20 ANIMAL MECHANICS 



upper part, or clerestory. From the upright part 

 of this masonry a pinnacle is raised, which at first 

 appears to be a mere ornament, but which is 

 necessary, by its perpendicular weight, to coun- 

 teract the horizontal thrust of the arch. 



By all this we see, that if the skull is to be 

 considered as an arch, and the parietal bones as 

 forming that arch, they must be secured at the 

 temporal and sphenoid 1 bones, the points from 

 which they spring. And, in point of fact, where 

 is it that the skull yields when a man falls, so as 

 to strike the top of his head upon the ground? 

 in the temples. And yet the joinings are so 

 secure that the extremity of the bone does not 

 start from its connections. It must be fractured 

 before it is spurred out, and in that case only does 

 the upper part of the arch yield. 



But the best illustration of the form of the head 

 is the dome. 



A dome is a vault rising from a circular or 

 elliptical base ; and the human skull is, in fact, 

 an elliptical surmounted dome, which latter term 

 means that the dome is higher than the radius 

 of its base. Taking this matter historically, we 

 should presume that the dome was the most diffi- 



1 In the Greek, sphenoid, in the Latin, cuneiform, like a 

 wedge, because it is wedged among the other bones of the head ; 

 but these processes, called wedges, are more like dovetails, which 

 enter into the irregularities of the bones, and hold them locked. 



