EXTERNAL STRUCTURE. VS 



From the back of the occipital bone (fig. f, p. 68), and inimodiately 

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

 of a ligamentous substance, and which is carried down and securely at- 

 taciied to the spines of the vertebrae, or bones of the back ; and by this lig- 

 ament (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 

 ligament. As it proceeds from the head, it is in the form of a round cord. 

 It passes over the atlas, or the lirst 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 in- 

 creased, 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 

 vertebrre. Each of these commiinications becomes a separate point of 

 support, as they approach nearer to the prop, or the centre of motion, the 

 mechanical disadvantage, or the force with which the weight of the head 

 and neck presses and acts, is materially lessened. 



The head, then, without any aid from muscular energy, is, while tho 

 animal is in a state of rest, supported by this ligament. 



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

 always elevated. The horse 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 animal to get at it. How is this effected ? This ligament, as \\e call 

 it, because it resembles in appearance the other ligaments of the body, pos- 

 sesses 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 pro- 

 perty. The pack-wax is elastic. It will yield to a force impressed upon 

 it, and it will resume its natural dimensions when that force is removed, 

 It sustains perfectly the weight of the head. That portion of tenacity or 

 strength is given to it, which will not yield to the simple weight of the 

 head, but will yield to a very little additional weight. Its resting power ia 

 so admiraljjy adjusted to that which it has to sustain, that when certain 

 muscles, whose action is to depressor lower the head, begin to act, and add 

 tiieir power to the previous weight it had to bear, the ligament stretches, 

 antl when the horse is browsing, it is full two inches longer than when the 

 head is erect. 



When the animal has satisfied himself, these depressing muscles cease to 

 act, and other muscles, which are designed to assist in raising the head, 

 begin to exert themselves; and, by their aid (but more by the inherent 

 ela (icity of the ligament), the head is more elevated, and remains so with- 

 out the slightest exertion of muscular power. This is one of the many 

 applications of the principle of elasticity which we shall have to observe 

 end admire in the construction of the animal frame. 



The ligament of the neck is inserted into the centre of the back part of 

 the occipital bone, and immediately below the vertex or crest of that bone, 

 and, therefore, the bone is so thick at this part (see fig. e, p. 68). 



Many, and large, and powerful muscles, however, are necessary to turn 

 the head in various directions, and to assist in raising it when depressed. 

 The occipital bone, as will be seen in this cut, presents a spine running 

 down the centre, B, and a large roughened surface for the attachment of 

 tl\ese muscles C C. 



liovver down, and still at the back of the occipital bone, are two roundeo 

 F 



