J 48 THE SENSORIAL FUNCTIOX. 



Let the line ABC I'epresent an elliptical arcli, composed of elastic mate- 

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



cannot compress it into smaller compass ; 

 but just in propoi-tion as it yields at B T\-ill 

 it spur or bulge out at D, and give way 

 sometimes as represented at E. In a dome 

 the weight of the materials constantly act- 

 ing may be considered as representing the 

 force applied at B ; and so great is the la- 

 teral 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 thi'ee times during its erection ; and the dome 

 of the cathedral of Florence stood unfinished a 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. 145), 

 is the occipital bone. Superioziy it covers and protects the smaller por- 

 tion of the brain, the cerebellum ; and as it there constitutes the 

 summit or crest of the head, and is particularly exposed to danger, and 

 not protected by muscles, it is interesting to see what tliickness it 

 assumes. The head of the horse does not, like that of the human being, 

 ride upright on the neck, with all its weight supported on the spinal 

 column, the only office of the muscles of the neck being to move the 

 head forward, or backward, or horizontally on its pivot ; but it hangs in a 

 slanting position from the extremity of the neck, and the neck itself pro- 

 jects a considerable distance from the chest, and thus the whole weight of 

 the head and neck are suspended from the chest, and requii-e very great 

 power in order to support them. In addition to the simple weight of the 

 head and neck, the latter projecting fi^om the chest, and the head hanging 

 from the extremity of the neck, act A^dth enormous mechanical force, 

 and increase more than a hundredfold the power necessary to support 

 them. 



The head and neck of the horse, and particularly of some horses of a 

 coarse breed, are of no little bulk and weight. It Avill hereafter be showia 

 in what breeds and for what pui'poses a hght or heavy head and neck are 

 advantageous ; but it may be safely affirmed that, projecting so far from 

 the chest, and being consequently at so great a distance from the fulcrum 

 or support, the h'ghtest head will act or bear upon the joint between the 

 last bone of \he neck and the first rib with a foi'ce equal to many thousand 

 pounds. 



How is this weight to be sujoported ? Is muscular power equal to the 

 task ? The muscles of the animal frame can act for a certain time with 

 extraordinary force ; but as the exertion of this power is attended with 

 the consumption of vital energy, the period soon arrives when their action 

 is remitted or altogether suspended. A provision, however, is made for 

 the purpose, simple and complete. 



From the back of the occipital bone, and inunediately below the crest, 

 proceeds a round cord of considerable bulk, and composed of a ligamentous 



