THE 
FUNCTION OF THE COCCYX 
IN THE 
MECHANISM OB’ LABOR 
BY 
HENRY D. FRY, M.D., 
Washington, D. C. 
[Reprinted from the American Journal of Obstetrics and Diseases of 
Women and Children, Vol. XXI., December, 1888.] 
NEW YORK : 
WILLIAM WOOD & CO., PUBLISHERS, 
56 & 58 Lafayette Place. 
1880. THE 
FUNCTION OF THE COCCYX 
IN THE 
♦ 
MECHANISM OF LABOR. 
BY 
HENRY D. FRY, M.D., 
Washington, D. C. — 
[Reprinted from the American Journal of Obstetrics and Diseases of 
Women and Children, Vol. XXI., December, 1888.] 
NEW YORK: 
WILLIAM WOOD & 0 0 ., PU B LIS H E RS., 
56 & 58 Lafayette Place. 
1889. THE 
FUNCTION OF THE COCCYX 
IN THE 
MECHANISM OF LABOR' 
It may seem like “ carrying coals to Newcastle” to introduce 
for discussion a subject connected with the mechanism of labor, 
yet there are many points relating to the phenomena of child- 
birth which are viewed differently by our most recent obstetric 
writers. 
Little more than half a century ago, the profession were en 
tirely ignorant of the laws governing the expulsion of the fetus 
through the maternal passages, and the act was as simple to the 
medical mind of that period as was the extrusion of a blood 
clot or uterine polypus. Step by step, mysteries were unrav- 
elled, and each successive investigator added his quota of 
knowledge to the stock already acquired; false impressions 
were corrected, and now the laws governing the phenomena 
of child-birth form a mechanism which challenges nature to 
show another more wonderful, or one which more beautifully 
demonstrates her resources for adapting a means to an end. 
In order to impress upon students a mental picture of the 
various movements imparted to the fetus during the mechanism 
of its birth, obstetric writers are accustomed to describe five 
successive steps or stages as flexion, descent, rotation, extension, 
and external rotation. Cazeaux, Cliarpentier, Barnes, and Par- 
vin add a sixth, the expulsion of the body. Playfair likewise 
makes six stages, describing as his third that of levelling or ex- 
tension of the head in the cavity of the pelvis. 
These are not so many separate or distinct acts executed with 
the precision of well-drilled troops, but they pass imperceptibly 
from one to another, and often combine two or more move- 
ments at the same time. Let us define what we understand by 
the expression “ stage ” of the mechanism of labor. 
Any distinct movement of the fetus during its passage through 
1 Read before Washington Obstet. and Gyn. Soc. meeting, June 15th, 
1888. 4 
Fry : Function of the Coccyx. 
the parturient canal may be termed a stage when it lias the ef- 
fect of : 1st, diminishing the diameters of the presenting part, 
and 
2d, bringing the longer diameters of the fetus coincident 
with the longer diameters of the pelvic canal. 
These are the fundamental principles involved. Accidental 
movements imparted to particular cases in consequence of 
anomalous conditions, either of the fetus or pelvic canal, are 
excluded. 
The first of these objects (diminution of the diameters of the 
presenting part), is confined to the movements of the head— 
the hard and incompressible nature of which demands move 
ments of flexion and extension in addition to that of rotation. 
The latter movement is common to all portions of the fetal 
body, and accomplishes the second object—that of bringing the 
longer diameters of the fetus coincident with the longer diame- 
ters of the canal. 
Moreover, flexion and extension are intimately associated 
with descent, the occiput or bregma descending in conjunction 
with the respective movements. 
Descent of the occiput beneath the pubes is associated with 
extreme flexion of the head at the pelvic outlet, while exten- 
sion sweeps the face over the perineum and delivers the head. 
These final steps which free the head from the pelvic outlet 
bring us to the consideration of the function of the coccyx in 
the mechanism of labor. 
.No function whatever is attributed to this little bone, situ- 
ated at the extremity of the spinal column, except to get out of 
the way of the advancing head, and thereby to increase the an- 
tero-posterior diameter of the inferior strait. It is not supposed 
to possess any obstetrical importance unless it rudely refuses to 
step aside. 
Premature ossification of its articular cartilages is mentioned 
as a source of dystocia, and instances are reported of fracture 
of the bone. 
I believe, however, that the coccyx has a distinct function to 
perform, and only after having performed it, does the bone re- 
cede before the advancing brow. To be placed at the pelvic 
outlet merely as an impediment, without having some office to 
perform, is contrary to nature’s law. 
In order to explain this function as I interpret it, the accom- Fry : Function of the Coccyx. 
5 
panying diagram is introduced to represent the movement of 
extreme flexion at the outlet. 
The forces producing flexion and extension of the head are 
recognized to be the expulsive power of the uterine and volun- 
tary muscles acting upon the fetal ovoid on one hand, and the 
opposing force or resistance offered by the structures to the 
passage of the head on the other. 
The line a b represents a part of the spinal column' of the 
fetus ; of, the occipito-frontal diameter of the fetal head; b', a 
point on this diameter opposite the foramen magnum; o b', 
the short end of the lever extending from the occiput to a point 
opposite the articulation of the spinal column; and f b', the 
long end of the lever extending from the brow to the same 
point. 
During uterine contraction the head is forced downwards by 
the power acting in the direction of a b. Flexion or exten- 
sion results according to the amount of resistance offered at the 
end of one or the other lever, that lever descending which 
meets least resistance. With equal resistance at both ends, o 
will descend, and flexion occur because o b' is the shorter 
lever. This movement takes place early in labor, and facilitates 
the passage of the head by substituting a shorter diameter for 
the occipito-frontal. 
In occipito-anterior positions, which, we are now considering, Fry : Function of the Coccyx. 
6 
the head passing downwards and backwards reaches the inferior 
strait in a position of partial flexion. The occiput (o) is behind 
the symphysis, the frontal end (f) of the occipito-frontal 
diameter is in the hollow of the sacrum, and the brow im- 
pinges against the coccyx (<?). Further descent in this direc- 
tion is prohibited by the coccyx, and the long end of the lever 
b' f is held up by the pressure of the bone against the brow 
at c. In consequence of this increased resistance posteriorly, 
the force of the uterine contractions is now exerted upon the 
short or occipital end. The head moves on its transverse 
axis, the occiput slips down under the symphysis to o', and 
then, by an exaggeration of the same movement, becomes fixed 
at o". 
The long end of the lever ascends slightly to(/v and later to 
f", whilst the body of the fetus being forced down in the 
direction of the axis of the pelvic canal, the sternum ap- 
proaches and later is in contact with the chin. 
The head is now at the inferior -strait in a position of extreme 
flexion, with the brow still kept up by the resistance of the 
coccyx. The occiput has slipped downwards and backwards 
and the nape of the neck is applied to the symphysis. The 
sub-occipito bregmatic diameter engages at the outlet. 
The short end of the lever (o") being fixed, motion is trans- 
ferred to the long end/*". 
The coccyx has performed its function and recedes before 
the advancing brow, the perineum distends as the chin leaves 
the sternum,1 the occiput rotates around the symphysis, and 
the head is born by a final movement of extension. 
According to this description, the function of the coccyx is 
to oppose the descent of the brow, thus forcing down the oc- 
cipital end of the lever beneath the symphysis / in other words, 
to produce extreme flexion of the head at the pelvic outlet. 
For several years I have taken advantage of every oppor- 
tunity to examine the final movements which give birth to the 
head, and claim to have clearly recognized, in all cases of oc- 
cipito-anterior positions, this flexion at the inferior strait, and 
further to attribute it to an arrest of the brow by the coccyx. 
The method employed is as follows: 
Having taken the precaution to see that the rectum is empty, 
1 Tarnier and Barnes both claim that the chin is applied to the sternum 
until the bregma is escaping from the vulva. Fry : Function of the Coccyx. 
7 
the patient is placed upon her back .with the knees elevated. 
When the head has descended into the pelvic cavity and is 
approaching the inferior strait, pass the index-finger of the 
right hand into the rectum until it comes in contact with the 
coccyx. Each uterine contraction approximates the head to 
the finger until finally it is driven forcibly against it. During 
the interval between the contractions, the linger readily passes 
between the coccyx and the head, but at each pain the finger 
is pushed aside, as the bregma is forced against the bone. 
Now let the thumb of the same hand be inserted between the 
vulva and behind the symphysis until it comes in contact with 
the occiput. Both ends of the lever are now placed under ob- 
servation and the movement of either is readily recognized. 
The head occupies the following relations to the pelvic outlet:1 
the occiput is behind and slightly to the left of the symphysis, 
the biparietal suture runs, not in the antero-posterior diameter 
of the outlet, but crosses it as it passes backwards and to the 
right. The anterior fontanelle is in the sacral cavity and to the 
right of the median line. The coccyx comes in contact with 
the left parietal bone behind the anterior fontanelle and near 
the biparietal suture. The bregma does not press forcibly 
against the perineum. The finger cctn easily pass between 
the head and perineum and the latter does not begin to distend 
until after the occiput is fixed and extension of the head is 
commencing. 
With the finger and thumb placed as described, it will be 
noticed that the continuance of uterine action forces the bregma 
against the coccyx, and as this offers a barrier to its progress, 
the occiput slips downwards and backwards until the nape of 
the neck is applied to the symphysis. Extreme flexion has 
occurred. 
This stage is represented by the cut and is that immediately 
preceding extension. The points I desire particularly to em- 
phasize are that the occiput passes downwards and backwards 
in the direction of the line a b, in order to reach this position, 
and that the perineum does not yet oppose the bregma. 
The continuance of uterine action, however, with now the 
opposing force of the perineal muscles and fixation of the 
occiput, produces extension, the resistance of the coccyx is 
1 Original position L. O. A. 8 
Fry : Function of the Coccyx. 
overcome, and the head is born, the occiput moving forwards 
and upwards under the symphysis. 
This flexion at the inferior strait which I have described as 
a normal stage of the mechanism of labor is either over- 
looked by obstetric writers, or if explained at all is done so on 
different principles. 
Charpentier recognizes its existence. He says 1 “ it is even 
the rule that flexion does not become complete until the head 
/ 
encounters the resistance of the pelvic floor.” Charpentier 
evidently does not mean that the resistance of the perineal 
muscles causes this flexion, because, as I shall show later, he 
recognizes what I have already stated, that the flexion precedes 
the contact of the bregma with the perineum. If, by re- 
sistance of the pelvic floor, he refers to the resistance of the 
coccyx, he then acknowledges the function of this bone. 
1 “Cyclopedia of Obstetrics and Gynecology,” Wm. Wood & Co., 
1887, Yol. I., p. 349. Fry : If unction of the Coccyx. 
9 
Tarnier1 also states “ that the movement of flexion is, at this 
juncture, at its utmost limit,” but he explains it by saying that, 
while “the occiput is beneath the pubic arch, all the soft parts 
which make up the perineum press the anterior part of the 
head against which they are applied upward and backward.” 
The same answer applies here ; the soft parts which make up 
the perineum are not applied against the anterior part of the 
head until the coccyx is pressed back and extension is commenc- 
ing. Flexion at the inferior strait (see cut) precedes contact 
with the perineum. I am gratified to find that Charpentier2 sup- 
ports this statement. ITe says : “ After its rotation, the occiput 
is not suddenly but progressively liberated, and it is not brought 
into close contact with the perineum until it is completely 
freed. Until that time it remains at a certain distance from the 
perineum, and if we introduce the finger posteriorly, we feel 
that the head is retained behind by its frontal eminences, on the 
sides by the biparietal eminences, and that there is a certain 
space between the perineum and the bregmatic region. In or- 
der that the bregma may be brought into close contact with the 
perineum, the occiput must be liberated from under the sym- 
physis, and the bregma must descend, which it cannot do at this 
moment unless the head becomes extended.” 
This statement also furnishes a key to the former quotation 
from Charpentier that flexion becomes complete only after “the 
head encounters the resistance of the pelvic floor.” It is not, 
according to him, the resistance of the coccyx which retains the 
head behind, but the opposition to the passage of the frontal 
and biparietal eminences. 
The reasons for attributing it to the former have already been 
given at length. How let us look at it with Charpentier’s spec- 
tacles—“ the head is retained behind by its frontal eminences, 
on the sides by the biparietal eminences.” 
How, in the position of the head as we are considering it, the 
frontal eminences are still above the inferior strait and in the 
hollow formed by the receding curve of the sacrum—a location 
offering little resistance to the passage of the brow ; and, on the 
other hand, the distance between the biparietal eminences is 
from three and one-lialf to three and three-fourths inches, while 
1 Cazeaux and Tarnier, “ Theory and Practice of Obstetrics,” Phila- 
delphia, 1884, p. 321. 
s Ibid., p. 352. 10 
Fry : Hunction of the Coccyx. 
the length of the transverse diameter of the pelvic outlet is four 
and one-fonrtli inches. 
Comparing the diameters of the fetal head with those of the 
pelvic outlet, we find ample room in all directions except one— 
the antero-posterior or coccy-pubic. But here a diameter of 
three and three-fourths inches is capable of being increased to 
four and one-half or five inches by recession of the coccyx ; the 
force expended in pushing back the bone, however, retains the 
bregma above until flexion is complete. 
Leishman 1 describes accurately flexion at the inferior strait, 
but instead of looking upon it as a normal movement, he con- 
siders it exceptional. He says: “ If the pelvis is at all under 
the average in point of size, the frontal region is arrested at the 
apex of the sacrum, and the occipital end of the lever is again 
driven downwards, so as to press upon and distend the perineum. 
If, however, the parts be ample, and the perineum not unduly 
resistant, this does not occur, and the whole bulk of the head 
follows the curve of the sacrum at every point, obviously at- 
tempting to effect an exit immediately under the pubic arch.” 
The curve of the sacrum to which he refers is the lower por- 
tion, i. <?., in a direction downwards and forwards. 
It is a movement which is the resultant of the force of the 
tissues of the floor of the pelvis and that from the uterine and 
abdominal muscles, and the direction is downwards and for- 
wards. Authors generally describe the descent of the occiput 
in this direction being guided to the vulvar opening by the 
perineum. 
There are several reasons why this is incorrect. In the first 
place, downwards and forwards is the direction of the move- 
ment of extension, while, as we have seen, the occiput passes 
behind the symphysis by one of flexion—downwards and back- 
wards (line a b). 
Again, it is only after this movement has taken place that the 
perineal muscles are called into play, and it is then that the 
reflected force of Solayres gives birth to the head by “ obviously 
attempting effect an exit immediately under the pubic arch.” 
Leishman2 once more refers directly to the movement of 
flexion at the inferior strait. He says : uTlie oscillations which 
the head in its course undergoes on its transverse axis are, first, 
1 “ A System of Midwifery,” Philadelphia, 1875, p. 296. 
2 Ibid., p. 297. Fry : Function of the Coccyx. 
11 
flexion; then partial extension prior to rotation; then flexion, 
if the forehead be arrested at the apex of the sacrum / and 
Anally, the movement of exaggerated extension, which is only 
completed with the birth of the head.” 
I have italicized the above words because they represent ex- 
actly what is my view of the mechanism in all cases of normal 
labor with occiput anterior. What I look upon as a distinct 
and necessary stage, Leishman speaks of as an occasional move- 
ment occurring, as he has said, when the pelvis is under the 
average size. So far, we have considered only the function of 
the coccyx in occipito-anterior positions, or such that have 
rotated into that position at the inferior strait. 
The same principle, however, is carried out in all cases, the 
coccyx retarding the end of the lever situated posteriorly until 
the opposite end slips down and becomes fixed at the symphy- 
sis. 
In some instances, consequently, the movement will be the 
opposite of flexion at the inferior strait, as in face presentations 
with chin forwards. The posterior end of the lever is kept up 
until extension forces the chin under the symphysis.