THORAX. 



1047 



1st. The movement of such levers, when 

 rotating. 



2nd. The effect of forces, oblique, perpen- 

 dicular, and decussating, upon such levers. 



1st. The movement of the levers. Let 

 fig. 677. A represent a series of parallel bars, 



Fig. 677. 



Fig. 678. 



Diagram representing the position of the ribs affected 

 by the position of the spine. 



simultaneously, the first four would approxi- 

 mate, whilst all the rest would recede from 

 each other. Therefore the positions of the 

 different parts representing the spine in fig. 

 679. command and regulate these changes. 



allowing of free rotation upon a rigid per- 

 pendicular body A a ; let the free extremities 

 of these bars be kept apart, so that the bars 

 may at all times be parallel to each other. 

 In this condition a certain distance exists 

 between the bars, and a certain distance be- 

 tween their free extremities and the perpen- 

 dicular body A a. Let B represent the same 

 bars moved into another position, resembling 

 that of the ribs ; in this position, the 

 two conditions seen at A are altered. The 

 perpendicular distances between the bars 

 are diminished, and the distance between the 

 free extremities of the bars and the body B b 

 is likewise diminished. If the direction of 

 this motion were still continued, the bars 

 would ultimately touch each other, and their 

 free extremities would be still nearer to the 

 body B b. But let the bars be elevated, as 

 in c c, and the same condition obtains as in 

 the bars at B b, viz., they approximate each 

 other, and the free ends come nearer to 

 the body, c c. In this case the bars only 

 have moved ; but the same effect can be 

 obtained without moving the bars. Let A B 

 (fig. 678.) be two bars at their maximum 

 distance, while horizontal ; at a b, and a' b' ', 

 they have nearly attained their minimum 

 perpendicular distances, though still hori- 

 zontal, because the rigid bodies c c arid c' c' 

 have been moved respectively. Now, if we 

 join these hree last figures into one, as in 

 fig. 679., an then move the bars simulta- 

 neously, some bars will approximate each 

 other, whilst others will recede. The superior 

 four are at their maximum perpendicular dis- 

 tance from each other ; while the 4th, 5th, 

 and 6th are at a medium perpendicular dis- 

 tance, and the 6th, 7th, and 8th bars at their 

 minimum distance. 



The distances of these bars are regulated by 

 the position of the rigid body representing 

 the spine. If all of them were moved upwards 



Fig. 679. 



Diagram as in fig. 678. with the three portions conjoined f 



From this we learn, that the bars cannot 

 rotate without changing their distances, and 

 that when they are at 90 with reference 

 to the body A a (fig. 677.), they are at their 

 maximum distance from each other, and as 

 they pass this position on either side, this dis- 

 tance diminishes. 



In the human body the spine may represent 

 the body to which the bars are attached 

 (fig. 677.). The movement of the ribs will 

 obey the same law in receding or approaching 

 each other, and whether they increase or 

 diminish their intercostal spaces, will depend 

 upon the relation they bear to the spine. 



Fig. 682. is a cast, from a dissection of the 

 thorax of a male subject, weight 1071bs., height 

 5ft. 4in. This correctly represents the natural 

 position of the ribs, when the thorax is in a 

 state of complete expiration, or with only the 

 residual air in the lungs. The position of the 



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