THE SENSORIAL FUN CTION. -5 



one on each side,//. These again are divided into two parts, or consist of two dis- 

 tinct bones ; the petrous portion, so called from its great or stony hardness, and con- 

 taining the wonderful mechanism of the ear, and the sjurtwiOT/s portion from the appear- 

 ance of its union with the parietal, overlapping it like a great scale. 



From the latter there projects a portion of bone, e, which unites with the frontal, 

 and forms a strong arch — the zyg jmatic — distinctly to be felt at the side of the head 

 immediately above the eye. This arch is designed to protect the upper part of the 

 lower jaw, the motion of which may very plainly be seen beneath it when the horse 

 is feeding. It is very strong, and it ought to be, for if it were depressed or forced 

 inward, the horse would starve. There is one species of violence which causes this 

 arch to require no common strength ; and that is, the brutal manner in which the 

 collar is often forced over the head. 



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 parietal, 

 not by a simple suture, as the lower part of the frontals, or the bones of the nose (see 

 fig. a and /, p. 70), 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 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 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 ren- 

 dered comparatively harmless ; and that surface is composed of the union of two 

 bones of dissimilar construction. The hard stoity structure of the parietal is very dif- 

 ferent 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 1 It is an elliptical or oval arch (see fig. ot, n, 0, p. 72). If pressure is made 

 on the crown of that arch — if a blow is received on the suture between the parietals 

 (sufllicient 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 figiire 

 will explain this : — 



Let the line ABC represent an elliptical arch, 

 composed of elistic 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 constantly acting may be considered 

 as representing the force applied at B ; and so 

 great is the lateral jtressure, 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 durino; 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 



