EXTERNAL STRUCTURE. 71 



motion should by accident be impeded, and the horse should starve. If 

 only one species of violence were considered to which this arch is too 

 frequently exposed, it would require to possess no common strength ; we 

 mean, the brutal manner in which the collar is forced over the head. At 

 the base of the arch is an important cavity not visible in our cut, receiving 

 into it, and forming a joint with the head of the lower jaw — we shall have 

 to speak of it presently. 



Having reached the base of the temporal bone, we find it united to the 

 parietal, not by a simple suture, as the lower part of the frontals, or the 

 bones of the nose (see fig. a and j, p. ^^^^ nor by a dove-tailed suture, 

 as the upper part of the frontals (see the same cut), but it is spread over 

 the parietal in the form of a large scale, and hence, as before observed, 

 called the squamous portion of the temporal bone. In fact, there are two 

 plates of bone instead of one. Was there design in this ? Yes, evidently 

 so. In the first place, to increase the strength of the base of the zygo- 

 matic arch. This extensive union between the temporal and parietal bones 

 answers to the buttress or mass of masonry attached to the base of every 

 arch to counteract its lateral pressure. The concussion likewise which 

 might be communicated by a blow on the top of the arch, is thus spread 

 over a large surface, and consequently weakened and rendered compara- 

 tively harmless ; and that surface is composed of the union of two bones 

 of dissimilar construction. The hard stony structure of the parietal is very 

 different from the tougher material of the temporal ; and thus, as a finger 

 acts on a sounding glass, the vibration communicated to the temporal is at 

 once stopped, and the brain receives no injury. 



But there is yet more admirable design. Where is this squamous por- 

 tion of the temporal bone situated ? On the side of the head. And what 

 is the figure of the cranium or skull, and principally that part of it which 

 contains the cerebrum or brain ? It is an elliptical or oval arch (see fig. m, 

 7iy 0, p. 68). If pressure be made on the crown of that arch — if a blow be 

 received on the suture between the parietals sufficient to cause the elastic 

 materials of which the skul] is composed to yield — the seat of danger and 

 injury is at the side. If a man receive a violent blow on the crown or 

 back part of the head, the fracture, if there be any, is generally about the 

 temple, and the extravasation of blood is oftenest found there. The fol- 

 lowing figure will explain this : 



Let the line ABC represent an elliptical arch, composed of elastic ma- 

 terials. Some force shall be applied at B sufficient to cause it to yield. 

 We cannot compress it into smaller compass, but just in proportion as it 

 yields at B, will it spur or bulge out at D. and give way sometimes as re- 

 presented at E. In a dome the weight of the materials constantly acting may 

 be considered as representing the force applied at B ; and so great is the 

 lateral pressure, or tendency to bulge out {vide D and E), that it isjieces- 



