Generally there are reckoned _ten_ diameters, and they are represented in the following Plate:-- [Illustration: PLATE XXV. _Diameters of the Head._ 1. The _Occipito Mental_, or from the back of the head to the chin, M, O, Fig. 1, five inches. 2. The _Mento Bregmatic_, or from the chin to the anterior fontanelle at the top of the head, M, O, Fig. 1, four inches. 3. The _Occipito Frontal_, or from the back of the head to the top of the forehead, O, F, Fig. 1, four inches. 4. The _Trachelo Occipital_, or from the throat to the back of the head, T, O, Fig. 1, four inches. 5. The _Sub Occipito Bregmatic_, or from the nape of the neck to the top of the head, S, B, Fig. 1, three inches and a half. 6. The _Trachelo Bregmatic_, or from the top of the throat to the top of the head, T, B, Fig. 1, about three inches and a half. 7. The _Trachelo Frontal_, or from the top of the throat to the top of the forehead, T, F, Fig. 1, about three inches. 8. The _Sub Occipito Frontal_, or from the nape of the neck to the top of the forehead, S, F, Fig. 1, about three inches. 9. The _Bi Parietal_, or across the head from one side to the other at the middle of the parietal bones, the widest part, B, P, Fig. 2, three inches and a quarter to three and a half. 10. The _Bi Temporal_, or across the head from one temporal bone to the other, B, T, Fig. 2, about two inches and a half.] The necessity for knowing these diameters will be obvious. It is only by being acquainted with them, and with the straits of the Pelvis, already described, that we can tell whether the child can pass or not in certain positions, and how we must change its position, when possible, to give relief. On comparing the diameters of the head with those of the Pelvis, it will be seen that some of them correspond in size while others do not, so that in one position delivery can take place spontaneously, while in another it will be difficult, and in some nearly impossible, without assistance. Thus, for instance, if the head should present by the occipito mental diameter, (O, M, Fig. 1,) it evidently could not pass while in that position, because by this diameter it is five inches in width, and the greatest diameter of the Pelvis is only about four inches and a half. Its position must therefore be changed, and the accoucheur must know how to change it with advantage. ATTITUDE OF THE FŒTUS AT FULL TERM. The attitude of the Fœtus is represented in the following Plate:-- [Illustration: PLATE XXVI. _Attitude of the Fœtus._ The arms, it will be seen, are crossed on the breast, upon which the chin is also bent; the thighs are close together, and brought against the Abdomen; the legs are close bent on the Thighs, and the feet are turned up against the front of the legs, the whole body being curved forward.]