398 TEXT-BOOK OF PHYSIOLOGY 



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

 these two layers of the pleura, the pulmonic and parietal or thoracic 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 movement of the lungs. 



THE MECHANIC MOVEMENTS OF THE THORAX 



As the blood flows through the pulmonic capillaries it yields carbon dioxid 

 to, and receives oxygen from, the air in the pulmonic alveoli. As a re- 

 sult, the intra-pulmonic air changes in composition which interferes to a 

 greater or less extent with the further exchange of gases. That this ex- 

 change may continue, it is of primary importance that the air within the 

 alveoli be renewed as rapidly as it is vitiated. This is accomplished by an 

 alternate increase and decrease in the capacity of the thorax, accompanied 

 by corresponding changes in the capacity of the lungs. During the former 

 there is an inflow of atmospheric air (inspiration), during the latter an out- 

 flow of intra-pulmonic air (expiration). 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 constitute 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 

 occupied by the heart and great blood-vessels (Fig. 185). This condition is 

 maintained by the pressure of the air within the lungs, the intra-pulmonic 

 pressure, which with the respiratory passages open, is that of the atmosphere, 

 760 mm. Hg. This relation persists so long as the thorax remains air- 

 tight. If the skin and muscles covering an intercostal space be removed the 

 lung can be seen in close contact with the parietal layer of the pleura gliding 

 by with each inspiration and expiration. If, however, an opening be now 

 made in the pleura sufficient to admit air, the lung immediately collapses and 

 a pleural cavity is established. A pleural cavity, therefore, does not exist in 

 physiological conditions, it is potential only. The pressure of air within 



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. 



