IX.] THE OSSIFICATIONS OF THE EAR. 237 



becomes part of the common vestibule in which the Eusta- 

 chian tubes meet in the middle line. 



One new splint bone, the vomer, begins to be formed 

 during this stage in the middle line about half way along 

 the palatines. 



The various membrane-bones of the skull are now much 

 more firm and consistent, and the frontals have sent down a 

 process which upon reaching the ethmoidal plate at once 

 sets up ossification in it. 



20. In the next stage, about the second day after birth, 

 a considerable number of changes have taken place. The 

 ethmoid has begun to be ossified by two ectosteal plates, one 

 on each side, and the ossification set up by the frontal process 

 in the ethmoid plate has increased. The ethmoid is now only 

 connected with the septum nasi by a narrow isthmus above. 



The septum still retains its anterior process, the pre- 

 nasal cartilage, but this is rapidly diminishing. 



In the ear there are now three ectosteal ossifications, the 

 prootic, which appeared in the last stage, and which is by far 

 the largest of the three ; the opisthotic, which lies between 

 the prootic and the exoccipital, but is distinct from both ; and 

 a third small ectosteal plate, the pterotic. 



A small ring-like ossification appears in the internal 

 angular process of Meckel's cartilage; its other process 

 rapidly diminishes and soon disappears. 



The supraoccipital ossifications have united in the 

 middle line ; the exoccipitals have increased very much, but 

 are quite separate from the opisthotics. 



The basioccipital ossification has begun to spread into 

 the condyles. 



21. The chief facts of importance in reference to the 

 splint bones, are, that the premaxillaries (Fig. 71, px) have 

 united in the middle line ; the vomer has considerably 

 increased in size; the lacrymals have developed, and 

 possess a large supraorbital and a thick anteorbital plate. 

 There is still a large fontanelle above, between the frontals 

 and parietals. The squamosals are very large. In the man- 

 dible all the splint bones have by this time become developed. 



At about the third week after birth two new centres 

 appear, one at the outside of the alisphenoid, forming the 

 centre for the post-frontal, and another over the posterior 



