes nearly plane; in this change of figure, it presses on 

 the abdominal viscera, so as to cause them to protrude, which they are ena- 

 bled to do by the relaxation of the abdominal muscles. In ordinary tranquil 

 breathing, the action of the diaphragm is alone nearly sufficient to produce 

 the necessary exchange of air; but, when a full inspiration is required, the 

 cavity of the chest is dilated laterally, as well as infcriorly. This is accom- 

 plished by the Intercostal muscles, the Scaleni, Serruti, and others ; which, 

 by elevating the ribs, bring them and their cartilages more nearly into the 

 same direction, and thus separate them more widely from the median line. 

 Expiration is chiefly effected by the contraction of the abdominal muscles, 

 which at the same time force up the diaphragm by their pressure on the vis- 



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



