1028 



THORAX. 



TABLE A. Relative Lengths and Weight of, and 

 Area of Thoracic Space encompassed by, the 

 respective Ribs, including the Space made up 

 by the Sternum and Costal Cartilages. 



Fig. 665. represents the above table by 

 curves. The perpendicular lines represent 

 the ribs; and the curves the characters re- 

 ferred to. By a general view it will be seen 

 that all the lines curve upwards, and are at 

 their highest at about from the 5th to the 9th 

 rib. We shall not treat of particular ribs, but 

 of certain characteristics as they run through 

 the ribs. A knowledge of their shape is ne- 

 cessary to comprehending the respiratory 

 movements in diagnosing thoracic disease. 



1. Length. The length of a rib may be 

 taken in three ways, its absolute length, 

 chord length, and versed-sine length. 



(a) Absolute length. In the length from the 

 anterior to the posterior extremity, (A c B, 



fig. 664.) the 12th rib is the shortest. The 

 1 1th rib is nearly double the length of the 12th 

 rib ; likewise the 2nd rib is nearly double the 

 length of the 1st rib ; therefore the 1st and 

 2nd, and the llth and 12th differ more re- 

 markably in their length than do any of the 

 other ribs. The length (curve a, fig. 665.) 

 suddenly increases up to the 4th rib ; and 

 then the difference is trifling to the 8th rib ; 

 after this the shortening is as rapid as in the 

 four superior ribs. 



(b) Chord length. The dotted line b,fig. 

 665, represents this measurement. This length 

 from tip to tip (A B, fig. 664.) of the rib is 

 the chief modifier of the different apparent 

 mobility of particular ribs. If we allow the 

 range of costal movement to be the same in 

 each rib, while each succeeding rib increases 

 in its chord length, the apparent mobility in 

 different ribs will increase exactly as their 

 chord measurement increases. If a rib be three 

 inches long, and if its free extremity by a 

 given movement passes through one inch of 

 space, the free extremity of a rib six inches 

 long will, with the same absolute movement, 

 pass through two inches. The chord length is 

 an important element in modifying thoracic 

 capacity. It is in this measurement that the 

 1st rib is the shortest and nearly one half the 

 length of the 2nd rib, as 2" is to 3-75. 

 (Table A.) 



The chord length (line b, fig. 665, com- 

 pare with line a,) increases and decreases less 

 abruptly than that of the absolute length. 



If we were to admit that all the ribs at 

 their fulcra possessed the same extent of mo- 

 tion, still the antero-posterior length of the 

 thorax would be unequally increased, and that 

 exactly in relation to the chord measurement ; 



Fig. 665. 



Absolute length 



Chord 3 

 Versed sine 



Area - - fl 



Weight - t e 



\. 



9 



10 



11 



and hence the 7th, 8th, 9th, and 10th ribs advances most, somewhat representing the 



would advance most. It is a fact that in deep curve line b,fig. 665. 



inspiration the lower part of the sternum (c) Versed Sine Length (D c, fig. 664.) 



