808 



SPECIAL PHYSIOLOGY. 



rotate them, so as to evert their lower borders. This slight rotation 

 of the ribs, which accompanies their elevation, increases the diameter 

 of the chest, and also widens each intercostal space along the sides of 

 the thorax, where the ribs are more movable than at their anterior 

 and posterior extremities. The elasticity of the costal cartilages is 

 highly favorable to this elevation and rotation of the ribs, rendering 

 both movements easier of execution than if the costal framework were 

 entirely composed of bone. Thirdly, besides the simpler action of 

 the levators of the ribs, and the more complex movements of the ex- 

 ternal intercostals, it is certain that a portion of the so-called internal 

 intercostals may, as will again be mentioned, also aid in elevating these 

 bones and the sternum. 



[The action of these muscles may be rendered plain by a reference 

 to the accompanying drawing, from Prof. Huxley's Lessons in Ele- 

 mentary Physiology, remembering that when a muscle contracts, it 

 tends to make the distance between its two ends as short as possi- 

 ble. Let a and b in the figure represent two parallel bars, movable 



Diagram L. 



[Diagram of models illustrating the action of the external and internal intercostal muscles. B, inspira- 

 tory elevation; C, expiratory depression. (From Huxley's Lessons in Elementary Physiology, page 89.)] 



by their ends upon the upright c, which may be regarded as the 

 back of the apparatus ; then a line directed from x to y will be in- 

 clined downwards and forwards, and one from w to z will be directed 

 downwards 'and backwards. It is obvious that there can be but one 

 position of the rods in which the points x and y are at their shortest 

 possible distance, and one position only in which the points w and z 

 are at the shortest possible distance; and these points are, for x 

 and y, the position B, and for w and -z the position C. These posi- 

 tions are respectively such that the points x, y, and w, 2, are at the 

 ends of a straight line perpendicular to both rods, for the shortest 

 distance between any two points is always in a line perpendicular to 

 the two points. And to bring x and y into this position, the parallel 

 rods in A must move upwards; and to bring w and 2 into it, they must 

 move in the opposite way. And it is thus proved that the external 

 intercostals raise, and the internal intercostals depress the bony ribs. 



F. G. S.] 



