466 



RESPIRATION 



of the costal cartilages. During inspiration these cartilaginous seg- 

 ments are subjected to an eversion and slight torsion. 



The Inspiratory Movement. It need scarcely be emphasized that 

 the space which is added to the thoracic cavity during inspiration, is 

 immediately taken up by lung tissue. In this way the more fully 

 expanded organs are capable of accommodating that extra amount 

 of air which is required for the oxidative processes of the body. As 

 has been stated above, the inspiratory movement is participated 

 in by a number of muscles which are designed as the muscles of inspira- 

 tion. They are classified further as intrinsic and extrinsic, because 



some of them are actual constituents of 

 the walls of the thorax, while others arise 

 elsewhere and are merely attached to its 

 framework. Furthermore, inasmuch as 

 many of these muscles are brought into 

 play only during forced or labored res- 

 piration, it is customary to divide them 

 into normal and accessory muscles of 

 inspiration. 



As normal muscles of inspiration are 

 to be regarded the diaphragm, the inter- 

 costales externi and the serratus posticus 

 superior, and as accessory muscles the 

 scalenus anticus, medius et posticus, the 

 sternocleidomastoideus, trapezius, pec- 

 torales major et minor, rhomboides 

 major et minor and serratus anticus. 

 The levatores costarum longi et breves, 

 which are sometimes classified under the 

 first heading, do not participate in the 

 raising of the ribs and belong to the 

 muscle system of the vertebral column. 1 



Under ordinary conditions, the action of the normal muscles of 

 inspiration is adjusted in such a way that the diaphragm assumes a 

 preponderating role; in fact, this muscle alone almost suffices to carry- 

 on a proper interchange of the gases as long as the body is only mod- 

 erately active. But when an additional supply of air is needed, other 

 muscles are brought into play; the lower intercostals being activated 

 first, and subsequently the upper intercostals and accessory muscles. 

 In this way, the previously diaphragmatic or abdominal type of respira- 

 tion is converted into the costal type. Hutchinson 2 states that in 

 abdominal breathing the abdomen is seen to bulge out before the 

 thorax is moved upward, while in costal breathing the elevation of 

 the ribs occurs first. 



FIG. 241 . SIXTH DORSAL VERTEBRA 

 AND RIB. (Reichert.) 



1 Du Bois-Reymond, Ergebn. der Physiologie, ii, 1902, 387. 



2 Todd's Cyclopaedia of Anatomy and Physiology, 1849. 



