36Q RESPIRATION [CH. XXIV. 



In ordinary quiet breathing, there is a negative pressure of only 

 1 mm. during inspiration, and a positive pressure of from 2 to 3 mm. 

 mercury during expiration. 



The instrument used by Hutchinson to gauge the inspiratory and expiratory 

 power was a mercurial manometer, to which was attached a tube fitting the nostrils, 

 and through which the inspiratory or expiratory effort was made. 



The greater part of the force exerted in deep inspiration is 

 employed in overcoming the resistance offered by the elasticity of 

 the lungs. 



In man the pressure exerted by the elasticity of the lungs alone is 

 about 6 mm. of mercury. This is estimated by tying a manometer 

 into the trachea of a dead subject, and observing the rise of mercury 

 that occurs on puncture of the chest-walls. If the chest is distended 

 beforehand so as to imitate a forcible inspiration, a much larger rise 

 (30 mm.) of the mercury is obtained. In the body this elastic force 

 is assisted by the contraction of the plain muscular fibres of the 

 alveoli and bronchial tubes, the pressure of which probably does not 

 exceed 1 or 2 mm. Hutchinson calculated that the total force to be 

 overcome by the muscles in the act of inspiring 200 cubic inches of 

 air is more than 450 Ibs. 



It is possible that the contractile power which the bronchial tubes 

 and air-vesicles possess, by means of their muscular fibres, may assist 

 in expiration ; but it is more likely that the chief purpose of this 

 muscular tissue is to regulate and adapt, in some measure, the 

 quantity of air admitted to the lungs, and to each part of them, 

 according to the supply of blood : the muscular tissue also contracts 

 upon and gradually expels collections of mucus, which may have 

 accumulated within the tubes, and which cannot be ejected by forced 

 expiratory efforts, owing to collapse or other morbid conditions of the 

 portion of lung connected with the obstructed tubes (Gairdner). 



The Nervous Mechanism of Respiration. 



In the central nervous system there is a specialised small district 

 called the respiratory centre. This gives out impulses which travel 

 down the spinal cord to the centres of the spinal nerves that 

 innervate the muscles of respiration. It also receives various afferent 

 fibres, the most important of which are contained in the trunk of the 

 vagus. The vagus is chiefly an afferent nerve in relation to respira- 

 tion. It, however, also is in a minor degree efferent, for it supplies 

 the muscular tissue of the lungs and bronchial tubes, and exercises a 

 trophic influence on the lung. 



The respiratory centre was discovered by Flourens ; it is situated 

 at the tip of the calamus scriptorius, and almost exactly coincides in 

 position with the centre of the vagus. The existence of subsidiary 



