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. 
Extracted front the Proceedings of the American Philosophical Society, 1893. 117 
1893.] 
[Dwight. 
Observations on the Psoas Parvus and Ifyramidalis. 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.) 
The 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, conies 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 
which Theile’s results on 20 bodies are quoted. 
Perrin f found the psoas parvus in 32 of 112 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 \ 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 
hundred 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 often 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 = 37.8% 
198 = 44% 
Present on neither side... 
Present on right side -i 
111 = 37 
| 1 
GO 
TJH 
II 
CQ 
183 = 40.6 
only 22 | 
Present on left side i 
only 23 
[■47 = 15.7 11 
11J 
[ 22 = 14.7 33 
l 88J 
69=15.4 
* This paper was read December 23, 1892, 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. 
t Edinburgh Medical and Surgical Journal, 1848, Vol. i, and 1849, Vol. ii. 
§ Beobachtungen aus der menschlichen und vergleichenden Anatomic, 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. 
REPRINTED APRIL 20, 1893, FROM PROC. AMER. PHILOS. SOC. VOL. XXXI. 118 
Dwight.] 
[March 17, 
Second Series. 
150 MALE. 
150 FEMALE. 
300 OF BOTH SEXES. 
Present on both sides 
.70 — 46.6% 
59 
= 39.3% 
129 = 43% 
Present on neither side... 
.56 == 37.3 
69 
= 46 
125 = 41.6 
Present on right side 
only 11 
Present on left side 
[ 24= 16.1 10 j 
| 
► 23 
= 14.7 
21 1 46 = 15.4 
1 
25 
only 13 
i 12 ' 
1 
The percentages are so nearly alike that there is no occasion to combine 
the series. The subjects came from all parts of Russia, and were mostly 
Sclaves. The following results are self-evident: 1. That, at least on one 
side, the muscle is more frequently present than wanting, although in 
more than half the cases it is wanting on at least one side. 2. That it is 
niore common in man than woman. 3. That the two sides are alike very 
much more often than unlike. This is directly contradictory of Hal let’s 
results. 4. That practically it is found as often on one side as on the other. 
Since this paper was first read the report of the Committee of Collective 
Investigation of the Anatomical Society of Great Britain and Ireland has 
appeared.* It shows that 457 bodies were examined and that the muscle 
was wanting in 270 (59 per cent.), that it was present on both sides in 139 
(30 per cent.), on one side in 48 (11 per cent.). It was found only on the 
right side 27 times against 21 on the left. It was therefore more often 
wanting than present. The sex was stated only in 171 instances. Both 
muscles were wanting in 54 per cent, of the males, and in 65 per cent, of 
the females. 
The observations about to be reported were made at the Harvard Medi- 
cal School. Those on negroes are not included in any series. As it often 
happened that the observation on one side of a subject had been neglected, 
or for some reason had to be discarded, I give first a series of observations 
in which only the number of sides of bodies is considered. It comprises 
608 sides, divided into 299 right sides and 309 left sides, and also by the 
sexes. It shows that considering sides only, the muscle is more often 
wanting than present; that it is more common in man than in woman, and 
that there is little difference between the sides. 
Frequence of Psoas Parvus in 608 Sides 
Male 
EIGHT. 
.. 89 
LEFT. 
83 
TOTAL. 
177 
Wanting 
,.. 102 
121 
223 
Total ... 
209 
400 
Per cent, present... 
.. 46.5 
42.1 
44.2 
* Journal of Anat. and Phys., Vol. xxvii, January, 1893. 1893.] 
119 
[Dwight. 
Female 
EIGHT. 
LEFT. 
30 
TOTAL. 
63 
Wanting 
.. 75 
70 
145 
Total 
100 
208 
Per cent, present... 
.. 30.5 
30 
30.2 
Both Sexes... 
118 
240 
Wanting 
191 
368 
Total 
.. 299 
309 
608 
Per cent, present... 
.. 40.8 
38.1 
39.4 
The next table shows the results of observations on both sides of two 
hundred and fifty subjects. 
Psoas Paryus. Records of Both Sides of 250 Subjects, 165 Male, 
85 Female. 
i : - 
MALE. 
female. 
BOTH SEXES. 
Present on both sides..... 
..60 
= 36.3% 
20 = 23.5% 
80 
= 32% 
Present on neither side.. 
..84 
= 50.9 
53 = 62.3 
137 
= 54.8 
Present on right side 
1 
1 
only 14 
121 
= 12.7 
5 
112 = 14.1 
19 
L 33 
= 13.2 
Present on left side 
only 7 
J 
7 
J 
14 
J 
Total 
165 
= 99.9 
85 =99.9 
250: 
= 100.0 
This shows the muscle decidedly less frequent than Gruber found it, 
and a little more frequent than would appear from the English observa- 
tions ; thus suggesting the influence of race. 
In order to study the relative variability of muscles,to ascertain .whether 
the presence of one occasional muscle renders the presence of another 
more probable, the pyramidalis was chosen as a starling point. It was 
chosen because it is generally held that while it is present in a large 
majority of instances it is not uncommonly wanting. Though this im- 
pression is, I believe, general, there is, so far as I know, no large series of 
observations on the pyramidalis ex cepting Hallet’s. In his two hundred 
subjects he found it wanting in the proportion of once in nine or ten.* In 
the following table of observations at the Harvard Medical School, indi- 
vidual sides only are considered. 
Pyramidalis in 673 Sides. 
Male 
EIGHT. 
LEFT. 
189 
TOTAL. 
369 
Wanting 
.. 45 
86 
81 
Total 
..225 
225 
450 
Per cent, present... 
.. 80 
84 
82 
* Loe. tit. It is worthy of notice that in his first hundred it was wanting once in every 
three subjects, and once in every fifteen in his second hundred. Dwight.] 
120 
[March 17, 
Female 
EIGHT. 
80 
LEFT. 
83 
TOTAL. 
163 
Wanting 
... 34 
26 
60 
Total 
.. 114 
109 
223 
Per cent, present... 
.. 71 
76.1 
73 
Both Sexes... 
... 260 
272 
532 
Wanting ... 
.. 79 
63 
141 
Total 
.. 339 
334 
673 
Per cent, present.,. 
.. 76.6 
81.4 
79 
The above table of sides only shows the muscle present only in 82 per 
cent, of the male sides, in 73 per cent, of the female, and in 79 per cent, 
of those of both sexes, which, however, were unequal. It was somewhat 
more common on the left. 
The next table shows the results in the same series of two hundred and 
fifty subjects which was used for the psoas parvus. 
Pvramidali$ Records of Both Sides in 250 Subjects, of Which 
165 Were Male and 85 Female. 
MALE. 
FEMALE. 
BOTH SEXES. 
Present on both sides 
,125 — 75.6% 
58=: 68.1% 
CQ 
CO 
II 
CO 
cc 
Present on neither side... 
to 
II 
Ol 
K-*- 
22 =: 25.9 
47 = 18.8 
Present on right side 
only 5 
Present on left side 
j 15=: 9.0 
°1 
5= 5.8 
5 1 
120=: 8.0 
only 10 
J 
5 j 
15 J 
Total 
165 = 99.7 
85 = 99.8 
250—100.0 
This shows, in addition to what was shown by the preceding table, that 
the two sides were alike in 92 per cent. 
We must now subdivide this series into smaller groups according to the 
presence or absence of pyramidalis and psoas parvus considered collec- 
tively. 
Records of Both Muscles on Both Sides in 250 Cases, of Which 
165 Were Male and 85 Were Female. 
GEOXJP. 
SEX. 
NUMBER. 
PER CENT. 
A. 
f Male 
48 
29 
Pyramidalis and 
Female 
16 
18.8 
psoas parvus on 
both sides. 
Both sexes 
64 
25.6 1893.] 
121 
[Dwight. 
GROUP. 
SEX. 
NUMBER. 
PER CENT. 
B. 
' 
Male 
18 
10.9 
Pyramidalis on both 
Female 
9 
10.5 
sides. Psoas par- 
vus on one side. 
Both sexes 
10.8 
C. 
Male 
59 
35.7 
Pyramidalis on both 
Female 
83 
38.8 
sides. No psoas 
parvus. 1 
Both sexes 
92 
36.8 
D. 
r 
Male 
4 
2.4 
Pyramidalis on one 
Female 
0 
0 
side. Psoas par- 
vus on both sides. 
Both sexes 
4 
2.4 
E. 
Male 
1 
.6 
Pyramidalis on one 
Female 
1 
1.1 
side. Psoas parvus 
on one side. 
Both sexes 
2 
.8 
F. 
c 
Male 
10 
6 
Pyramidalis on one 
Female 
4 
4.7 
side. No psoas 
parvus. 
Both sexes 
14 
5.6 
Gr. 
r 
Male 
8 
4.8 
N o pyramidalis. 
Female 
4 
4.7 
Psoas parvus on 
both sides. 
Both sexes 
12 
4 8 
H. 
r 
Male 
1.2 
N o pyramidalis. 
Psoas parvus on 
one side. 
Female 
2.3 
Both sexes 
4 
1.6 
I. 
r 
Male 
15 
9.6 
No pyramidalis. No 
Female 
18.8 
psoas parvus. 
Both sexes 
31 
12.4 
These tables show, as would naturally be expected, that the largest 
group (C) is that of two pyramidales and no psoas, and that this is the 
largest in either sex. For the next largest groups we have to examine the 
sexes separately, for we find that 29 per cent, of the men (A) have all four 
muscles, while precisely an equal number of women (18.8 per cent.) (A Dwight:] 
122 
[March 17, 
and I) have all four, and none of the four. Only 9.6 (I) per cent, of the 
men have all four wanting. 
By way of a more exact method of determining what relation there may 
he between the presence or absence of one muscle and that of the other, I 
have put together all the cases of presence of both pyramidales, groups A, B 
and C, into one series, and reckoned the percentages of instances of a psoas 
parvus on both sides and on neither side, to compare them with the per- 
centages of the normal series. 
We find 125 men and 58 women, 188 in all, having both pyramidales ; of 
these 48 men and 16 women have the psoas parvus on both sides. Calcu- 
lating the percentages we find that 38.4 per cent, of these men, 27.6 of 
the women and 34.9 of the whole, have the psoas parvus on both sides, 
against 36.3, 23.5 and 32 respectively as percentages of the whole series 
of 250. We find, therefore, that a subject with two pyramidales is a little 
more likely to have the psoas on both sides than one which has not. The 
next step is to take as a basis the cases of no pyramidalis, and to find 
whether in them the percentage of psoas parvus on both sides is difierent 
from that of the entire series. We find that 25 men and 22 women, 47 in 
all, had no pyramidalis ; of these, 8 men and 4 women, 12 in all, had the 
psoas parvus on both sides, giving percentages of 32,18.4 and 25.5 respec- 
tively, against 86.3, 23.5 and 32 in the entire series, showing that subjects 
with no pyramidales are less likely to have the psoas parvus on both sides 
than others. Thus we seem to have found a tendency, though a slight 
one, in variations by excess and variations by defect to go together re- 
spectively. 
By way of further comparison I have counted the number in these 250 
subjects in which the palmaris lohgus was known to be absent on both 
sides. I find this is recorded in 21 cases, and very probably occurred in 
some others. Let us see whether in these 21 cases there was a more fre- 
quent deficiency of either pyramidalis or psoas minor than one would 
expect. We find nothing of the kind. As for the pyramidalis we find it 
was wanting in three cases only on both sides and once on one side. The 
psoas parvus was present on both sides seven times, almost precisely the 
percentage of the series of 250. 
Finally I found among these 250 cases, 12 in which a sternalis was 
present on one or both sides (twice on both sides and ten times on one). 
This is what may be called an anomaly by excess. Let us see whether 
the series of subjects presenting it was marked by more than ordinary 
frequency of the pyramidalis and psoas parvus. The result is certainly 
remarkable, for it is the very reverse of what might be expected. In these 
12 the pyramidalis was wanting on both sides in 4, and on one side in 3, 
leaving it therefore present on both sides in only 5. The psoas parvus 
was more normal, being present on both sides in 5, on one side in 1, and 
wanting in 6. If we take these two muscles together we find that both 
were absent on both sides 3 times, a percentage of 25, which is about twice 
that in the entire series. It cannot be denied that this series of twelve 123 
1893. J 
[Dwight. 
subjects with sternales is too small, and yet there is some reason to think 
that a larger series would give the same result only more strikingly. 
Thus there are three other cases of sternalis which could not be included, 
because in each the record of one psoas was wanting. The first had 2 
pyramidales, one psoas was wanting, the other unaccounted for ; in the 
second and third both pyramidales and the recorded psoas were wanting. 
As we have found that the two sides generally agree it is probable that 
the unrecorded psoas was in all these instances wanting. In any case if 
these could have been added to the series they would materially have 
diminished the frequency of psoas parvus and pyramidalis when the sterna- 
lis was present. The criticism may he made that the sternalis should not 
he reckoned as an instance of excessive differentiation, for it is found with 
abnormal frequency in anenceplialous foetuses and there is reason in some 
cases at least to look upon it as a displacement of some fibres of the pec- 
toralis major. It is to be remarked, however, that it is in these monsters, 
rather than in adults, that the deficiency of the pectoral is common. No 
one would think of including them in a series for the study of the fre- 
quency of variations. In normal bodies the sternalis may, I think, be 
called an anomaly by excess. 
These two sets of cases, namely those of absence of the palmaris longus 
and presence of the sternalis, the latter especially, tend to disagree with 
the conclusions reached on the series of the psoas parvus and pyramidalis. 
None the less I am disposed to allow the series of psoas and pyramidalis a 
certain weight. But granting that there is a tendency for variations by 
excess and deficiency to go together respectively, the fact seems to be that 
the tendency is a slight one and that probably the cause, whatever it may 
be, that determines the differentiation of muscles must be considered as 
acting in most cases on each independently of the others. 
Again the only large series of observations bearing on this point is by 
Gruber. It deals with the psoas parvus, the palmaris longus, and the 
plantaris of 300 subjects equally divided between the sexes. His system 
of tabulation is not easy nor altogether satisfactory. He discusses the rel- 
ative frequency of defect of these muscles, but apparently from rather a 
different standpoint. In no case were all three muscles wanting on both 
sides. In three cases only were two of them wanting in both sides and 
one on one side. Absence of palmaris and plantaris in the same subject 
was also very uncommon. In twro cases both palmares and one plantaris 
were wanting. In no case, apparently, were both wanting on both sides.