Some difficulties may arise however with twins, which it is necessary to be prepared for. Thus the two heads may come together, and mutually impede each other. In this case the one which moves the easiest must be pushed up till the other is descended sufficiently low. One head may also descend with one or two feet; in which case, if the feet cannot be returned, the head must be pushed up, and they must be brought down. The force exerted however, must not be very great at first, because _one may belong to each of the children_, and much injury may be done; a little gentle traction will soon detect this however, with ordinary care. If two arms, or one arm and a foot descend, the same care is also required, before pulling upon them, to ascertain that they are not parts of the _two_ children. Sometimes when the head of one twin descends along with the feet of the other they may, if small, descend together. But if this is impossible, and interference is needed, we must first try to push up the head; and if this cannot be done, _it_ must be drawn upon, not the feet; because if the feet were drawn down the two children would soon occupy the passage together, body and head, and would perhaps become firmly wedged. In nearly every case one of the twins presents by the head and the other by the feet, as formerly shown. EXCESSIVE SIZE OF THE FŒTUS, OR THE DISEASED DEVELOPMENT OF CERTAIN PARTS. _Fœtus too large._--It is very rarely the case that the Fœtus is so large as not to pass easily through a well-formed Pelvis, though such cases have been known. The mode of proceeding is of course precisely the same as if the pelvis were too small. If no means will succeed in abstracting the Fœtus whole, it must be made less; but Nature should be first allowed full time to act with all her force. _Hydrocephalus._--This consists of an accumulation of water in the head of the child, and is usually termed watery head. The bones of the cranium will sometimes be widely separated by it, and the head be made so large that it cannot possibly be born till made less. The causes which produce this disease before birth are unknown. In cases of hydrocephalus the head does not descend into the straits, owing to its size, and is felt to be full and firm, during a pain, but soft and yielding during the intervals, especially at the fontanelles and sutures, which are also very large. The bones are usually very wide asunder, or even totally separated, as if floating in the fluid.