If the fœtus is not at full term, and of course is under the full size, its expulsion may be left to nature safely, but not otherwise. SECTION V. THE PHYSIOLOGY OF SPONTANEOUS DELIVERY, OR CHILDBIRTH, AND THE MANNER OF CONDUCTING A NATURAL LABOR. Having now completed the description of the _Mechanism of Delivery_, in all the various presentations and positions, it is necessary to explain the physiological phenomena attending a natural labor, and the duties of the accoucheur when conducting it, and to show what assistance he can render, and when he should or should not interfere. CHAPTER XV. OF DELIVERY IN GENERAL. DIFFERENT KINDS OF DELIVERY. When the child is brought into the world by the unaided efforts of nature, and without any accident to itself or the mother, it is called a _Natural Delivery_. When it occurs by the efforts of nature alone, but not advantageously for both, it is not called natural, but simply _Spontaneous Delivery_. And when assistance is required it is called an _Artificial_, or _difficult Delivery_. It is also called _precocious_, or _tardy_, according as it comes before or after the full term. CAUSES OF LABOR. What it is that causes labor to commence, and proceed, is not fully known. At the proper time the Uterus prepares to cast out the fœtus it has so long retained, in the same manner that the tree casts off its fruit, and from some efficient cause which we have not yet discovered. It is probable that, when the fœtus attains a certain size, it presses upon the nerves of the neck of the Uterus and irritates them, and they react again upon the muscular fibres of the Womb and cause them to contract, and so expel its contents. This is much the same action, in fact, as vomiting. When any body very repugnant to the stomach is swallowed, it irritates the nerves of that organ, and then they excite its muscular fibres, which, by forcible contractions, expel the offender. It is possible, also, that the fœtus itself may instinctively assist in bringing about its own delivery, as was supposed of old by Hippocrates, and more lately by Harvey and others. It is certain that labor is both more difficult, and more dangerous, when the child is dead; though it may take place as usual after the death of the mother, providing the child be still alive. Several instances of this kind have been known, when the living child was expelled from the Womb, by the natural process, sometime after the mother had ceased to breathe. The contraction of the muscular fibres of the Womb however, must be regarded as the immediate or efficient cause of fœtal expulsion, let them be brought on how they may. The muscles of the Abdomen, and the diaphragm, also assist, in the last stage, but are not essential.