THE SKULL THE CEANIUM 287 



VARIATIONS IN FORM AND DEVELOPMENT. 



The frontal presents variations in the number and distribution of 

 its foramina. In about one-fourth of the specimens examined the 

 supraorbital ridge was pierced near the middle by a small opening 

 into the cancellous tissue. A distinct suborbital foramen just under 

 the orbital arch was present in two-thirds of the specimens ; rarely this 

 foramen was double. The ethmoidal foramina near the lower border 

 of the vertical plate were usually present as a large notch and just 

 in front of it a small foramen which emerged on the nasal surface ; in 

 some specimens, however, there were two notches, in others two fora- 

 mina, in others a single notch or a single foramen, and in many no 

 distinct notches or foramina were visible. The postorbital foramen 

 was absent in about one-third of the specimens ; in a very few instances 

 it pierced the superior surface of the process. 



The presence of a temporal line was observed in two-thirds of the 

 bones ; it was rarely entirely absent, but was usually obscurely defined. 

 In many cases, probably about a third of all studied, the bevelling on 

 the superior surface for the parietal was extended in a point to the root 

 of the postorbital process. The size of the postorbital process varies 

 greatly : in some specimens it is long and either massive or very 

 slender, in others it is reduced to a mere tubercle. 



The nasal surface of the vertical plate presents such changes as are 

 dependent upon the variations in the lateral ethmoid, to which it fur- 

 nishes lines of attachment. In the majority of the specimens studied 

 there were six oblique ridges for the ethmoidal partitions ; in a few 

 this number was increased to seven or even eight, showing an increased 

 complexity in the ethmoidal scrolls. In only one case was the number 

 reduced to five. 



In three left frontals out of one hundred and fifty of both sides, 

 the frontal sinus was entirely absent, and its place was occupied by 

 firm cancellous tissue. 



HUMAN FRONTAL BONE. 



When a child is about six years old the frontal bones begin to 

 unite along the metopic suture ; the adult skull therefore usually pre- 

 sents but a single bilaterally symmetrical frontal bone. In comparing 

 it with the frontal of the cat, of course we need only to study one-half 

 of it. 



