PARTURITION. 
the child lies across and presents with the arm 
or shoulder, a position in which it cannot be 
born. The two first are therefore called natural, 
the last unnatural presentations. In the human 
subject neither the antero-posterior nor the 
_ transverse are the longest, but the oblique dia- 
meters, both of the brim cavity and outlet of 
the pelvis, and it is only in the directions of 
these diameters that the pelvis is of a tolerably 
uniform size throughout. They are named the 
right and left oblique diameters according to 
that sacro-iliac synchondrosis from which they 
are drawn. 
The great peculiarity in the mechanism of 
human parturition is that in either of the natu- 
ral presentations the presenting part enters the 
pelvis obliquely, not only as to the transverse 
diameter, but as to the axis of its brim; it 
passes through the cavity and outlet nearly in 
the same position ; so that it not only takes that 
direction in which the pelvis is most roomy, 
viz. in the oblique diameters, but in which it 
will itself occupy the least possible space. 
Having stated this law, it will now be neces- 
sary to describe these presentations of the child 
in illustration of it. 
The cephalic end of the child may present 
in two ways, either with the head or the face; 
the former is by far the most common; it is also 
the most favourable for mother and child, and 
at one time was looked upon as the only natu- 
ral and favourable presentation. The head 
presents either with the right or left parietal 
protuberance, the sagittal suture running pa- 
rallel with the right or left oblique diameters, 
and in both cases, at the beginning of labour, 
crossing the os uteri. 
These three facts at once confirm the law 
above mentioned, viz. that the head enters the 
pelvis obliquely both as to its long and perpen- 
dicular diameters, or, as before expressed, ob- 
liquely as to the transverse diameter and axis 
of the brim; for if (as is well known to be the 
case) the os uteri at the beginning of labour is 
Situated at the upper part of the hollow of the 
sacrum, the vertex of the head will be turned 
towards this part of the pelvis, and the parietal 
protuberance being that part which is lowest 
and in the centre of the pelvis, it follows that 
the perpendicular diameter of the head will 
run obliquely upwards and forwards with the 
axis of the brim. 
The first position, viz. where the right pa- 
rietal protuberance -presents, and the sagittal 
suture corresponds to the right oblique diameter 
of the pelvis, is known by the posterior or 
small fontanelle being felt in the vicinity of the 
left foramen ovale, the anterior or large fonta- 
nelle in the opposite direction near the right 
sacro-iliac synchondrosis: this has been called 
the first position from occurring more frequently 
than the other, viz. in the proportion of five 
to two. As the head approaches the pelvic 
outlet, the occiput turns somewhat more for- 
wards, so that instead of the protuberance, the 
posterior and superior quarter of the right pa- 
_ rietal bone presents: this is the part of the 
__ head which the finger at this period of labour 
_ first touches upon during examination, which 
907 
first passes under the pubic arch, and first dis- 
tends the os externum ; the position of the 
head is nevertheless still oblique, for the right 
branch of the lambdoidal suture will be felt 
parallel with the left descending ramus of the 
pubic arch. In still further proof of what has 
now been stated, we may mention, that if the 
head be some time in its passage through the 
vagina, it becomes so tightly encircled by it as 
to produce a considerable obstruction to the 
circulation in the scalp ; hence we shall feel a 
tumefaction of the cranial integuments on that 
part of it which presents. On examining, 
therefore, the head of a new-born child which 
has presented in the first position, it has a cir- 
cular swelling of the scalp situated upon the 
posterior and superior quarter of the right pa- 
rietal bone. This is the caput succedaneum, 
the Vorkopf of the German authors, and, as 
was pointed out by the late Professor Chaussier 
of Paris, is a distinct evidence of the manner 
in which the child has presented during labour. 
The shoulders enter the pelvis in the contrary 
oblique diameter to what the head does, so that 
if the head in the first position has passed 
through with its long diameter corresponding 
to the right oblique diameter of the pelvis, the 
shoulders will be found in the left oblique dia- 
meter—from this circumstance, after the head 
is born, the face is turned backwards and to 
the right. 
The second position of the head is the reverse 
of the first. The left parietal protuberance 
presents. During the descent of the head 
through the brim into the cavity of the pelvis, 
the sagittal suture is in the right oblique dia- 
meter as in the first position, only now the 
posterior fontanelle is directed towards the right 
sacro-iliac symphysis, the anterior one to the 
left foramen ovale. The head descends in this 
position until it approaches the pelvic outlet, 
when it makes the quarter of a turn and passes 
from the right into the left oblique diameter, 
the anterior fontanelle now corresponding to 
the left sacro-iliac symphysis, the posterior one 
to the right foramen ovale. As the head enters 
the vagina and begins to pass under the pubic 
arch, it is the posterior and superior quarter of 
the left parietal bone which now presents, and 
upon which the puffy swelling of the scalp is 
situated ; as in the first position it was the right 
lambdoidal suture which corresponded to the 
left branch of the pubic arch, so here it is the 
reverse, the left lambdoidal suture at this moment 
will be found parallel with the right branch of 
the pubic arch ; in like manner, the face when 
born turns backwards and to the left. This 
change in the position of the head from one ob- 
lique diameter to the other is not peculiar to the 
second position, for we meet with it occa- 
sionally in the first, the anterior fontanelle being 
turned to the right foramen ovale, the posterior 
one towards the left sacro-iliac synchondrosis, 
the change in this case usually taking place at 
a much earlier period of labour than in the se- 
cond position, whether it is owing to the posi- 
tion of the rectum or not is difficult to say. 
The uniformity with which this change occurs 
in the position of the head from one oblique 
