[Extracted from tlie American Journal of the Medical Sciences for October, 1870.] 
ON THE 
“STNCLITISM” 
OF THE 
FCETAL HEAD IN NATUEAL LABOUE. 
HUGH L. HODGE, M.D., 
EMERITUS PROFESSOR OF OBSTETRICS AND DISEASES OF WOMEN AND CHILDREN IN THE 
UNIVERSITY OF PENNSYLVANIA. 
(with two wood-outs.) 
The number of the Edinburgh Medical Journal for June last contains 
some critical remarks by Dr. J. Matthews Duncan, on what M. Kueneke 
terms the synclitic movement of the foetal head during its descent through 
the cavity of the pelvis and vagina, in natural labour. Not having seen 
M. Kueneke’s work, my only knowledge of his views of the mechanism of 
labour is derived from Dr. Duncan’s paper, and I am, therefore, unable to 
express any opinion as to how far these views may be consistent with the 
idea of the synclitic movement of the head, or to what extent he is amen- 
able to criticism. 
The whole subject, however, is most interesting and important, and its 
practical value can hardly be estimated. This circumstance, with a sincere 
desire of having this subject fully examined, induces me to offer to the 
consideration of the profession my views in regard to it. 
In all such scientific discussions it is very difficult to express our ideas 
with so much precision as to be clearly understood. Hence differences of 
opinion are often more apparent than real, a little explanation often 
resolving the difficulty. Perhaps, therefore, the theory of M. Kueneke 
may be reconcilable to correct views upon the mechanism of labour. 
Synclitism, or parallelism of the plane of the child’s head, in cases of 
natural presentation, to the planes of the pelvis and of the vagina, is one 
of my favourite doctrines, and has been taught by me with more or less 
fulness and precision, since the year 1832. My confidence in its truth has 
been continually strengthened by careful observation, and the ideas I en- 
tertain in regard to it are detailed at length, and to a considerable extent 
illustrated, in my work on the Principles and Practice of Obstetrics. 2 
I know, however, that it would be impossible for Dr. Duncan and myself 
to agree upon this point, unless, what seems to be equally impossible, we 
could come to some prior agreement as to the axis and planes of the 
pelvis and vagina, and as to the mode in which the head presents and 
moves through the obstetric canal. 
First. As to the planes of the pelvis. There can be no doubt that the 
planes through which the child’s head descends till the top of the head 
reaches the floor of the pelvis, are all parallel to each other as low down 
as the top of the arch of the pubis, and of course at right angles to the 
axis of the superior strait, or brim ; but below this it is universally agreed 
that the planes are oblique ; as to the degree of obliquity authors differ. 
The most usual representation is 
Flg> 1- given by lines drawn from vari- 
ous points on the sacrum and 
coccyx, below the third sacral 
vertebra, through the symphysis 
pubis, converging to a point ex- 
terior to the pelvis (see Fig. 1). 
Certainly the head does not pass 
perpendicular to such planes. 
If, however, from the same 
points in the posterior part of 
the pelvis, the lines should con- 
verge to the subpubic ligament, 
or top of the arch of the pubis 
they would represent planes of 
the pelvis through which the 
child descends, I believe, perpendicularly, during the latter part of labour. 
Lines similarly converging to the pubis, drawn from the posterior wall 
of the distended vagina even to the frmnum vaginae, would represent the 
successive planes of this canal, through which the head passes synclitically 
until the parietal protuberances pass the vulva. My reason for thus de- 
scribing these planes is that the child’s head revolves under the arch of the 
pubis at the termination of labour; its sub-occipital or cervical region being 
applied to the top of the arch of the pubis. In other words, the cervico- 
bregmatic diameter of the child’s head may be regarded as a radius de- 
scribing the segment of a circle, the centre of which is at the subpubic 
ligament. 
Second. As to the obstetric axis. The direction of this is perpendi- 
cular to the plane of the superior strait or brim, and then to each succes- 
sive plane of the upper part of the pelvis, to the third sacral vertebra. 
Then it varies so as to correspond with the perpendicular of each plane 
through which the head passes. As these inferior planes represent radii of 3 
a circle, the obstetric axis will here, of course, be represented by the circum- 
ference of a smaller circle, passing 
through the centre of each plane to - - fhir. I 
that of the distended orifice of the 
vagina. The first part of the axis, 
therefore, is straight; being coinci- 
dent with that of the brim; the 
lower part is the segment of a circle; 
the whole representing the line which 
the head follows during the progress 
of descent and delivery (see Fig. 2.) 
As to the “curve of Cams,” which 
has been often quoted with appro- 
bation by many high authorities in 
this country and Europe, I think it 
must be admitted to be erroneous. 
He describes a circle with a radius of 
two and a quarter inches, from the 
middle of the posterior margin of the 
symphysis pubis. This would indicate that the child’s head describes a 
segment of a regular circle from the brim to the outlet, and is altogether 
inconsistent with the fact that the child descends directly in the axis of 
the brim till the sinciput reaches the floor of the pelvis, and, moreover, it 
would not be at the centre of the plane of the inferior outlet. It would 
approach too near the pubis. 
Third. The lateral inclined planes of the pelvis, have not been brought 
into notice either by Dr. Duncan, or by the gentlemen whom he quotes. 
These planes exert a most important influence upon the progress of the 
head, especially as regards rotation. They are, I believe, four in number, 
two anterior and two posterior. Many authors ignore these planes, and 
few describe them with any degree of minuteness. In my work I have 
gone into details upon the subject, and endeavoured to show that the ex- 
tremities of the spines of the ischia, indicate the boundaries between the 
anterior and posterior inclined planes, and also that these planes are greatly 
prolonged by the distension of the perineal tissues, to the middle line of 
the perineum. Rotation, therefore, is partially affected by the occiput 
pressing against the side of the pelvis and then against the levator ani 
muscle, covered by its fascia and other tissues, as it descends towards the 
rectum. I do not perceive how all the movements of descent in the oblique 
positions of the head can be explained without taking cognizance of these 
inclined surfaces and their influences upon the movements of the foetal 
head. 
Fourth. As regards the word “vertex,” obstetricians ought certainly to 
come to some understanding, if anything like precision is to be expected 4 
as to the mechanism of labour. Nasgele virtually includes the whole 
upper part of the cranium in the idea of a vertical presentation. Others 
wonld restrict it to the central portion of the sagittal suture, and others 
again would refer it to the occipital region of the head. The true mean- 
ing of the word “ vertex” is the apex or top of the head. Hence, if a 
person stood erect, with the head in a horizontal direction, the real top or 
apex of the head would be about the middle of the sagittal suture. But 
if the chin be approximated to the breast, the head being in a state of 
flexion, the apex will then be at the posterior extremity of the sagittal 
suture. Now, as the child descends in natural labour, with the head 
flexed, the region of the posterior fontanel, or “ crown of the head,” is 
usually called the vertex. This region may be described as a circle, the 
centre of which is the posterior termination of the sagittal suture, and its 
radius extending from this point to the occipital protuberance. This circle 
will extend as far forward as the middle of the sagittal suture, and con- 
stitutes, I believe, the real presenting part of the head, in favourable 
labours. 
The word “ Sinciput” is still more indefinitely employed. It may be 
considered as representing the region of the anterior fontanel. 
Fifth. As to the presenting part, not “ point,” of the child’s head, which 
has elicited so many definitions and given rise to so much discussion, and, 
I may add, to many errors, in the study of the mechanism of labour, Dr. 
Duncan’s definition and my own are very similar. He defines it as “ that 
point on the surface of the child’s head through which the axis of the de- 
veloped pelvic canal passes.” My definition is, “ that portion of the fcetal 
ellipse which is recognized toward the centre of the canal of the pelvis 
and vagina.” This portion, in cases of favourable labour, is the vertex; 
not merely a point in the sagittal suture, but any point between this and 
the occipital protuberance, according to the degree of flexion or extension 
of the child’s head as it passes through the various planes of the obstetric 
canal. 
I cannot help, therefore, agreeing with M. Kueneke, that this portion 
of the child’s head, or vertex, presents towards the centre of the canal 
from the beginning to the end of descent, and, of course, differ from Dr. 
D. and most others, in transferring this portion from the line of the sagit- 
tal suture to the anterior parietal bone. 
All these definitions involve many points of great importance, in which 
again Dr. D. and I appear to differ. The vertex, according to my views, 
cannot present, unless the head be in a state of flexion, so that while the 
vertex is the most dependent portion, the chin, at the breast of the 
child, is the most elevated point of the head. Hence the occipito- 
mental diameter, and not the vertical, as Dr. D. mentions, is parallel to 
the axis of the brim at the commencement of descent. Hence, also, the 
cervico-bregmatic circumference is parallel to the plane of the superior 5 
strait, and not the occipito-frontal circumference, which would be the case 
if the vertical diameter of the head was coincident with that of the brim. 
This occipito-bregmatic circumference is represented by a line drawn 
over the base of the occiput, the parietal protuberances, and the anterior 
extremity of the sagittal suture, and may be regarded, practically, as a 
circle; its two important diameters being the transverse or bi-parietal and 
the perpendicular, so-called, or cervico-bregmatic diameter. Hence a sec- 
tion of the head, through these points, may be called the plane of this 
circumference, and it is this plane which, in my judgment, presents parallel 
to all the planes of the pelvis and vagina, from the superior strait or 
brim, until the delivery at the vulva. The axis of this cervico-bregmatic 
plane is the occipito-mental diameter of the head, which diameter, there- 
fore, is coincident with the axis of the obstetric canal, as above defined ; 
that is, coincident with the axis of the brim, until the top of the head 
reaches the floor of the pelvis, and then with the axis of each of the suc- 
cessive oblique planes of the lower part of the pelvis and of the distended 
vaginal canal to its extreme orifice. 
These views, which I have taught during the whole of my professorial 
course in the University of Pennsylvania, are, I suppose, in unison with 
what M. Kueneke terms the “ synclitic” movement of the child’s head in 
natural labour, although they may be inconsistent with some of his state- 
ments and with his ideas of the mechanism of labour, as Dr. Duncan has 
endeavoured to show. I did not originate these views, although I may 
have presented them with more precision and detail than my predecessors— 
they belong to the French school of obstetrics. Yelpeau speaks of the 
parallelism of the head at the inferior strait in opposition to the views of 
the Heidelberg professor, M. Hsegele, which have been so generally adopted 
in modern times, especially in Great Britian. Cazeaux, one of their latest 
writers, although perfectly acquainted with Naegele’s theory upon this point, 
also insists upon this parallelism of the head. In this country, my predecessor 
in the obstetric chair, Dr. Win. P. Dewees, who was familiarly called the 
“ Baudelocque of America,” taught the same doctrine more than fifty years 
ago, and by him it has been transmitted to the thousands of students of 
the University, and to the physicians who have read his invaluable writ- 
ings. 
Perhaps it will not be a vain repetition if I should endeavour to illus- 
trate my views of the mechanism of labour in the occipito-anterior posi- 
tions of the vertex, by tracing the descent of the child’s head through the 
os uteri and the obstetric canal in a “ given” position of a vertex presen- 
tation, in a primiparous woman. 
I will simply premise that the child’s head may be regarded as having 
passed through a plane when its parietal protuberances have passed, as 
these are the most prominent points upon its lateral surfaces. The child’s 
head may be said to be in a plane when these protuberances are opposed 6 
to any part of the. circumference of such plane. It has not entered a 
plane when the parietal protuberances are still above its level, however 
much of the head may project through the plane. All this is exemplified 
to the touch as well as to the eye during the last act of delivery. The 
occiput may project for some time at the vulva before this orifice is suffi- 
ciently distended to admit the whole cervico-bregmatic circumference; but 
as soon as the dilatation is adequate the parietal protuberances pass the 
lateral margins of the orifice, and delivery is virtually accomplished. 
For our illustration, as presenting the most simple view of the mechanism 
of labour, we still take, not an oblique position of the head, which has 
some complications, but a direct one, namely, the “ occipito-pubic position” 
of the “ vertex presentation,” the third of Baudelocque, but which, as we 
think, has been unnecessarily thrown out of view by M. Nmgele. 
By the time the os uteri is nearly or quite dilated, in a primiparous 
patient, the bag of waters usually ruptures, and the whole force of the 
contractions of the uterus are immediately directed upon the body of the 
child, which is therefore compressed into a comparatively small space, the 
limbs towards the abdomen, while the imperfect flexion of the head be- 
comes gradually increased. If the resistance of the os uteri be consider- 
able, this flexion becomes more complete, so that the occipital protuberance 
will be observed towards the pubis, the lambdoidal sutures converging on 
either side towards the sagittal suture. In a longer or shorter time the 
anterior margin of the os uteri slips over the protuberance of the occiput 
and retracts over the parietal protuberances to the face or neck of the child. 
The head is now left engaged in the superior strait, with the base of the 
occiput towards the top of the pubis, the parietal protuberances opposed 
to the sides of the brim, while the anterior fontanel will, with more or less 
precision, be opposite the promontory of the sacrum. Here there will 
probably be some delay, as the head is attempting to enter through the 
short or conjugate diameter of the brim, but in proportion to such delay 
will be necessary the increase of flexion, for the whole bearing-down force of 
the mother is, as Dr. Duncan well represents it, directed chiefly upon the 
short arm of the lever of the head, through the medium of the spine of the 
child. The os occipitis, therefore, descends, while the os frontis is resisted 
by impinging against the promontory of the sacrum. Hence flexion is 
perfected ; the posterior part of the sagittal suture, or vertex, will be found 
at the centre of the pelvis, the base of the occiput to the pubis, and the 
anterior fontanel, or anterior extremity of the sagittal suture, at the 
sacrum. Hence the cervico-bregmatic diameter of the head is parallel to 
the sacro-pubic diameter of the superior strait, while the bi-parietal, or 
transverse diameter of the head, is parallel to the bis-iliac or transverse of 
the brim, and the occipito-mental is coincident with the axis of the supe- 
rior strait. Hence the plane of the occipito-bregmatic circumference is 
parallel to the plain of the superior strait. This is synclitism. I can hardly suppose that the statement thus given will meet with any 
objection, inasmuch as if the head be not in a state of flexion, as I maintain, 
the protuberance of the occiput will be at the pubis, and the anterior por- 
tion of the os frontis, or forehead, will be at the promontory of the sacrum, 
in which case the head will be arrested, because the occipito-frontal diameter 
will be parallel with the short or conjugate diameter of the brim. Then 
the sagittal suture would run nearly in a direct line from the anterior to 
the posterior part of the pelvis, and the two fontanels would be at the 
same level. When, however, flexion has occurred, the posterior fontanel 
will be found lower down and nearer the centre of the strait, and the ante- 
rior higher up, and nearer the promontory, while the sagittal suture will 
be found to run not only backward, but obliquely upward. 
The process of labour continuing, the child’s head descends precisely In 
the same direction till the sinciput reaches the floor of the pelvis, and the 
occiput has glided along the posterior or inner surface of the body of the 
pubis. The whole head descending thus far through a cylinder, as M. 
Schroeder would express it, the occipito-mental diameter coincident with that 
of the axis of the brim, and the cervico-bregmatic plane parallel to each 
plane of such cylinder, till it reaches the level of the third sacral vertebra 
behind, and the top of the arch of the pubis in front. By the time this 
is accomplished the os frontis has descended below the promontory of the 
sacrum, and the occiput appears under the arch of the pubis, when, as Dr. 
Dewees would express it, the chin begins to leave the breast, or, more 
accurately, the process of extension commences. How the further descent 
of the head is resisted by the sinciput striking against the floor of the 
pelvis, and anteriorly, by the neck of the child pressing against the pubis. 
The driving force continues to operate and chiefly upon the occiput through 
the medium of the spine. This portion of the head then descends towards 
the perineum as far as practicable, but the resistance here being also great, 
the head passes off diagonally between these opposing forces, and describes 
a circular motion on the base of the occiput under the arch of the pubis, 
where it is comparatively at rest. Hence the process of extension is con- 
tinually increasing until the occiput has somewhat dilated the vulva and 
the parietal protuberances will be perceived on either side at the tubers of 
the ischia, while through the rectum the anterior fontanel can be detected 
at the perineum or coccyx. The head, now partially extended, presents 
at the inferior strait, with the same diameters and the same planes which 
were recognized at the brim, and the occipito-mental diameter, which was 
coincident with the axis of the brim, is now in unison with that of the 
inferior strait; the chin pointing towards the concavity of the sacrum, 
while the vertex, as represented by the posterior fontanel, is at the centre, 
not of the vulva, but of the inferior strait. The plane, therefore, of the 
occipito-bregmatic circumference is parallel to the plane of the inferior 
strait. 8 
Now, as the head moved in this regular manner through the upper or 
cylindrical portion of the pelvis, presenting its equatorial plane, as Dr. 
Barnes might term it, parallel to the planes of the cylinder, till the sinciput 
reached the floor of the pelvis, so, during the gradual process of extension, 
through the curved portion of the canal, the same cervico-bregmatic plane 
became parallel to each successive oblique plane of the curved portion of 
the canal, to the inferior strait. The parietal protuberances will always 
be found at the sides of the pelvis, and as the occiput gradually advances 
under the arch of the pubis, the posterior fontanel is found towards the 
centre of each plane till it reaches the outlet of the pelvis. 
The bearing-down efforts of the mother now force the child more and 
more against the perineum, the process of extension continues, till eventu- 
ally the occiput gets in front of the pubis, the parietal protuberances on 
either side of the vulva, and the posterior fontanel is observed clearing the 
perineum at the centre of the dilated orifice of the vagina, so that again 
we have the cervico-bregmatic plane engaged, but now parallel to the plane 
of the orifice of the vagina. Hence the head passes through the canal of 
the vagina in this circular manner, presenting its equatorial plane parallel 
to the various oblique planes of the vagina. 
If this representation be correct, the foetal head, “at term,” notwith- 
standing its apparent irregularity of form, traverses the obstetric canal 
with as much readiness and precision, continually presenting the same 
diameters to the walls of the canal, and the same diameter of the head, 
to the axis of the obstetric canal, as if it were a perfect sphere of about 
three and a half inches in diameter. 
This seems to me a correct representation of the synclitic movement of 
the child’s head—in other words, the parallelism of the plane of the great 
occipital extremity—Tin relation to those of the pelvis and vagina. 
If we now consider the First position of the vertex, the left occipito- 
anterior, the process is rather more complicated, from two causes. The 
first is that the head is no longer direct, but oblique, at the superior strait, 
and subsequently in the cavity of the pelvis; and the second, resulting 
necessarily from the first peculiarity, is the necessity of a rotatory move- 
ment of the head, that the occiput may pass under the pubis. 
If the propositions already assumed be granted, these difficulties imme- 
diately vanish. For if the head passes through the os uteri in a state of 
flexion, presenting its cervico-bregmatic circumference to the circle of the 
os uteri and that of the superior strait, it may practically be regarded as 
a sphere or ball to be driven through the planes of the canal. Hence it 
would be a matter of no moment whether it did or did not rotate upon its 
axis, for diameters of the same length would always be coincident with 
those of the obstetric passages. Parallelism of planes, therefore, would 
ensue. 
To be, however, more precise. In this first position, as in the former, 9 
the resistance of the os uteri, or the margin of the superior strait, against 
the anterior parts of the head of the child, while the uterine forces are 
driving down the occiput through the medium of the spine, necessitates 
flexion of the head to a greater or less degree. Hence, after the os uteri 
has retreated, the head will be found oblique, not only because it extends 
diagonally from the left ramus of the pubis to the right sacro-iliac sym- 
physis, but also because the occiput is lower in the pelvis than the forehead 
and the face. Hence the posterior fontanel will be found not on the same 
level with, but lower than the anterior fontanel; and the sagittal suture, 
therefore, not running directly across the pelvis, but obliquely upward as 
well as backward. 
The parietal protuberances of course present obliquely to the right and 
left sides of the pelvis, while the base of the occiput will be towards the 
left anterior part of the pelvis, and the anterior fontanel towards the right 
sacro-iliac symphysis. Hence we have the cervico-bregraatic diameter par- 
allel to one of the oblique, and the bi-parietal to the other oblique diameter 
of the brim, while the posterior extremity of the sagittal suture will be found 
towards the centre of the pelvis, and the chin, being directly opposed to it, 
will be the most elevated part of the head. 
I can hardly expect that all this will be conceded by Dr. Duncan, as he, 
in common with almost all British authorities, speaks of the “vertical” 
diameter as being coincident with the axis of the uterus and of the brim, 
and of the two fontanels as being nearly upon a level, all of which would 
involve the idea that the occipito-frontal diameter is parallel with the 
oblique diameter of the brim, and, of course, that there is a synclitism of 
the plane of the brim with that of the occipito-frontal circumference, and 
not with that of the cervico-bregmatic circumference, as I believe. The 
truth, of course, of this question can only be ascertained by careful ob- 
servation. That the head will be found in the position Dr. D. describes, in 
many cases of labour, there can be no doubt, owing to the greater length 
of the oblique diameter of the pelvis, the relaxation of the os uteri, or 
the diminutive size of the head. In exact proportion, however, to the 
resistances of the os uteri or of the bones of the pelvis, will be the greater 
flexion of the head, and, of course, the accuracy with which its cervico- 
bregmatic circumference becomes parallel with the plane of the superior 
strait, and hence the occipito-mental diameter coincident with the axis of 
the brim. 
The head descending into the cavity of the pelvis to its floor, presents 
virtually the same diameters, and soon the process of extension commences. 
This is often slow, inasmuch as another process, that of rotation, must 
precede and accompany it. It is not necessary to inquire at this time how 
this rotation is effected, as all acknowledge its necessity as well as its 
existence. As already intimated, I believe it is dependent upon the inclined 
planes of the pelvis, and their prolongation by the soft tissues to the 10 
middle line of the perineum. The question arises—and here, lam sorry 
to say, Dr. D. and I again differ—On what diameter does the head rotate ? 
Not on the vertical, as he declares, as this would indicate that the head 
was situated directly across the pelvis, that is, the occipital protuberance 
towards the left foramen ovale and the forehead opposite the sacro-ischiatic 
oramen, and the anterior fontanel opposed to the coccyx. I think it will 
be found that the posterior fontanel is nearer the centre of the floor of 
the pelvis, and the anterior fontanel, and not the forehead, opposed to the 
sacro-ischiatic foramen. Hence the occipito-mental diameter constitutes 
the axis upon which the head rotates. If this be true, it seems to me to 
necessitate the parallelism of the cervico-bregraatic plane with the planes 
of the lower part of the pelvis during the process of rotation. This 
rotation is never complete until the head is fairly engaged in the inferior 
strait. By this time the process of extension, which had been retarded by 
the impingement of the occiput upon the side of the pelvis, advances with 
more rapidity until the parietal protuberances are observed at the tubers 
of the ischia, and the base of the occiput at the sub-pubic ligament. 
The small or equatorial plane of the head now presents parallel to that 
of the inferior strait, as was noticed when speaking of the third position 
of the vertex. If, therefore, we adhere to Dr. Duncan’s definition of pre- 
sentation as “that point of the surface of the child’s head through 
which the axis of the developed pelvic canal passes,” I think it will be 
found that the posterior extremity of the sagittal suture, or the angle 
formed by the two parietal bones, will be found towards the centre of 
the canal of the pelvis. Of course it gradually advances as the occiput 
approaches the arch of the pubis, and then more rapidly in proportion to 
the degree of extension that ensues, the presenting part being always cen- 
tral to that particular plane through which the head is passing. 
This rejects altogether the usual mode of judging as to what part pre- 
sents. We cannot determine this question by the position of the caput 
succedaneum, for reasons Dr. D. has well detailed. Neither can the question 
be settled by examining what part of the child’s head is felt at the orifice 
of the vagina, because, in the first place, the orifice of the vagina is never 
parallel with any plane of the pelvis, and should be regarded as situated 
at the anterior part of the cavity; and in the second place, the part of 
the head felt towards the orifice of the vagina is continually changing 
during the process of rotation, so that in many cases the anterior part of 
the parietal bone, its posterior part, and then the side of the occiput may 
be felt in succession at the opening of the vagina before the occipital pro- 
tuberance fairly emerges. If, however, the finger be directed to the true 
vertex, this part will be found nearly stationary, during the process of 
rotation, except that it advances slowly along the perineum. If, therefore, 
the occipito-mental diameter be thus constantly coincident with the obstetric 
axis, and the whole posterior portion of the head be represented by the 11 
figure of a ball, each of its diameters measuring three and a half inches, 
it follows, I think, that there is a necessary parallelism of this equatorial 
plane with the planes of the pelvis. This view is not invalidated by the 
fact, recorded by Ntegele, that in many instances the right parietal pro- 
tuberance escapes first at the inferior strait, as this would only show that 
the head is small enough to be delivered before rotation is complete, but is 
not really inconsistent with the synclitism for which we contend, I must, 
however, believe that careful observation will show that such an escape of 
the parietal protuberance occurs in easy labours only; that when the cus- 
tomary resistances are present, as in first labour, the head being of normal 
size, each parietal protuberance finds its exit upon the same level at the 
tubers of the ischia, while the sagittal suture and anterior fontanel will be 
found in the middle line of the perineum. 
Dr. Duncan’s declarations that “continued synclitism is conceivable 
during and after rotation,” and, moreover, “ that there arises from the 
mechanism a direct tendency to the production of the synclitic condition 
there can be no doubt,” gives me much pleasure. But in another place 
he observes, “ The tendency to the synclitic movement is prevented by two 
causes. First, the driving force operates chiefly on the occipital portion 
of the head, through the spine; and, second, the greater resistance of the 
pelvis posteriorly, as compared with that anteriorly towards the symphysis. 
These prevent the synclitic movement.” 
How these two circumstances prevent the tendency to a synclitic move- 
ment, I am at a loss to comprehend; for, on the contrary, they seem to 
me to necessitate synclitism of the head by increasing the flexion of the 
child’s head when it is high up, and its extension when low down in the 
pelvis or in the vagina. 
Respecting the quotation given by Dr. Duncan from Dohrn, as to the 
“shearing” of the child’s head, I do not, perhaps, comprehend his full 
meaning, but I must agree with Dr. Duncan in believing that the vertical 
shear to which he alludes does not occur at the superior strait in cases of 
natural labours, but I have observed it in cases where the conjugate 
diameter was contracted. In respect to the second or lateral shear, it in- 
cludes, if I understand it, those circumstances which determine the rotation 
of the head in the inferior part of the pelvis, and may, as he observes, 
give rise to the lateral shearing of the head, as described by Dohrn. 1 
should not, however, regard this as of any practical importance. My own 
belief is that the head of the child, during descent, in all the occipito- 
anterior positions, impinges much more firmly against the anterior por- 
tions of the cavity, and that the rotation of the head is dependent, in these 
cases, upon one or other, as the case may be, of the anterior inclined 
planes of the pelvis, as formed by the bones of the pubis and ischium, and 
as they are prolonged by the levatores ani muscles, &c. But these are 
points not now under review. 12 
I thus have been endeavouring to maintain the synclitic movement of 
the child’s head in cases of natural labour, as true, both theoretically and 
practically. I cannot insist upon its mathematical accuracy, for the ever- 
varying size of the child’s head, and the degree of resistance it meets with 
from the os uteri and from the bones of the pelvis, which must influence 
continually its degree of flexion or extension, defy any attempts at mathe- 
matical accuracy. I would only repeat that the head passes more readily 
when it presents its cervico-bregmatic plane, and that the greater the re- 
sistance the more perfect will be the flexion, and the more exact will be 
tile parallelism or synclitism of the equatorial plane of the head with the 
various planes of the pelvis and vagina. 
If this fact be substantiated, its practical importance can hardly be esti- 
mated. This assertion needs no demonstration to any experienced ac- 
coucheur. If the great occipital extremity of the head descends, in a 
natural labour, in such a manner that the occipito-mental diameter always 
coincides with the axis of the obstetric canal, whether straight or curved, 
and that the cervico-bregmatic plane of the head, with its diameters of 
equal length, is parallel to the successive planes of the pelvis through 
which it passes, it results : That it is the great business of the accoucheur 
during the whole process of descent, to insure this parallelism by facili- 
tating, during the first portion of the descent, the process of flexion of the 
head, and during the last, by facilitating its extension. 
Hence, also, in all obstetric operations the same principle must regulate 
the hand of the practitioner. The blades of the forceps, for example, 
should always be applied as nearly as practicable in the direction of the 
occipito-mental diameter, and traction effort be made in the axis of the 
obstetric canal, inasmuch as then the longest diameter of the head will be 
coincident with the obstetric axis, and the cervico-bregmatic circumference 
will be parallel with the planes of the pelvis and vagina. This, as all 
experience demonstrates, is not only the most natural, but also the easiest 
mode for the transit of the head.