[Reprinted from The American Journal op Obstetrics and Diseases op Women and 
-— Children, Vol XVIII., No. III., March, 1885.] 
OBSERVATIONS UPON THE APPARENT CURVE OF THE 
TAL SUTURE IN VERTEX PRESENTATIONS. 
BY 
CHAS. F. WITHINGTON, M.D. (Harv.), 
The importance of being able to recognize by digital exam- 
ination the exact position of the presenting head with refer- 
ence to the maternal pelvis, is sufficiently insisted upon by all 
obstetrical writers as being essential at once to the proper ap- 
prehension of the successive stages in the mechanism of normal 
labor, and also to the intelligent application of the forceps or 
any other artificial aid that may be necessary. Yet it cannot 
be denied that a very large number of practitioners habitually 
conduct labors without obtaining from their successive exam- 
inations any distinct idea of the relation of the sagittal suture 
to the different parts of the pelvis, and not infrequently ne- 
glect to gain this information even before using the forceps. 
The ability to make this diagnosis is not altogether easy to 
gain, by reason of certain disturbing elements in the problem 
which have not, to the writer’s knowledge, been alluded to in 
the current text-books, and which will repay investigation. 
As every one knows, the chief guide to a knowledge of the 
position of the head is in the sagittal suture, which extends 
from the posterior or blunt angle of the anterior fontanelle to 
the anterior angle of the posterior fontanelle. The examining 
finger, extended through the partially dilated cervix to the 
presenting head, usually finds somewhere within the circle of 
the os some portion of this suture, which appears as a curve 
passing across from one lip of the os to the other. For ourpur- 
pose, we may consider the head to be a sphere, on the surface 
of which is cut the segment of a great circle with sufficient 
distinctness to be readily palpable, even in many cases’ through 
intervening membranes. The considerations to be now ad- 
vanced may, perhaps, be more readily followed if the reader 
will make use, either in imagination or in reality, of a ball, hav- 2 
Withington: Sagittal Suture 
ing an arc of a great circle described upon it. The circle of the 
os may be represented by the circumference of a round hole 
cut in a piece of cardboard, which is held with the central point 
of its orifice tangential to the sphere. If the finger be allowed 
to trace the portion of the great circle included within the 
circle of cardboard, it will be of assistance in verifying some 
of the points to be referred to. 
A portion of the curve of the suture can be felt during labor 
by the finger through the circle of the os. From this portion 
of the curve the observer is obliged mentally to construct the 
remainder of it. In order to do this, lie must consciously or 
unconsciously project that curve upon the plane of the os, or, 
as will be seen below to amount to the same thing, on the 
plane which, for the time being, is tangential to the most ad- 
vanced point of the head. Geometrically this is done by drop- 
ping perpendiculars from the successive points of this curve 
upon the plane. The bases of these perpendiculars, taken to- 
gether, indicate the line of the projected curve. Now, it is 
evident that if the plane of the suture is so placed as to be per- 
pendicular to the plane of the os, the projection upon the latter 
of the curve of the suture will be a straight line. This 
straight line passes through the centre of that circle which 
would be formed by the projection of the whole sphere upon 
the tangential plane. Consequently, the rectilinear projection 
of the suture passes through the middle point ol the plane of 
the os, or nearly so. The problem of determining from a certain 
accessible portion of a line the remainder of it, is, of course, 
much easier if the given line be straight than if it be curved. 
If, then, the curve of the sagittal suture touches the plane of 
the os, in which case it is perpendicular to it, the projection of 
the curve is a straight line, and the conditions are most favor- 
able for easily prolonging that line beyond the limits of the oa 
on each side, determining definitely the direction of the two 
fontanelles, and, by adding the knowledge of their distance 
apart from one another, diagnosticating definitely their posi- 
tion. 
Suppose, however (and this is what is generally the case), 
the plane of the suture is not perpendicular to that of the os, 
then the projection of that curve, instead of being a straight 
line, is a curve which no longer passes through the centre of the in Vertex Presentations. 
3 
plane of the os. The nearer the suture comes to being tan- 
gential to the plane of the os, and the more nearly perpendicu- 
lar the two planes to each other, the greater is the radius of the 
projected curve, and the nearer that curve is to a straight line ; 
while the more obliquely the two planes cut each other, the 
further the projection of the suture lies from the centre of the 
circle of the os, and the shorter the radius of its curve. More- 
over, the concavity of the projected curve always looks toward 
the larger of the two portions into which it divides the circle 
of the os (vd. Fig. 1). 
If the projection of the suture is a curved line and one 
capable from one period of the labor to another of altering its 
radius of curvature, it is plain that there is much greater diffi- 
culty in prolonging in imagination that curve beyond the 
Fia. 1. 
Fig. 1.—Diagram representing curve of sagittal suture projected upon various pelvic 
planes, seen from below; a, b, c, d, and e represent different degrees of dilatation of the 
os. 1,2,8, 4, and 5 successive degrees of rotation. The curve in each case is pro- 
longed by a dotted line beyond the limits of the dilatation which corresponds with its 
particular extent of rotation. The curves, it will be noted (the position being O. L. A.), 
have their concavity forward and to the right. The gradual disappearance of the anterior 
fontanelle and advance of the posterior towards the reach of the examining finger, 
shows the increasing flexion of the head. 
limits of the os than if the projection were a straight line. 
Now it will presently appear that this rectilinear projection of 
the suture is possible only after rotation is complete, and that 
before that time the plane of the suture is never perpendicular 4 
Witiiington : Sagittal Suture 
to the plane of the os, so that the projection is always curvi- 
linear during the whole progress of the rotation. 
Of course, the accoucheur, during actual labor, possessing, as 
he does, no better instrument of precision than his linger, can- 
not carry out the process we have described with anything 
like the accuracy of a geometrical method ; but unless he is 
able in a rough way to project the curve of the sagittal suture 
upon the plane of the os, he can form no adequate idea of the 
direction of the long diameter of the head across the mother’s 
pelvis. The secret then of accuracy in diagnosticating 
position consists in the ability to describe with the finger-tip 
perpendicular lines from the various accessible points of the 
sagittal suture to the plane of the os, and to estimate the 
position of the line in that plane to which these perpendiculars 
lead us. As the os becomes more fully dilated, portions of 
the head higher up become accessible to the touch, and the per- 
pendiculars from the higher points on the sagittal suture must 
be longer than those from points lying in or near the plane of 
the os. The chance for error in drawing these perpendiculars 
must, therefore, be greater than in the shorter ones. So that 
though a more dilated os gives us a greater arc of the circle to 
work from, it also increases the liability to mistake which is 
due to the rough means employed. 
The expression “plane of the os,” now requires further con- 
sideration. It evidently has no material existence in the form 
of any maternal structure. As used in this paper it refers to 
the plane which corresponds, during the first stages of labor, 
to the rim of the os uteri. As it is only by feeling of the edge 
of the lips of the os that the examiner can locate the position 
of that plane upon which he must project the curve of the 
suture, the less the amount of dilatation the easier it is to 
recognize the data for determining that plane. On the other 
hand, when dilatation is complete so that the uterine lip can 
no longer be felt, the plane of the os can be defined only as 
that particular pelvic plane which for the time being happens to 
be tangential to the fetal head. Now, it is important to re- 
member the constant change of direction of these planes due 
to the unequal length of the anterior and posterior pelvic 
walls, and the constantly changing direction of the pelvic axis. 
From the time when the fetal head engages at the superior in Vertex Presentations. 
5 
pelvic strait until it comes in sight at the vulvar orifice it un- 
dergoes a revolution upon a horizontal transverse axis of about 
90°. By reason of the progress of the head down through 
the pelvic canal which is going on simultaneously with this 
revolution it happens that it revolves not about its own intrin- 
sic axis (as is the case in the maneuvre technically known as 
“ rotation,” to be referred to hereafter), but about a point in a 
horizontal line outside the sphere, corresponding in position 
probably neafly to the inner side of the pubic symphj’-sis. 
Hence we have termed this motion a revolution as distinguished 
from the “ rotation,” technically and properly called, of the 
head about its own verbal axis. This axis of rotation, how- 
ever, on account of the synchronism of the rotation with the 
revolution just spoken of, is constantly changing its direction 
through an arc amounting in all to about 90°. 
For an illustration of these relative motions we may look not 
to the rotation and revolution of the earth, because the axis of 
that body remains constant or nearly so. If, however, the 
diurnal rotation of the earth remaining as it is, we were to 
imagine the earth’s axis to be constantly pointed toward the 
sun, we should have an example in kind, though by no means 
in relative degree, of the two circular motions of the fetal 
head. 
These two motions, though occurring simultaneously, require 
to be estimated separately by the accoucheur. The revolution of 
the head is about a centre which is pretty nearly coincident with 
that part of the head most readily within reach of the examin- 
ing finger. In any revolving system, of course, the amount of 
motion is less in proportion as we approach its centre of revolu- 
tion. If we place the finger at the lowermost point of a sphere, 
and then, keeping the finger motionless, revolve the sphere around 
that point 90° backward, we shall see that but little impres- 
sion has been communicated to the finger of a motion which 
has brought the centre of the sphere from a point directly 
above to a point directly behind the finger (vd. Fig. 2). So 
in the successive examinations during the expulsive stage 
of labor, while the finger is always passing in the same direc- 
tion and is constantly impinging on nearly the same part of the 
head, it is difficult to appreciate the fact that during this time 
the direction of the centre of the head from the finger tip has 6 
Withington: Sagittal Suture 
entirely changed, the arc of revolution being about 90°. 
The information obtained through the finger, then, requires to 
be supplemented by a knowledge of about the angle of that 
plane of the pelvis which from time to time becomes tan- 
gential to the advancing head, and upon which consequently the 
sagittal suture is to be projected. It is fortunate that a 
moderate error in the location of this plane is not fatal to an 
exactness in diagnosis of position sufficient for practical pur- 
poses. 
Now on examining the phenomenon of rotation as affecting 
the direction of the sagittal curve, we first notice an aid to the 
diagnosis of position, in the fact that the pole of the axis lies 
Fig. 2. 
Fig. 3. 
constantly at about the mid-point from side to side of the pelvis. 
Hence the finger examining jper vaginam impinges upon the 
sagittal suture at a point very near the pole of rotation. If 
from this pole the finger, during the process of rotation, makes 
from time to time a short excursion in either direction along 
the meridian of the sagittal suture, a change in direction of 
that meridian corresponding to the degree of rotation is 
evinced in two ways, viz.: by a progress or motion (for 
example), from left to right of that portion of the meridian 
which lies on one side the pole, and also by a simultaneous 
retrogression or motion from right to left of that part of the 
meridian lying on the other side of the pole (vd. Fig. 3). 
But it must be remembered that as the head comes down through 
the successive pelvic planes, what is the lowest point with in Vertex Presentations. 
7 
reference to them, that is, what is the first point to reach them, 
is not the lowest point with reference to the horizontal pi ne by 
which the examiner instinctively judges of “ high” and “ low.” 
Let us suppose the woman to be lying upon her back and let 
directions be indicated as right or left, according to the corre- 
sponding side of the mother, upward or downward from a 
horizontal plane, and headward or footward from a vertical 
transverse plane. Suppose the head to be at the pelvic 
brim and the position exactly transverse. Then the polar 
point on the sagittal suture is absolutely the lowest point 
of that suture, but not the lowest point of the sphere, 
which is (supposing the inclination of the plane of the 
brim to the horizon, the woman being recumbent, to be 
30°), 30° distant from that pole. From the latter point 
the finger tracing the sagittal suture passes either to right or 
left, upward and headward. If, however, the position is not 
exactly transverse, the pole is no longer the lowest point of the 
suture, and thus, while one is tracing the suture in a continuous 
line from the pole, he may be surprised to discover a change in 
direction as he reaches this lowest point and passes up beyond 
it. A condensed statement of the direction traversed by the 
examining finger in passing from the pole in the various posi- 
tions at the pelvic brim is as follows: 
O. L. A. 
and O. II. P. 
right, downward, then upward, headward ; 
left, upward, footward. 
O. R. A. 
and O. L. P. 
right, upward, foot ward ; 
left, downward then upward, headward. 
Were it possible for the sagittal suture to lie in the sagittal 
plane of the mothers body while the head was still at the 
brim, it is obvious that the examining finger, as it moved from 
its pole footward, would pass downward over an arc of 30° 
to reach the lowest point of the suture, which would then co- 
incide with the lowest point of the sphere. On the other 
hand, to reach the most “ footward ” point of> the suture and 
of the head, the finger would have to traverse an arc of 60°. 
But in point of fact, while the process of rotation is bringing 
the plane of the sagittal suture more and more toward coinci- 
dence with the sagittal plane of the mother’s body, a concur- 
rent process is so altering the direction of the axis that the 
pole is departing further and further from the lowest point of 8 
Withington: Vertex Presentations. 
the great circle, and approximating more and more to its 
footward point. 
The alteration in direction of the axis of rotation is not, 
however, sufficiently rapid to keep that axis perpendicular to 
the successive pelvic planes as they become tangential to the 
fetal head. Whether we hold that the head on entering the 
pelvic brim is synclitical, i. e. (having its biparietal diameter 
parallel with the plane of the brim), or asynclitical, in either 
case the initial degree of inclination of the axis remains unal- 
tered until the most advanced portion of the head has passed 
some distance into the pelvic canal; probably, indeed, until the 
maximum diameter of the head comes to the plane of the 
brim. During all this time, the plane of the suture has 
been perpendicular to no one of these successively tan- 
gential planes, except only (on the synclitical view), the 
plane of the brim. So that it is only at the brim that 
the projection of the curve of the suture upon the plane of the 
os is a straight line. From that moment on until rotation has 
become complete, the projection is always a curve, having its 
concavity forward, and when the head occupies the right 
oblique diameter of the pelvis, to the right, while, when the 
head lies in the left oblique pelvic diameter, the concavity of 
the projected curve looks toward the left (Fig. 1). 
It is not until rotation is complete that the plane of the sa- 
gittal suture becomes perpendicular to the tangential plane of 
the head ; but this holds true alike whether rotation be early or 
late, and for all possible pelvic planes after it occurs the pro- 
jection of the curve is a straight line. Of course, as the rota- 
tion is approximating completion, the projected curve is ap- 
proximating a straight line. But it remains a fact that the only 
circumstances under which this projected line is rectilinear are, 
on the one hand, while the head is tangential to the pla e of 
the brim (a condition which has ceased before the degree .of 
dilatation admits of recognition of the suture by the finger) 
and, on the other hand, after rotation is complete.