804 RESPIRATION. 



SECTION II. STUDY OF THE MODE OF ACTION OF THE MUSCLES OF 

 RESPIRATION. 



In man, the entry of air into the chest in tranquil breathing 

 is accomplished exclusive!}* by the diaphragm. In the dog, it 

 is effected partly by the descent of the diaphragm, partly by 

 the widening of the chest. In the rabbit, the respiratory 

 movements resemble in their general character those of man, 

 on which account this animal is preferable to any other for 

 the purposes of study. From the fact just stated, it is ob- 

 vious that in our examination of the action of the muscles of 

 respiration, we must not confine ourselves to the normal 

 breathing, for if we were to do so, our studies would relate 

 almost exclusive!}' to one muscle. To observe the action of 

 the others, we must direct our attention to the excessive tho- 

 racic movements of animals affected more or less with dyspncea, 

 the phenomena of which condition, so far as they relate to 

 the action of muscles, must therefore be entered upon here, 

 although its nature and cause will form the subject of a special 

 section. 



The muscular movements by which the chest is expanded, 

 must be studied in their relation to a certain definable position 

 of the thorax, which is called the position of equilibrium. It 

 is the position assumed by it at the end of normal expiration. 

 For as no muscle takes part in the normal expiratory act, the 

 whole thoracic muscular apparatus is at that moment in a 

 state of rest, the bones and cartilages assuming that position 

 which results from the balance of the opposed elastic forces, 

 which act upon them from within and from without. Of these 

 elastic forces, the most important is that of the lungs ; which 

 organs, being contained in a cavity much larger than they are 

 themselves, to the inner surface of which their external sur- 

 face is inseparably applied, constantly draw together its walls 

 with a force to be investigated in a future paragraph. Next 

 in importance is the elasticity of the ribs and cartilages, by 

 virtue of which the thoracic wall ever tends to be larger than 

 it is, in opposition to the contractile influence of the lungs. 

 Co-operating with these, we have, thirdly, the "tonus" of the 

 thoracic muscles, different indeed in its nature, but indis- 

 tinguishable as regards its action. All the muscles by which 

 the chest is enlarged beyond the position of equilibrium are 

 called inspirator}' ; all those by which it is contracted to a 

 capacity less than that which it possesses when in equilibrium 

 are called expiratory. 



91. Inspiratory Muscles. The Diaphragm. To demon- 

 strate the action of the diaphragm, several methods may be 

 used. The most striking is to expose it in an animal made 

 completely insensible by the injection of from twenty to forty 



