THE SEXSOKIAL FUXCTIOX. 14^ 



substance, wliicli reaches do^vn and is securelj attached to the spines of 

 the vertebrae, or bones of the back ; and by this ligament— the Una- 

 nientmn colli, Hgament of the neck, commonly called the vaclcwax—ihe 

 head is supported. 



There are, however, some admirable contrivances connected ^^th the 

 arrangements of the ligamentmn colli. As it proceeds from the head it is in 

 the form of a round cord. It is connected with the atlas, or first bone of the 

 neck, and then, attaching itself strongly to the second bone, principally 

 supports the head by its union mth this bone. The mechanical disadvan- 

 tage IS mcreased ; but the head is turned more freely on the first and second 

 bones. The prmcipal stress is on the dentafa, or second bone, so much so 

 that, m poll-evil, this hgament may be divided without serious inconvc-' 

 nience to the horse. It then suddenly sinks deeper, and communicatps 

 with all the other vertebrae. Each of these communications becomes a 

 separate point of support, and as they approach nearer to the base the 

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

 and neck presses and acts, is materially lessened. 



The head, then, whde the animal is in a state of rest, is supported by 

 tins hgament, mthout any aid from muscular energy. 



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

 always elevated. The anhnal has his food to seek. In a state of nature 

 this food hes principally on the ground, and the head must be lowered to 

 enable the horse to get at it. How is this eff-ected ? This ho-ament as 

 it has been called, because it resembles in appearance the other ligaments 

 ot the body, possesses a property which they have not, and which they 

 must not have, or they would be useless, ^o well-knit joint could exist 

 il it had this property. It is eJastic. It will ^-ield to a force impressed 

 upon it,_and tviU 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 give way to the simple weight of the 

 head, but which will yield to a very httle additional weight. Its resisting 

 power IS so admu-ably adjusted to that which it has to sustain, that when 

 certam muscles, whose action is to depress or lower the head beo-in to act 

 and add their power to the previous weight it had to bear,' the ligament 

 stretches, and when the horse is brousing, it is full two inches lono-er than 

 when the head is erect. ° 



T\aien 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 

 elasticity of the ligament— the head is once more elevated, and remains 

 so mthout the slightest exertion of muscular power. This is one of the 

 many appHcations of the principle of elasticity which will be discovered 

 and adniired 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 the bone • 

 and therefore the bone is so thick at this part. ' 



Many largeand powerful muscles are necessary to turn the head in 

 various directions, as Avell as to assist in raising it when depressed The 

 occipital bone presents a spine ranning down the centre, and a laro-e 

 roughened surface for the attachment of muscles. * 



Jiower down, and stHl at the back of the occipital bone, are two rounded 

 protuberances, by which the head is connected with the atlas or upper or 

 first vertebra, or bone of the neck ; and these are called the condyloid 

 processes of the occipital bone. All the pei-pendicular motions of the 

 liead are performed by this joint. 



Between them is a large hole, the foramen magmm, or great apertui-e. 



