RESPIRATION 193 



9. Expiration is a less powerful act than inspiration. The 

 diaphragm relaxes, and ascends in the form of a dome; the 

 ribs descend and contract the chest; while the lungs them- 

 selves, being elastic, assist to drive out the air. The latter 

 passes out through the same channels by which it entered. 

 At the end of each expiration there is a period of repose, last- 

 ing about as long as the period of action. 



10. Frequency of Respiration. It is usually estimated that 

 we breathe once during every four beats of the heart, or about 

 eighteen times in a minute. There is, of course, a close rela- 

 tion between the heart and lungs, and whatever modifies the 

 pulse, in like manner affects the breathing. When the action 

 of the heart is hurried, a larger amount of blood is sent to the 

 lungs, and, as a consequence, they must act more rapidly. 

 Occasionally, the heart beats so very forcibly that the lungs 

 cannot keep pace with it, and then we experience a peculiar 

 sense of distress from the want of air. This takes place when 

 we run until we are "out of breath." At the end of every 

 fifth or sixth breath, the inspiration is generally longer than 

 usual, the effect being to change more completely the air of 

 the lungs. (Head Note 1.) 



1. Experiment. The Lungs Respiration. Obtain from your 

 butcher a pair of sheep's or calf's lungs ; they should be cut out with 

 great care, so as not to be injured in any part. Insert the nozzle of a 

 pair of bellows into the trachea and tie it securely ; then inflate the lungs 

 fully, and tie the trachea just below the point of the nozzle very securely, 

 so that no air escapes. Now hang the lungs up to dry. If there is no 

 hole in any part of them, they will retain the air sufficiently long to allow 

 the air-cells to dry in this distended state. During the drying the lungs 

 will shrink somewhat, but that will not interfere with the demonstration 

 of the specimen. When thoroughly dry, cut through both lungs length- 

 wise (see Fig. 50, page 180) with a long, sharp knife. You will then obtain 

 a very interesting and valuable specimen, which will keep for a long time 

 if protected from dust. It shows the trachea and its structure, bronchi 

 and their subdivision,' and the terminal air-cells. 



The presence of carbonic acid gas in the expired air may be demon- 

 strated by blowing air through some perfectly clear lime water, by means 

 of a glass tube. Carbonic acid gas produces a chemical change in the 

 lime contained in the water, transforming it into carbonate of lime, which 



9. Expiration ? The mechanism of expiration ? 

 10. Frequency of respiration ? Effect of hurried action of the heart? 



