These articulations, or joinings, become much softened during labor, and give way a little, but not to any extent sufficient to assist delivery. It is a mistake to suppose that the bones separate at that time. The only part which gives way is the sacro coccygeal symphysis, which does relax, and allows the Os Coccygis to be pushed back by the child's head a full inch or more, thus enlarging the inferior strait.—(See c. and o. Plate VIII.) Sometimes this little bone will be even broken off, when there is great disproportion between the head and the strait. I have heard it snap like a stick breaking. There is nothing serious nor alarming in this, however, unless it be a first delivery late in life, though it may cause some pain at the time, and a little difficulty in sitting for some time after. In young persons the symphysis is soft, and gives way easily, so that they have little difficulty during delivery from this cause; but if a female marry late in life, after it becomes hardened, she may suffer considerably. In this case the coccygis is usually curved inwards considerably, and being firmly fixed the head cannot push it back, and on that account cannot pass, without great difficulty, and with the risk of rupturing some of the soft parts, or breaking the coccygis completely off. There is in fact great difficulty, and some danger, if the first pregnancy takes place late in life.
The Pelvis is usually divided into two parts,—the great pelvis, or upper part, enclosed between the wide flanges of the Ossa illii and the upper part of the sacrum; and the small pelvis, or basin, which is enclosed between the lower part of the sacrum and coccygis behind, and the ossa ischii and ossa pubes in front. The basin is nearly cylindrical, larger in the middle, and curved towards the front.
The Straits of the Pelvis.—The bones of the Pelvis, it will be seen, form a kind of broad ring, or cylinder, particularly in the basin; and the straits are two passages, one by which the child passes into the basin from the upper Pelvis, and the other by which it passes out from the basin into the world.
In Plate VIII. the line marked † is the antero posterior diameter of the upper strait, through which the child first passes, called also the brim, or entrance to the Pelvis. The line marked ‡ is the diameter of the lower strait, through which the child passes into the world, called also the outlet of the Pelvis. In Plate VII. the line marked † crosses the upper strait, or brim of the Pelvis.
The diameters of the Pelvis are the distances between the prominent points of each strait, and are four in number for each, those for the upper strait being represented below.
PLATE IX.
Diameters of the Upper Strait.
A B, which extends from the most prominent point of the Sacrum, to the top of the Symphysis pubes, is called the antero posterior diameter, or that from before to behind.—C D, and E F, are called the two oblique diameters; they extend from each sacro iliac symphysis, to the most prominent point of the Os Ilium on the opposite side.—G H, is called the Transverse, or bis iliac diameter, it crosses the Pelvis nearly from one hip joint to the other.
The Sacro Antero posterior diameter measures four inches. The two oblique diameters four inches and a half each. The bis iliac diameter measures five inches.