116 THE SENSORIAL FUNCTION. 



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 the 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 hori- 

 zontally on its pivot _: but it hangs in a slanting position from the extremity oi' 

 the neck, and the neck itself projects a considerable distance from the chest, 

 and thus the whole weight of the head and neck are 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 conse- 

 quently at 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 ? 

 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 com- 

 plete. 



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

 the crest, proceeds a round cord of considerable bulk, and composed of a liga- 

 mentous substance, which reaches down and is securely attached to the spines 

 of the vertebra, or bones of the back ; and by this ligament — the ligamentum 

 colli, ligament of the neck, commonly called the pack-wax — the head is 

 supported. 



There are, however, some admirable contrivances connected with this liga- 

 ment. 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 strongly 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 dentata 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 other vertebra?. Each of 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 on the ground, and the head must be lowered to enable the horse 

 to get at it. How is this effected ? This ligament, as it has been called, 

 because it resembles in appearance 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 this property. It is elastic. 

 It will yield to a force impressed upon it and will resume its natural dimensions 



