INSPIRATION. 367 



etc.) the respiratory acts are very infrequent ; and in some, 

 the oxidation of the blood is more effectually performed by 

 the general surface than by the lungs. 



A glance at the physiological anatomy of the thorax in 

 the human subject makes it evident that the action of certain 

 muscles will considerably increase its capacity. In the first 

 place, the diaphragm mounts up into its cavity in the form 

 of a vaulted arch. By contraction of its fibres, it is brought 

 nearer a plane, and thus the vertical diameter of the thorax 

 is increased. The walls of the thorax are formed by the 

 dorsal vertebrae and ribs posteriorly, by the upper ten ribs 

 laterally, and by the sternum and costal cartilages anteriorly. 

 The direction of the ribs, their mode of connection with the 

 sternum by the costal cartilages, and their articulation with 

 the vertebral column, are such that by their movements the 

 antero-posterior and transverse diameters of the chest may be 

 considerably modified. 



Inspiration. 



The ribs are somewhat twisted upon themselves, and have 

 a general direction forwards and downwards. The first 

 rib is nearly horizontal, but the obliquity progressive- 

 ly increases from the upper to the lower parts of the 

 chest. They are articulated with the bodies of the -vertebrae, 

 so as to allow of considerable motion. The upper seven ribs 

 are attached by the costal cartilages to the sternum, these 

 cartilages running upwards and inwards. The cartilages of 

 the eighth, ninth, and tenth ribs are joined to the cartilage 

 of the seventh. The eleventh and twelfth are floating ribs, 

 and are only attached to the vertebras. 



It may be stated in general terms that inspiration is effect- 

 ed by descent of the diaphragm and elevation of the ribs ; 

 and expiration by elevation of the diaphragm and descent 

 of the ribs. 



Arising severally from the lower border of each rib, and 



