PHYSIOLOGY. 



in living animals, and from experiments imitating their action. 

 In more violent and laborious inspirations, the raising of the 

 ribs is assisted by many muscles attached to the ribs, and arising 

 from the clavicle, humerus, scapula, and vertebrae of the neck 

 or back. 



CLXXXVI. By the enlargement of the thorax, a dilatation 

 of the lungs is produced, in proportion to the bulk of air 

 entering into them ; but the dilatation may often be greater by 

 the air that enters into the lungs being heated or rarefied ; and 

 the greatest distention of the lungs is obtained by a constriction 

 of the glottis confining the air that has already entered into the 

 lungs. 



CLXXXVI I. As inspiration, or the admission of air into 

 the lungs, depends upon the enlargement of the thorax, the di- 

 minution of it must expel the air, or produce expiration. The 

 capacity of the thorax is diminished, while the muscles dilating 

 it are spontaneously relaxed by the elasticity of the ligaments 

 connecting the ribs with the vertebrae, and by the elasticity of 

 the cartilages and ligaments connecting the ribs with the ster- 

 num ; both which powers, commonly assisted by the weight of 

 the ribs themselves, bring the ribs and the sternum into the po- 

 sition they were in before respiration. At the same time, the 

 elasticity of the mediastinum draws the diaphragm upwards ; 

 and the contraction of the abdominal muscles both presses the 

 diaphragm upwards and pulls the ribs downwards ; and, in the 

 last, they are assisted by the sterno-costal and infra-costal mus- 

 cles. While these powers concur in diminishing the capacity 

 of the thorax, the expulsion of the air from the lungs is assisted 

 by the elasticity of the lungs themselves, and by the contraction 

 of the muscular fibres of the bronchiae. 



CLXXXVIII. These are the ordinary powers of expira- 

 tion, which, depending upon the reaction of elastic parts, is per- 

 formed slowly, and with little force ; but when it is necessary to 

 perform it with more velocity and force, some other and very 

 powerful muscles, as the quadratus lumborum, sacrolumbalis, 

 and longissimus dorsi, concur in pulling down the ribs ; and, at 

 the same time, the abdominal muscles, actuated by the animal 

 power, are contracted with greater velocity and force than in 

 spontaneous expiration. 



