Primary Causes, Multipara3.—a. An abnormal separation of the chin from the chest, first degree of extension. b. Excessive amount of liquor amnii, and small volume of the foetus leading to exaggerated mobility of the head. c. Intra-uterine death of the foetus. (Asphyxia, recent death, maceration). d. Uterine obliquity. e. Presentation of the lateral plane of the foetus, and above all of the belly. .f. Rapid outflow of the liquor annul. g. Sudden changes of position in pregnant females, in oblique and ventral presentations.
Secondary Causes, Multip«rce.—Do not exist in primiparse. a. Ab normal contraction of the internal os, which opposes the engagement of the neck. b. Uterine or peri-uterine tumors. c. Placenta prsevia. d. Rigidity and tumefaction of one or both lips of the os. e. Obstacles formed by the lines innominata.
1st. The foetus descends perpendicularly, the foetal axis being parallel to the vertebral column; if, for a primary cause, there is a projecting occiput, this occiput will be arrested by the lines innominata, and it will ascend while the forehead descends. 2d. The foetus lies obliquely with reference to the superior strait, and then occurs a presentation of the fore head or of the face, the belly looking more or less directly downwards; then either the occiput will be stopped by the innominata ridge, or the forehead will be arrested. j. Procidentia of an arm, more rarely of a foot. g. Too large or too long iliac spines; the sciatic ligaments and tu berosities; small inclination of the pelvis; all these things may arrest the occiput. i. Exostoses or tumors of the pelvis. j. Persistence of the hymen. Atresia of the vulva ! ! Hemicephalia always causes face and not forehead presentations.
2. Jbrehead Foetal hydrothorax or ascites. 2 Exaggerated development of the thorax, or cedema of the thorax. 3. Hydrocephalus. 4. An abnormally short neck. 5. Tumors of the neck, congenital strums, bands of the cord around the neck or forehead. 6. A foetus before term, feeble, or macerated, the increased weight of the head not being compensated by muscular action. .
Secondary Causes.--1. Interference with descent of the occiput, the uterine pressure working upon the other arm of the lever. 2. Con
tractions of the pelvis; in some relatively frequent cases, resistance of the os.
We have seen that the presentation of the vertex results from the per fect accommodation of the fcetus to the uterine cavity, which does not take place until the sixth or seventh month. Indeed, until then, the amount of liquor amnii leaves the fcetus very mobile; so that the ftetus may find itself, as it were, surprised and fixed in a sitting position, with out being able to return to a vertex presentation, in consequence of uterine resistance or contractions. We may mention, among the things that increase f(rtal mobility and breech presentations, a special conforma tion of the uterus, multiparity, premature labor, smallness of the fcetus, death of the fcetus, contractions of the pelvis, tumors of the uterus, vicious insertion of the placenta, hydramnion, hydrocephalus, twin preg nancies, mobility of the limbs, and finally, premature rupture of the mem branes, which may render the presentation incomplete.
According to Depaul, the right shoulder presents a little oftener than the left; as 76:69. When the fetus presents by the trunk, the presenta tion is never exactly transverse; one end, usually the breech, is the more elevated. The child always lies more or less obliquely, and it is not, as Tarnier and Chantreull say, only during labor and under the influence of uterine contractions that trunk presentations occur. Transverse presen tations can happily be detected before labor sets in, thus enabling us to transform them into vertex cases (except where there are twins) by means of external version.
The causes of trunk presentations are many, and, besides the peculiar shape of the uterus to which Herrgott has called attention, multiparity, relaxation of the uterine and abdominal walls, uterine obliquity may be mentioned. We may also note pelvic contractions and fibromata, a vicious insertion of the placenta, small size of the foetus, death of the ketus, premature births, dropsy of the amnion, twin pregnancy—in a word, all the things that hinder a normal accommodation.