76 THE SENSO RIAL FUNCTION. 



than this, in order to make assurance doubly sure, she has placed this jffectual 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. 70, and fig. &, 

 p. 72), is the occipital bone. Superiorly it covers and protects the smaller portion 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 thickness it assumes. The head of the horse does not, like that of t. 3 

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

 column, and 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 projects a considerable distance from the 

 chest, and thus the whole wsight of the head and neck is suspended from the chest, 

 and require very great power in order to support them. In addition to the simple 

 weight of the head and neck, the latter projecting from the chest, and the head hanging 

 from the extremity of the neck, act with enormous mechanical force, and increase more 

 than a hundred-fold 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 will hereafter be shown in what breeds and for 

 what purposes a light or heavy head and neck are advantageous ; but it may be safely 

 affirmed, that, projecting so far from the chest, and being consequently^ so great a 

 distance from the fulcrum or support, the lightest head will act or bear upon the joint 

 between the last bone of the neck and the first rib with a force equal to many thousand 

 pounds. 



How is this weight to be supported ? Is muscular power equal to the task 1 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 pro- 

 vision, however, is made for the purpose, simple and complete. 



From the back of the occipital bone (fig./, p. 72), and immediately below the crest, 

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

 which reaches down and is securely attached to the spines of the vertebrae, or bonea 

 of the back ; and by this ligament the ligamentum colli, ligament of the neck, com- 

 monly called the pack-wax the head is supported. 



There are, however, some admirable contrivances connected with this ligament. 

 As it proceeds from the head, it is in the form of a round cord. It passes over the 

 atlas, or first bone of the neck, without touching it, and then, attaching itself stronglj 

 to the second bone, principally supports the head by its union with this bone. The 

 mechanical disadvantage is increased ; but the head is turned more freely on the first 

 and second bones. The principal stress is on the denlata or second bone, so much so, 

 that, in poll-evil, this ligament may be divided without serious inconvenience to the 

 horse. It then suddenly sinks deeper, and communicates with all the ether vertebrae. 

 Each cf these communications becomes a separate point of support, and as they 

 approach nearer to the base, the mechanical disadvantage, or the force with which the 

 weight of the head and neck presses and acts, is materially lessened. 



The head, then, while the animal is in a state of rest, is supported by this ligament, 

 without any aid from muscular energy. 



There is, however, something yet wanting. The head must not be always elevated. 

 The animal has his food to seek. In a state of nature this food lies principally 01: 

 the ground, and the head must be lowered to enable the horse to get at it. How is 

 this effected 1 This ligament, as it has been called, because it resembles in appear- 

 ance the other ligaments of the body, possesses a property which they have not, and 

 which they must not have, or they would be useless. No well-knit joint could exist 

 if it had tins property. It is elastic. It will yield to a force impressed upon it, and 

 will resume its natural dimensions when that force is removed. It sustains perfectly 

 Jie weight of the head. That portion of tenacity or strength is given to it which will 

 not give way to the simple weight of the head, but which will yield to a very littlu 

 additional weight. Its resisting power is so admirably adjusted to that which it has 

 to suntain, that when certain muscles, whose action is to depress or lower the head, 

 negin to act, and add their power to the previous weight it had *o beaj, th Hgamenl 



