204 THE FCETUS. 



with the vitiated diameters of the pelvis through which the child has 

 to pass, we can ascertain whether delivery will be practicable or not. 



523. Varieties. It is not necessary for me to observe that these 

 measurements should be taken merely as mean terras, and that there 

 is no more uniformity in the size of the foetal head than in the other 

 parts of the body; but I ought not to fail to remark, that they are 

 susceptible of various degrees of reduction or elongation, whether 

 by means merely of the uterine contractions, or by the mechanical 

 action of the instruments that are sometimes employed in midwifery. 

 Thus the occipito-frontal diameter, when compressed at its extre- 

 mities, may be shortened several lines,' by the over-riding of the 

 corresponding edges of the frontal, parietal, and occipital bones. 

 The same is true of the transverse and occipito-bregmatic axes, 

 whenever the pressure bears chiefly on the two opposite points of 

 their circumference; but in order to form a clear idea of the changes 

 that the head may undergo in this regard, it is indispensably neces- 

 sary to obtain correct notions as to the arrangement of the bones of 

 the head at birth. 



524. Bones of the skull. In the foetus at term, the eight bones 

 of the skull, the frontal, occipital, the t7X)0 parietal, the tivo tem- 

 poral, the sphenoid, and ethmoid bones are far from possessing the 

 same firmness as those of the adult; those of the vault are still quite 

 flexible, and separated from each other by membranous spaces of 

 greater or less size; the frontal is formed of two symmetrical pieces; 

 the thin or flattened portion of the occipital and the squamous portion 

 of the temporal are pretty frequendy separate from the petrous and 

 condyloid portions respectively; the basilar apophysis, the body of 

 the sphenoid; the petrous portion of the temporal, and the several 

 pieces which constitute the base of the cranium, are on the contrary 

 almost completely ossified, or at least form with the cartilages which 

 unite them an incompressible mass. 



525. From the above arrangement it follows, 1. That the diame- 

 ters of the vault of the cranium alone are reducible in labor; 2. 

 That whenever the diameters of the pelvis are smaller than those of 

 the base of the foetal skull, delivery is physically impossible without 

 the aid of art; 3. That in this way the medulla oblongata, the 

 pons varolii, the tubercula quadrigemina, and the peduncles of the 

 cerebrum and cerebellum, are completely protected, while the cere- 

 bral and cerebellar lobes, which are almost unconnected with the 

 maintenance of the vegetative life of the foetus, are alone liable to be 

 slightly compressed. 



The numberless varieties of which the dimensions of the head are 

 susceptible has long since convinced accoucheurs of the necessity 



