296 PULMONARY MUCOUS TISSUE. 



The effect of this mechanical phenomenon is to introduce 

 a certain quantity of air into the lung. Indeed, the principle 

 which governs the movements of the gases in respiration is 

 the same as that which regulates the circulation of the fluids ; 

 that is to say, the consequence of inequality of pressure. 

 From the moment when, by means of the enlargement of the 

 pulmonary or thoracic cone (we shall, in future, regard these 

 two words as synonymous), the gases in the pulmonary 

 reservoir are rarefied, a blast of air rushes in from the ex- 

 terior, as the lung is in free communication with the air, and 

 this produces a current from without inwards. We have 

 already observed that the velocity of this current differs, in 

 different zones of the respiratory reservoir, owing to the form 

 of the pulmonary cone (see p. 287). 



B. Expiration. 



All this is, however, only a part of the act of respiration : 

 the introduction of air, or inspiration, is quickly followed by 

 expiration or the expulsion of air by a current flowing in a 

 contrary direction. 



This latter movement takes place by means of a mechanism 

 which differs entirely from that already described, and, in the 

 normal condition, does not require the intervention of any 

 extra-muscular effort. In order to form a clear idea of it, 

 we must remember the exact structure of the pulmonary 

 parenchyma, and the properties of its tissue. The envelope 

 of the alveoli is formed of elastic tissue; it may, perhaps, 

 contain some muscular tissue ; but, if this be so, the latter 

 very seldom gives rise to any phenomena of contraction. 1 

 On this point experimenters are not agreed. Williams made 

 the experiment of passing an electric current through the 

 lung of a dog, the bronchus being connected with a rnano- 

 metric apparatus; and observing the variations which took 

 place in the column of mercury under the influence of the 

 current he found that there was contraction of the smooth 

 muscular fibres, either of the lung properly so called 

 (alveoli), or of the bronchi. We have repeated this experi- 

 ment several times without success, but, in spite of our failure, 2 



1 The name of muscles of Reisseisen is often applied to these 

 muscular fibres, they having been first described by this author. 

 (Reisseisen, " De Fabrica Pulmonuin. " Strasbourg, 1822.) 



a Paul Bert (Lemons sur la Physiologic Comparee de la Respira- 

 tion Professees au Museum d'Histoire Naturelle." Paris, 1870), 

 having made a number of experiments on the contractility of 



