RESPIRATION OF TETRAPODS 57 



ribs. Contraction of the circular and radial fibres lowers 

 the dome and increases the volume of the pleural cavities. 

 In man this movement is normally about 1-5 cm. downwards 

 and, from its surface area, estimates have shown that its 

 movement can account for nearly the whole of the 500 ml. 

 of air which is taken into the lung with each breath. The central 

 tendon of the diaphragm does not dome upwards so much as 

 the two sides so that during relaxation the right and left sides of 

 the diaphragm are elevated above the central tendon by the 

 abdominal pressure. 



The thoracic cavity may be increased in volume by changing 

 its cross-sectional area as a result of the action of the ribs. In 

 most mammals these are inclined posteriorly and ventrally from 

 their articulation with the vertebrae. This articulation is by two 

 heads, the tuberculum and capitulum, and movement is restricted 

 to the axis of their joints with the transverse process and cen- 

 trum respectively. The external intercostal muscles are attached 

 between the ribs, the attachment to the anterior rib being closer 

 to the vertebral column than the insertion on the more posterior 

 rib (compare fig. 17). These muscles are largely inspiratory in 

 character, and the internal group of intercostals which are used 

 during expiration have a diagonally opposite arrangement. Both 

 diaphragmatic and intercostal activities are sufficient to produce 

 ventilation of the lungs by themselves. Human infants rely 

 entirely on the diaphragm because their ribs are oriented at right 

 angles to the body axis so that any movement of them results in 

 a decrease in volume of the thoracic cavity. Adult females use 

 the intercostal muscles to a greater extent than do males who 

 rely more on diaphragmatic breathing. In quadrupeds which 

 depend on the thoracic cage to suspend the fore-limbs through 

 the shoulder blade, the diaphragm is used to a far greater 

 extent than the rib cage. Correspondingly, aquatic mammals 

 use the intercostal musculature much more because the body 

 is supported by the water. Aerial and bipedal mammals use 

 the intercostals and the diaphragm in a way similar to that 

 of humans. 



The lung is contained within the true pleural cavity formed by 



