THE DEVELOPMENT OF THE HUMAN SKULL. 155 



Lastly, the third ossicle, " scutum ovale referens," is that 

 developed upon the posterior part of the posterior vertical 

 semicircular canal, which gives rise to the mastoid process. 

 (Fig. 61, B, Ep.O.) 



Thus, in a foetus between the fifth and sixth months, the 

 "pars mastoidea ' : exhibits the appearance represented in 

 Fig. 61, B. Its upper part is cartilaginous, but its lower part 

 is occupied by the three " ossicula ' of Kerckringius, which 

 have now come into contact, and begun to unite, though their 

 primitive contours are perfectly distinct. 



The "pars mastoidea" of human anatomy is therefore not 

 a single bone, but one, the " scutum ovale," combined with 

 parts of two others; and as the "scutum ovale" is certainly 

 the homologue of the bone I have termed Epiotic in the ovi- 

 parous Vertebrata* I propose to get rid of the confusing term 

 " mastoid " altogether, and to call the specially " mastoid " part 

 of the pars mastoidea, Epiotic. 



Of the three periotic bones thus developed, the pro-otic 

 gives rise to most of the pars petrosa, which is visible in the 

 interior of the skull (Fig. 59, A), investing, as it does, the 

 roof of the cochlea, the superior, and part of the posterior, 

 vertical semicircular canals, the internal auditory meatus, and 

 forming the tegmen tympani. To it, in addition, is due the 

 upper half of the circumference of the fenestra ovalis, and a con- 

 siderable portion of the pars mastoidea, as has been stated above. 



The opisthotic bone constitutes all the pars petrosa visible 

 on the base of the skull, furnishes the floor of the cochlea, 

 surrounds the fenestra rotunda, and contributes half the contour 

 of the fenestra ovalis ; gives rise to the carotid canal by develop- 

 ing a lamella of bone, which gradually wraps itself round the 

 carotid, and so converts the primitive groove for the vessel 

 into a complete tube, at the same time furnishing the inner 

 part of its floor to the tympanum. 



The lower edge of the squamosal is at first nearly straight, 



* Croonian Lecture. Proceedings of the Royal Society, 1858. In the absence of 

 a sufficient knowledge of the development of the human temporal bone, I followed 

 Hallmann in identifying the opisthotic of oviparous vertebrates with the mastoid 

 of Mammals at the lime this Lecture was delivered. 



