98 



MAMMALIAN ANATOMY 



FIG. 400. 



THE SKULL, AT BIRTH. 

 (Natural size.) 



In the skull at birth (Fig. 400) the supraoccipital shows a faint 

 lambdoidal crest ; the paroccipital processes begin to appear. The 



mastoid portion of the petro-niastoid of the 

 temporal is visible in the lower part of the 

 posterior lateral fontanelle as a club-shaped 

 rod of bone extending upward and back- 

 ward. The ectotympanic ring is stouter ; 

 f m the entotympanic sac has become cartilagi- 

 nous. Viewed from the cranial cavity, the 

 petrous is seen to have ossified behind the 

 appendicular fossa, around the superior semi- 

 circular canal and a small part of the poste- 

 rior semicircular canal. The tentorial process 

 of the parietal has grown wider. 



On the under surface of the sphenoid 

 there are still distinct traces of the pterygo- 

 alisphenoidal sutures. The fontanelle at the 

 pterion and the strip of membrane on the 



inner wall of the orbit are well developed. The anterior ossification 

 of the lachrymal has increased to a narrow crescentic band. 



The ossifications in the lateral ethmoid at the end of the fourth 

 piece, in the terminal medial part of scrolls 4 and 4' (Fig. 219), have 

 extended laterally and upward along the posterior boundary of the 

 ethmoidal cleft, and also slightly upward into the anterior ends of 

 the first and second pieces. The ossification in the sixth piece has 

 increased in size. A new separate and distinct centre has appeared 

 on the medial side of the seventh piece. The anterior end of the 

 mesial piece, including part of the cleft plate, appears to contain 

 some osseous elements. The fifth piece is entirely cartilaginous. 



The skull at seven days (Fig. 401) does not differ decidedly from 

 the skull at birth. 



The mastoid has extended further upward into the fontanelle. 

 The squamous, petro-mastoid, and tympanic are still separate elements ; 

 the ectotympauic ring has grown thicker, the endotympauic sac is 

 cartilaginous. The ossification of the petro-mastoid has extended 

 further behind the appendicular fossa, which is very large and widely 

 open below ; the semicircular canals are very plainly outlined, and 

 almost completely surrounded by bone. 



