Fig. 6.
The Back-part of the Skeleton of a Foetus of about Four Months after Conception.
In the Embryo, or First Rudiments of the Foe∣tus within the Womb; those Parts which after∣wards become Bones, are than Intirely Cartilagi∣nous, till about the End of the First Month after Impregnation; at which Time divers Bony Specks or Ossifications begin to Appear about the Middle of the Larger Bones of the Limbs, especially in the Claviculae. Two Months after Conception, the Whole Head do's not afford any Bony Appear∣ance, except the Third Pair of Bones of the Upper Jaw, and the Two Bones which Frame the Lower Jaw, which about this Time Appear Distinct. The Middle-parts of the Claviculae are Bony after the First Six Weeks. The Shoulder-blades are with∣out any Proper Figure about the Second Month after Conception, at which Time their Ossifica∣tions begin in their Middle-parts: About the Third Month their Spinal Process's begin to be Bony, as well as their Coracoidal and Short Pro∣cess's: Near this Time the Whole Spine, or Ver∣tebrae of the Neck, Back, Loins, Os Sacrum and Coccygis, begin to be Bony. The Sixth Vertebra of the Back Internally, some Bony Specks in each Vertebra Appear, and Gradually Lessen them∣selves to the Fifth Vertebra of the Neck; the like Ossifications may be Observ'd to become Less and Less in each Vertebra, towards the Lower-part of the Spine, to the Third Vertebra of the Os Sa∣crum. The Incurvation or Bending Forwards of the Whole Spine at this Time, is Remarkable. The Four Upper Vertebrae of the Neck, afford some Bony Appearances Laterally, about the Third Month; near which Time the Bodies of the Three Upper Vertebrae of the Os Sacrum, seem to Frame One Bone, but its Lateral Parts are not Bony till the Fourth Month; at which Time the Bodies of the Third and Fourth Vertebrae of the Neck begin to Appear. The Fifth and Sixth Month, the Epistropheus or Second Vertebra of the Neck has a Bony Speck; but its Tooth-like Process is yet Cartilaginous: The Fore-part of the Atlas or First Vertebra is yet wanting. The Os Coccygis is Cartilaginous till about the Eighth or Ninth Month; at which Time, in its Internal Part, call'd its Body, Two Offeous Specks Appear about the Bigness of Two great Pin's Heads. All the Vertebrae of the Spine of the Foetus (at this Time) Appear Compos'd of Three Bones; First that of the Bodies of the Vertebrae Forwards; Secondly its Two Lateral Parts which Frame their Transverse Process's: Their Spinal Process's not Appearing Bony till some Time after the Par∣tus; whence (as Spigelius Observes) Rope-Dan∣cers, Tumblers, &c. by early Practice whilst they are Children, the Spines of the Back-bone give way to the Inflection of their Vertebrae Back∣wards; the common Position of the Spines being Obliquely Descending, they do thereby incline more Horizontal, and their Points are also ren∣dred more Obtuse.
The Os Innominatum about the Second Month after Conception is Cartilaginous, except that Part of it call'd Ilium, where it Frames the Upper-part of the Acetabulum, it has a Bony Speck about the Bigness of a Common Pin's Head. In the Fourth Month the Os Coxendicis or Ischium, (another Part of the Os Innominatum) has a Bony Appear∣ance, where it meets the Ilium within the Aceta∣bulum, not exceeding the Head of a common Pin in Magnitude; the like may be Observ'd of the Os Pubis within the Acetabulum: These Three Bones which Compose the Innominatum, remain Distinguish'd in the Foetus by a Cartilaginous In∣terposition, which continues till the Seventh Year, at which Age those Cartilaginous Marks Disappear. In the Second Month all the Ribs except the First and Last, are so harden'd, that the Channels (Exprest Fig. 4. Tab. 94. E,) or Si∣nus's for the Intercostal Blood-Vessels and Nerves, Appear.
The Time of the Ossification of the Sternum is uncertain; but Eustachius is mistaken in saying, It's altogether Cartilaginous in Children Newly Born. Kerckringius affirms he never Diffected a Foetus of Four Months, but he found some Little Bony-Bodies in the Sternum. Their Number and Figure Varying in most Subjects, we need not say more of them in this place. The Ossification of the Artus is very Early, as Appears by the First and Second Figures of this Table, where the Mid∣dle-parts of the Bones First Appear in little White Specks a Month after Conception, as above No∣ted; but some of their Appendages are Intirely Cartilaginous for some Months after the Birth. The Eight Cartilages of the Carpus become Bony some Time after the Birth. The Appendages of the Bones of the Metacarpus and Fingers, conti∣nue Intirely Cartilaginous some Months after the Birth; the like may be Observ'd of the Feet and Toes; the Patellae in like Manner are Intirely Cartilaginous some Months after the Birth.