THE SENSORIAL FUNCTION. n 5 



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

 into it, and forming a joint with, the head of the lower jaw — it will be presently 

 described. 



Having reached the base of the temporal bone, it is found united to the parie- 

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

 nose (see fig. a and j, p. 110), 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 

 3 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 zygomatic arch. This extensive union between the 

 temporal and parietal bones resembles the buttress or mass of masonry attached 

 to the base of every arch, in order to counteract its lateral pressure. The con- 

 cussion, likewise, which might be communicated fly a blow on the top of the 

 arch, is thus spread over a large surface, and consequently weakened and ren- 

 dered comparatively harmless j 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. 



There is another proof of admirable design. Where is this squamous portion 

 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, n, o, p. 111). 

 If pressure is made on the crown of that arch — if a blow is received on the 

 suture between the parietals sufficient to cause the elastic materials of which 

 the skull is composed to yield — the seat of danger and injury is at the side. 

 If a man receives a violent blow on the crown or back part of the head, the 

 fracture, if there is any, is generally about the temple, and the extravasation of 

 blood is oftenest found there. The following figure will explain this : — 



Let the line ABO represent an elliptical 

 arch, composed of elastic materials. 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 represented at E. 

 In a dome the weight of the materials con- 

 stantly acting may be considered as repre- 

 senting the force applied at B ; and so great 

 is the lateral pressure, or tendency to bulge out (vide D and E), that it is 

 necessary either to dove-tail the materials into one another, or to pass strong 

 iron chains round them. For want of sufficient attention to this, " the dome 

 of St. Sophia, in Constantinople, built in the time of the Emperor Justinian, 

 fell three times during its erection ; and the dome of the cathedral of Florence 

 ^stood unfinished an hundred and twenty years, for want of an architect." 



Nature, in the construction of the horse's head, has taken away the pressure, 

 or removed the probability of injury, by giving an additional layer of bone, or 

 a mass of muscle, where alone there was danger, and has dove-tailed all the 

 materials. Farther than this, in order to make assurance doubly sure, she has 

 placed this effectual girder at the base, in the overlapping of the squamous 

 portion of the temporal bone. 



Above the parietals, and separated from them by a suture (fig. g, p. 110, 

 and fig. e, p. Ill), is the occipital bone. Superiorly it covers and protects 



i 2 



