1893.] -^ -'■ * [Dwight. 



Observations on the Psoas Parvus and Pyramidalis. A Study of Variation. 



By Thomas Dwight, M.D., LL.D., 



Parkman Professor of Anatomy at Harvard University. 



(Read before the American Philosophical Society, March 17, 1893.) 



Tlie only anatomist who has made observations as to the presence or 

 absence of the psoas parvus on a really large scale is the late Prof. Gruber, 

 of St. Petersburg. After him, at a considerable interval, comes Mr. Hallet, 

 and again, at an interval, Mr. Perrin. So far as I am aware no other anato- 

 mist has published observations sufficiently numerous to discuss.* As 

 evidence that this deficiency really exists, I may refer to the zeal with 

 wliich Theile's results on 20 bodies are quoted. 



Perrin f found the psoas parvus in 32 of 113 subjects. Of these 32, 21 

 were males and 11 females. He states that the sexes were about evenly 

 divided in the whole number examined. He gives no details as to the 

 occurrence of the muscle on one or both sides. 



Hallet X gives the results of two years' observations in the dissecting 

 room at Edinburgh, on about one hundred subjects each year. In his 

 first set of observations the psoas parvus was more frequently present than 

 absent, 61 to 54, and this proportion was preserved throughout the second 

 liundied subjects. He remarks that when it is present it is most fre- 

 quently found on one side, and that the right one. "In the first hundred 

 subjects dissected it was found as ofien deficient in the female as in the 

 male, but in the second hundred it was more frequently present in the 

 proportion of seven to six." 



We come now to the only satisfactory statistics, those of Gruber. § He 

 gives first a series of observations of both sides of 450 bodies, of which 300 

 were male and 150 female. He made later observations for a particular 

 purpose on 300 subjects in which the sexes were equally divided. The 

 close correspondence between these two series is very remarkable. 



First Series. 



300 MALE. 150 FEMALE. 450 OF BOTH SEXES. 



Present on both sides 142 = 47.3% 56 = 37.3% 198 = 44% 



Present on neither side... Ill = 37 72 = 48 183 = 40.6 



Present on right side "| 1 1 



<*°ly ^^'[^47 = 15.7 ^^[23 = 14.7 ^^J.69 = 15.4 



Present on left side i j 



only 25^ ll-" 36^ 



* This paper was read December 2*', 1S92, at the meeting of the Association of American 

 Anatomists at Princeton. The context will show that one addition has been made since 

 the reading. 



t Medical Times and Gazette, 1872. 



I Edinburgh Medical and Surgical Journal, 1848, Vol. i, and 1849, Vol. ii. 



§ BeobacMungen aus der menschlichen und vergleichenden Anatomie, Heft 1, 1879. I have 

 transposed Gruber' s figures so as to show the presence rather than the absence of the 

 muscle, and have substituted percentages for his rather crude proportions. 



