394 TEXT-BOOK OF PHYSIOLOGY 



in parallel bundles, are directed from above downward and from behind 

 forward. The point of attachment, therefore, of any given bundle of fibers 

 to the rib above, lies nearer the vertebral column, nearer the fulcrum, than 

 the point of attachment below. 



The intercostales interni, eleven in number, occupy the spaces between, 

 and are attached to the ribs from the tubercle to the anterior extremity of 

 the cartilages. Their fibers, which are also arranged in parallel bundles, 

 are directed from above downward and backward (Figs. 186 and 187). The 

 portions of the internal intercostals between the cartilages are frequently 

 termed intercartilaginei. 



The levatores costarum are twelve in number on either side. They arise 

 from the tips of the transverse processes of the last cervical and the thoracic 

 vertebrae with the exception of the last. From the point of origin the fibers 

 pass downward and outward in a diverging manner to be inserted into the 

 ribs between the tubercle and the angle. Their action, as their name im- 

 plies, is to elevate the posterior portion of the ribs. 



The triangularis sterni arises from the side of the posterior surface of the 

 lower third of the sternum and is inserted by fleshy slips into the cartilages 

 of the ribs from the second to the sixth. 



From the fact that the inferior opening of the thorax as well as the inter- 

 costal spaces are completely closed by the foregoing muscles, and from the 

 further fact that the superior is closed by fascia except at those points through 

 which pass the trachea, blood-vessels and esophagus, the cavity of the thorax 

 is absolutely air-tight. 



The Pleurae. Each lung is surrounded by a closed invaginated serous 

 sac, the pleura, of which the inner portion is reflected over and is closely 

 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 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 



