1166 PHYSIOLOGY 



a ring directed obliquely from behind downwards and forwards. With 

 each inspiratory movement the ribs are raised, the obliquity becomes 

 less, and the horizontal distance between sternum and spinal column 

 is therefore increased. Moreover the ribs from the first to the seventh 

 increase in length from above downwards, so that when they are 

 raised, the sixth rib, for instance, occupies the situation previously 

 taken by the fifth, and the transverse diameters of the thorax at this 

 height are increased. With each inspiration there is a rotation of the 

 ribs. In the expiratory condition they are so situated that their 

 outer surfaces are directed not only outwards but also downwards. 

 As they are raised by, the inspiratory movements, they rotate on an 

 axis directed through the fore and hind ends of the rib, so that their 

 outer surfaces are turned directly outwards. In this way a certain 

 enlargement of the thoracic cavity is produced. As the thorax is 

 raised there is always some stretching of the rib- cartilages. 



In expiration the processes are reversed, and the cavity of the 

 thorax is diminished in all three dimensions. 



The movements of the thorax are effected by means of muscles. 

 Inspiration is performed by the following muscles : 



The diaphragm, which is the most important, and almost suffices 

 alone to carry out quiet respiration. 



The external intercostal muscles, which shorten and so raise the 

 ribs. 



The serratus posticus superior. 



It is probable that an important part is played even under normal 

 circumstances in the respiratory movements by the extension of the 

 spinal column. This movement, which is specially marked at the 

 upper part of the thorax, causes an increase in all three diameters of this 

 cavity. The levatores costarum, which are often included in inspiratory 

 movements, are so inserted into the ribs as to be unable to influence 

 their movements. They are concerned, not in respiration, but in 

 lateral movements of the spine. 



These muscles are the only ones normally engaged in carrying out 

 inspiration. When, in consequence of muscular exertions or from 

 any other cause, the inspiratory efforts become more forcible, a large 

 number of accessory muscles are brought into play. These are : 



The scaleni, 



Sterno-mastoid, 



Trapezius, 



Pectoral muscles, 



Rhomboids, and 



The serratus anticus. 



Normal expiration is chiefly effected passively. When the in- 

 spiratory muscles cease to contract, the lungs, which were stretched 



