MECHANICAL PHENOMENA OF PESPJRAT1ON. 295 



functions at the time of gestation, as the diaphragm could 

 not then, without injury, press upon the gravid uterus. 



In short, in inspiration the thorax is dilated in every direc- 

 tion, the action of the diaphragm serving chiefly to produce 

 this effect. In making the complete inspiration which is 

 necessary when some extraordinary effort is demanded, all 

 the inspiratory powers, and all the mobility of which the ribs 

 are capable, are brought into activity ; the sternum is also 

 raised by the muscles inserted at its upper extremity. But, 

 under ordinary circumstances, when the breathing is quiet 

 and unconsciously performed, it is found that, in the same 

 person, some of the ribs possess an extraordinary freedom of 

 movement, while others are nearly motionless, and that in 

 the case of different persons, under similar conditions, the 

 same ribs are not always found to be influenced by movements 

 of the greatest extent. In some cases, also, the whole 

 thoracic cage seems nearly motionless, no movement of the 

 ribs can be detected. These facts have led to the establish- 

 ment of three types of inspiration (Beau and Maissiat) : the 

 abdominal type, the costo-inferior type, and the costo-supe- 

 rior. In children, of both sexes, respiration is abdominal 

 (see above); in man, it is costo-inferior; in woman, it is 

 generally costo-superior. This distinction, however, must 

 not be looked upon as absolute : the diaphragm, even when 

 it acts alone, evidently raises the lower ribs ; in the costo- 

 superior type, on the other hand, the lower ribs are also 

 elevated to a certain degree, the sternum being unable to 

 move without drawing them as it rises. 



What is the state of the lungs during these movements in 

 the thorax? We have se*en that the pulmonary cone com- 

 municates with the exterior air: between the external surface 

 of the lung and the internal surface of the cavity of the 

 thorax, however, there exists a cavity, entirely closed, which 

 is called the pleural cavity. By means of this empty space, 

 the lung adheres to the cage of the thorax, and follows its 

 every movement, exactly as a stone to which a piece of 

 moistened leather is fastened by suction, follows the leather, 

 when it is lifted up : this well known child's toy, exactly 

 represents tne mechanism, by means of which the thoracic 

 cone, being actively enlarged, forces the pulmonary cone to 

 follow all its changes in size, and, in short, to dilate. This is 

 the mechanism of inspiration : the lung is quite passive ; the 

 thoracic cage dilates actively, and the lung is obliged to 

 follow suit. 



