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VETERINARY PHYSIOLOGY 



In expiration this dome-like muscle, rising (a) from the 

 vertebral column, and (6) from the lower costal margin, 

 arches forwards, lying for some distance along the inner 

 surface of the ribs, and then curving inwards to be inserted 

 into the flattened central tendon, to which is attached the 

 pericardium containing the heart. 



In inspiration the muscular fibres contract. But the 

 central tendon being fixed by the pericardium does not 

 undergo extensive movement. The result of the muscular 



Fig. 209. — Vertical- tangential, Transverse, and Vertical Mesial Sections 

 of the Thorax in Inspiration and Expiration in man. Similar 

 changes occur in quadrupeds. 



contraction is thus to flatten out the more marginal part of 

 the muscle and to withdraw it more or less from the chest 

 wall — thus opening up a space, the complemental pleura, into 

 which the lungs expand (fig. 209). 



The vertebral part of the diaphragm pulls downwards, 

 the costal part pulls backwards, and together they act like a 

 piston extending the vertical diameter of the thorax (fig. 168). 



2nd. An Increase in the Thorax in the transverse 

 and vertical diameters. 



This is brought about by the pulling forwards of the 

 ribs which rotate round the axes of their attachments to the 

 vertebral column. 



To understand this, the mode of connection of the ribs 



