RESPIRATION. 



577 



The most important muscles of inspiration, which act by elevating 

 the ribs, are the levatores costarum. These are small muscles which 

 rise from the upper sides of the cervical and dorsal vertebrae, and are 

 inserted in the posterior surfaces of the ribs. Although these are small 

 muscles, they are inserted near to the axis of rotation of the ribs, and, 

 consequently, but a slight degree of contraction, the lever being so long, 

 will produce considerable elevation of the anterior extremity of the ribs. 

 At the anterior extremity of the upper two ribs are inserted the scalene 

 muscles, which rise from the cervical vertebra, and which in their con- 



Fig. 249.— Scheme of Action op the Intercostal Muscles. (Landois.) 

 I. When the rods a and 6, which represent the ribs, are raised the intercostal space must be widened 

 (c/> c d). On the opposite side, when the rods are raiBed, the line a h is shortened (i ft < ,9 h, the direc- 

 tion of the external intercostals) ; I m is lengthened (lm< o n, the direction of the internal intercostals.'. 

 n. When the ribs are raised the intercartilaginei, indicated by g h, and the external intercostals, in- 

 dicated by I k, are shortened. When the ribs are raised the position of the muscular fibres is indicated by 

 the diagonals of the rhombs becoming shorter. 



traction serve to elevate the first ribs, and so also tend to elevate the 

 entire thoracic wall. 



Between the ribs are found the intercostal muscles, which form two 

 layers, the external and internal. The external intercostal muscles are 

 attached to the adjacent margins of each pair of ribs, and extend from 

 the tubercles of the ribs, behind, to the commencement of the cartilages 

 of the ribs, in front, where they terminate into a thin, membranous aponeu- 

 rosis, which is continued forward to the sternum. They arise from the 

 outer lip of the groove on the lower border of each rib, and are inserted 

 into the upper border of the rib below. Their fibres are directed obliquely 



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