THE OS COCCYGIS is also in _Adults_, for the most part, but one entire _BONE_; tho’ in _younger Persons_ it is compos’d of 3 or 4 small _Divisions_; Of which the _Lower_ is still less than the _Upper_; till the _Last_ ends in a small _Cartilage_. It is join’d in its _Glenoide Cavity_ to the Extremity of the OS SACRUM; being short and bent _inwards_: It supports the INTESTINUM RECTUM, and yields to the _Pressure_ of the _INFANT_ in _Travail_: But _MIDWIVES_ ought not to thrust it back or repel it with _Violence_; No, they should rather handle it _gently_, if they would prevent dangerous _Consequences_, as well as great _Pain_ to the _Woman_ in _LABOUR_. FROM hence it is manifest, that _they_ are mistaken who imagine that the _Opening_ and _Enlargement_ of the _PELVIS_, in making way for the _INFANT_, does depend upon the _Separation_ of the OSSA PUBIS: For it much more depends upon the _yielding_ of the OS SACRUM, or its giving way naturally; especially _This Part_ of it call’d OS COCCYGIS. Neither doth the _Straitness_ of the _Upper Part_ of the _PELVIS_ so much occasion a difficult _BIRTH_, as the _small Distance_ that is betwixt the _Points_ of the OSSA PUBIS, call’d OSSA SEDENTARIA or _Seat-Bones_, and the OS COCCYGIS: No indeed, neither of _These_ can be any great _Hindrance_ to the Passage of the INFANT; since all BONES, never so closely knit together with LIGAMENTS, may be moved _extensively_ upon occasion, by carefully and gently _stretching_ the said LIGAMENTS. But, in short, it most commonly happens, that the _Ill Position_ of the INFANT itself, or the bad _Condition_ and _Situation_ of the WOMB, or _Both_, occasion a difficult or preternatural BIRTH. BUT I would here farther observe yet, that as these BONES differ frequently both in _Form_ and _Size_, according to the different _Constitution_ of the Body; so neither are _they_ always of the _same Substance_: For in _some Women_, we find a great many _Nervous_ and _Cartilaginous_ LIGAMENTS, which penetrate into the _solid Substance_ of the BONES themselves; in which the LIGAMENTS are so fast bound together, that it is hard to distinguish whether they are _One_ or _More_ BONES. From whence, however, it will hereafter appear, that _One Woman_ is more easily delivered than _Another_; the BONES in _One_ being more firm and immoveable, altogether resisting any _Relaxation_; which in _Another_ are more loose and pliable, easily give way and yield freely to the _Force_ of the Endeavouring and Struggling _INFANT_.