PARTURITION. 



907 



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 



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 fjrSt. 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 



