RESPIRATION. 385 



adherent to the surface of the entire lung as far as its root; the outer portion 

 is reflected over the inner wall of the thorax, the superior surface of the dia- 

 phragm, and the viscera of the mediastinum. Under normal conditions 

 these two layers of the pleura, the visceral and parietal, are in contact, or 

 at most separated only by a thin capillary layer of lymph. The presence of 

 this fluid prevents appreciable friction as 

 the two surfaces play against each other 

 in consequence of the movements of the 

 lungs. 



THE MECHANIC MOVEMENTS OF THE 

 THORAX. 



The blood receives oxygen from, and 

 yields carbon dioxid to, the alveoli of the 

 lungs, as it flows through the pulmonic 

 capillaries. That this exchange of gases 

 may continue, it is of primary importance 

 that the air within the alveoli be renewed FlG< jgs. VIEW FROM BEHIND OF 

 as rapidly as it is vitiated. This is accom- FOUR DORSAL VERTEBRA AND 

 plished_by an alternate increase and de- ^ n ^ OT I ^ I > 

 crease in the capacity of the thorax, ac- MUSCLES OF THE RIBS AND THE 

 companied by corresponding changes in INTERCOSTALS. i. Long and short 

 the capacity of the lungs. During the for- 

 mer there is an inflow of atmospheric air 

 (inspiration), during the latter an out-flow of intra-pulmonic air (expira- 

 tion). The continuous recurrence of these two movements brings about 

 that degree of pulmonic ventilation necessary to the normal exchange of 

 gases between the blood and the air. The two movements together con- 

 stitute a respiratory act or cycle. 



In the course of the respiratory cycles the thorax presents alternately a 

 short period of rest viz., between the end of an expiration and the beginning 

 of an inspiration and a relatively long period of activity, including both 

 inspiration and expiration. The former may be regarded as the static, the 

 latter as the dynamic condition of the thorax. In the static condition, the 

 thorax and its contained and associated organs sustain a definite relation 

 one to another; in the dynamic conditions these relations undergo a change 

 the extent of which is proportional to the extent of the movements. 1 



THE STATIC CONDITION. 



Relation of the Thoracic Organs. Intra-pulmonic Pressure : Intra- 

 thoracic Pressure. In the static condition of the thorax the lungs, by 

 virtue of their distensibility, completely fill all parts of the thorax not 



1 It is a matter of discussion as to whether or not there is an absolute cessation of movement 

 of the thoracic walls at the end of expiration. A graphic record of the movement shows that 

 if there is no absolute cessation, the movement is so slight that, for the purposes here intended, 

 a pause may be admitted. With this admission it is however, recognized that the forces, both 

 elastic and muscular, which are always acting on the thoracic walls, though in opposite directions, 

 have not ceased to act, but have become so nearly equal that for a brief period they are practically 

 in a condition of equilibrium, during which the thoracic walls are stationary. 



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