PECTORALIS MAJOR AND DELTOID INSERTION 165 



the pectoralis major muscle was the same on both sides in only- 

 three out of one hundred and five individuals. The unusually 

 large percentage of asymmetrical cases is on the average equally 

 distributed on the two sides. One finds only small differences 

 between the averages on the right and left side in table 3. The 

 largest individual asymmetry was found in a negro whose index 

 of position was 6.0 greater on the right than on the left side. 



The index which has just been discussed gives a clear idea 

 of the position of the median point of the muscle insertion, and 

 it will therefore be of interest to devote our attention briefly to 

 the length of the insertion of the pectoralis major, from which 

 the median point was obtained. The absolute value of this in- 

 sertion length is represented by the difference between measure- 

 ment 2 and 3 in figure 1. In order to make this measurement 

 independent of the individual size of the upper arm it has been 

 expressed in percents of the humerus length. The formula of 

 this relative measurement is as follows : 



measurement 3 — measurement 2 



XlOO 



measurement 1 



The averages and the conditions of variability of this index 

 are tabulated in table 5. One notices a tremendous range of 

 variation from 8.8 to 23.1, and the variation coefficients also are 

 unusually large. It seems inadvisable therefore to attach any 

 particular significance to the slight differences in sex and 

 race, such as the relatively longer attachment of the muscle in 

 females and in whites. There is no correlation between the rela- 

 tive insertion length and the muscle strength nor the position of 

 the insertion. The measurement which has just been discussed 

 is somewhat longer in whites on the right side, and in negroes on 

 the left. There is a very marked tendency to asymmetry in the 

 relative insertion lengths in the different individuals, as has 

 already been found to be the case for the position of the insertion. 

 The relative attachment length was equal on both sides in only 

 four cases, and in only one case did the absolute length of at- 

 tachment show no asymmetry. 



THE AMERICAN JOUBMAL OF ANATOMY, VOL. 23, NO. 1 



