1894.] NATURAL, SCIENCES OF PHILADEf.PHIA. 33 



The prevailing view among anatomists at the present day appears 

 to be that the petromastoid portion of the temporal bone is developed 

 in Man from three distinct centres of ossification, named respectively 

 prootic, opisthotic and epiotic; that the prootic and opisthotic coalesce 

 to form the pars petrosa, the epiotic giving rise to the specially mas- 

 toid part of the pars mastoidea, and later joining the other two. 



According to this view the homologues of these three centres of 

 ossification of the petromastoid of Man exist in the skull of the 

 lower vertebrata either as distinct bones or more or less coalesced with 

 each other or with adjacent bones. The bone formerly considered as 

 the homologue of the great wing of the sphenoid or the alisphenoid, 



Figs. 1, 2, a is regarded 

 as the prootic or the homo- 

 logue of the upper part of 

 the human pars petrosa. 

 The alisphenoid proper, t, is 

 often but little developed 

 or even absent in the skull 

 of the lower vertebrates ; 

 the bone formerly named 

 occipital externe or par- 

 FiG. 1. Side view of human skull. occipital is regarded as 



the homologue of the opisthotic or epiotic, while the latter is 

 often represented by a small ossification coalesced with the supra oc- 

 cipital. 



So far as known to the author Kerckringius is usually regarded as 

 ,^,=ss^ having been the first anatomist to describe 



the petromastoid portion of the temporal 

 1^ bone as developed from three centres of ossi- 

 fication. Unless the author has misunder- 

 stood the meaning of the text, however, that 

 which Kerckriuirius refers to as being devel- 

 oped from three centres of ossification is not 

 the whole petromastoid but only that part of 

 the pars petrosa which constitutes the mara- 

 millary process or pars mastoidea, the pars 

 petrosa proper having been more or less de- 

 FiG. 2. Interior view veloped before and independent of the three 

 of humau skull. centres. 



