168 1>HYSIOI,OGV. 



CHAP. IV. -OF RESPIRATION. 



CLXXXI. Respiration consists of the motion of inspira- 

 tion, or the admission of air into the lungs, and of expiration, or 

 the expulsion of air from the same, alternately happening. 



CLXXXI I. Respiration takes place in man, and other anal- 

 ogous animals, soon after the infant is taken from the uterus of 

 the mother, and is exposed to the air. After it has taken place 

 for a little time, it is ever after necessary to the continuance of 

 life, as it is absolutely necessary to the continuance of the cir- 

 culation of the blood. 



CLXXXIII. The lungs are a hollow spongy mass, cap- 

 able of confining air, and readily dilatable by it. By the wind- 

 pipe they are open to the atmosphere ; and they are so situated 

 in the thorax that the air must enter into them, if the cavities 

 of the thorax, in which they are placed, are enlarged. For, as 

 there is no air in these cavities, and the external air cannot en- 

 ter into them, the enlargement of the thorax must form a va- 

 cuum around the lungs, which the external heavy and elastic 

 air will supply by entering into and dilating the lungs, while 

 these do not allow the air to pass through them into the cavities 

 of the thorax. 



CLXXXI V. Inspiration therefore depends upon the enlarge- 

 ment of the capacity of the thorax ; and this is performed chiefly 

 by the contraction of the diaphragm. This, in its relaxed state, 

 is suspended by the mediastinum, and its middle tendinous 

 part is raised high in the thorax; wherefore, as this middle 

 part, by the contraction of the muscular, is moved downwards, 

 the thorax is thereby considerably enlarged. 



CLXXXV, The capacity of the thorax is also enlarged by 

 the motion of the ribs upwards, whereby the curvatures of op- 

 posite ribs are set at a greater distance from each other ; and, 

 by the same motion, the sternum is moved outwards, and set at 

 a greater distance from the vertebrae of the back. The mo- 

 tion of the ribs upwards is caused by the contraction of both 

 layers of intercostal muscles. That the muscles called inter- 

 nal intercostals concur with the external in raising the ribs, ap- 

 pears from the situation of those muscles, from the greater mo- 

 bility of the inferior ribs, from the inspection of those muscles 



