256 RESPIRATION 



The muscles involved in these movements of the ribs are the external 

 intercostals and the part of the internal intercostals situated between the 

 costal cartilages. Their action is to widen the intercostal spaces. The 

 scaleni fix the first and second ribs, thereby making a fixed point of action 

 for the other muscles involved. The serrati postici superiores assist the above 

 and also raise the third, fourth, and fifth ribs. The levatores costarum longi 

 and brevi elevate and evert all the ribs from the first to the tenth. 



In extraordinary or forced inspiration, which may be due either to violent 

 exercise or to interference with the due entrance of air into the lungs, all the 

 above muscles act more strongly. The diaphragm descends lower, the scaleni 

 raise the first and second ribs instead of merely fixing them, as in ordinary 

 respiration, as do also the sterno-cleido-mastoids. These, together with the 

 erector spina, which straighten the spine, increase the vertical diameter. 

 The trapezii and the rhomboidii assist in increasing the antero-posterior and 

 lateral diameters by fixing the shoulders and thus giving a fixed point for the 

 action of the pectorals and latissimi dorsi. 



The enlargement of the chest during inspiration presents peculiarities 

 in different persons. In children of both sexes the principal muscle in- 

 volved seems to be the diaphragm, and this type of breathing is known as 

 abdominal breathing. In men, the chest and sternum, together with the 

 front wall of the abdomen, are subject to a wide movement; this type of 

 breathing is called the inferior costal. In women, the movement appears 

 less extensive in the lower and more extensive in the upper part of the chest, 

 which is called the superior costal type. This has been shown to be due 

 rather to mode of dress than to a real difference in the sexes (Mosher). 



Expiration. Quiet expiration is a passive act due to the return 

 of the thorax and its contained lungs to their normal position when the mus- 

 cles involved in inspiration relax. This elastic recoil is sufficient in ordinary 

 quiet breathing to expel air from the lungs. In forced expiration, however, 

 which may occur to a slight degree in speaking, singing, etc., as well as in 

 the case of many involuntary and reflex acts, such as coughing, sneezing, 

 etc., other muscles are involved. Of these the principal are the abdominal 

 muscles, obliquus externus and internus, rectus abdominis, transfer salis ab- 

 dominis and pyramidalis. These act, first, by pressing the abdominal 

 viscera against the diaphragm and thereby forcing it up, their descent into 

 the pelvic cavity being prevented; second, by their attachments to the lower 

 ribs and cartilages, they draw these downward and inward, thereby lessening 

 the size of the thoracic cavity; lastly, by their contraction, they form a fixed 

 point for the action of that part of the internal intercostals, not involved in 

 inspiration, to approximate the ribs. 



When by the efforts of the expiratory muscles the chest has been squeezed 

 to less than its average diameters, it again, on relaxation of the muscles, 

 returns to the normal dimensions by virtue of its elasticity. The construe- 



